Novel Control of Run-around 
Heat Recovery System 
Evaluation of the novel control method’s performance 
through laboratory experiments 

 
Master’s thesis in the Master’s Programme  
Sustainable Energy Systems 
 

 
 
SUJITH PRIYANTHA 
LORENSU HEWA WELLE KAMKANAMGE 
 
 
 
 
 
 
 
 
 
 
 
 
 
DEPARTMENT OF ARCHITECTURE   
AND CIVIL ENGINEERING 

CHALMERS UNIVERSITY OF TECHNOLOGY 
Gothenburg, Sweden 2025 
www.chalmers.se



  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 



 
 
 
 
 

 
 

MASTER’S THESIS ACEX30 

 

 

 

 

 

 

 
Novel Control of Run-around Heat Recovery System 

Evaluation of the novel control method’s performance through laboratory experiments 

Master’s Thesis in the Master’s Program Sustainable Energy Systems 

SUJITH PRIYANTHA, LORENSU HEWA WELLE KAMKANAMGE 
 
 

EXAMINER and SUPERVISOR: 

Jan-Olof Dalenbäck 

 

 

 
 
 
 
 
 
 

 
 
 

Department of Architecture and Civil Engineering 
Division of Building Services Engineering 

CHALMERS UNIVERSITY OF TECHNOLOGY 
Gothenburg, Sweden 2025 



 
 
 
 
 

 
 

 
 
 
 
 
 
 
 
 
 
 
Novel Control of Run-around Heat Recovery System 
Evaluation of the novel control method’s performance through laboratory experiments 
 
 
Master’s Thesis in the Master’s Program Sustainable Energy Systems 

SUJITH PRIYANTHA, LORENSU HEWA WELLE KAMKANAMGE 
 
  
© SUJITH PRIYANTHA, LORENSU HEWA WELLE KAMKANAMGE, 2025  
  
 
Examensarbete ACEX30 
Institutionen för arkitektur och samhällsbyggnadsteknik 
Chalmers tekniska högskola, 2025    
 
 
Department of Architecture and Civil Engineering 
Division of Building Services Engineering 
Chalmers University of Technology 
SE-412 96 Gothenburg 
Sweden  
Telephone: + 46 (0) 31-772 1000  
 
 
 
 
 
 
  
 
 
 
 
 
Department of Architecture and Civil Engineering. 
Gothenburg, Sweden 2025



 CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30                                                 i 
 

Novel Control of Run-around Heat Recovery System 
Evaluation of the novel control method’s performance through laboratory experiments 

Master’s thesis in the Master’s Programme Sustainable Energy Systems 

SUJITH PRIYANTHA, LORENSU HEWA WELLE KAMKANAMGE 

Department of  Architecture and Civil Engineering 
Division of Building Services Engineering 
Chalmers University of Technology 
 

ABSTRACT 

Run-around coil (RAC) systems are particularly well-suited for applications requiring complete 
separation between supply and exhaust airflows, such as hospitals, laboratories, and other 
facilities with stringent hygiene or contamination control requirements. Unlike rotary or plate-
type heat exchangers, RAC systems ensure full airstream separation while offering design 
flexibility and modular installation, making them appropriate for complex and specialized 
ventilation scenarios. However, their efficient operation critically depends on the accurate 
control of the circulating liquid flow rate to achieve optimal heat recovery performance. 

Traditional control strategies regulate liquid flow based on balancing the heat capacity flow rates 
of air and liquid media, which requires accurate real-time knowledge of their thermophysical 
properties. This approach becomes challenging in dynamic operational environments, especially 
under demand-controlled ventilation (DCV) conditions, where airflows fluctuate in response to 
changing occupancy. Moreover, accurately measuring flow rates and properties in real-time 
often increases system complexity, limiting the practicality of such control strategies. 

To address these challenges, a novel temperature-based control method was developed and 
evaluated through laboratory experiments. This new approach relies on temperature 
measurements, rather than flow measurements or thermophysical property estimations, to 
regulate the system. The method aims to enhance system adaptability, reduce measurement 
uncertainties, and maintain stable heat recovery performance across a wide range of ventilation 
demands and external conditions. The experimental evaluation focused on testing this strategy's 
effectiveness, robustness, and responsiveness compared to conventional flow-based control 
methods. 

The results demonstrate that the temperature-based control method (Xt) offers a reliable and 
efficient alternative, particularly in applications where fluid properties are variable or where 
accurate flow sensors are impractical. With appropriate tuning of control parameters, 
specifically the proportional and integral settings, the method achieved consistently high 
effectiveness and system stability across variable conditions. The findings suggest strong 
potential for integrating this novel control approach into both new and existing RAC 
installations, supporting improved energy efficiency and simplified operation in real-world DCV 
systems. 

Key words:  Run-around Coil (RAC), Heat Recovery, Temperature-Based Control, 
Demand-Controlled Ventilation (DCV),  Energy Efficiency 

 



ii                                  CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 



 CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30                                                 iii 
 

Table of content 
Abstract ........................................................................................................................................ i 

Table of content .......................................................................................................................... iii 

List of tables ............................................................................................................................... iv 

List of figures ............................................................................................................................. iv 

Preface ....................................................................................................................................... vii 

Abbreviations ........................................................................................................................... viii 

1. Introduction ......................................................................................................................... 1 

1.1 Background ....................................................................................................................... 1 

1.2 Aim of the thesis ............................................................................................................... 2 

1.3 Literature review ............................................................................................................... 2 

1.3.1 Air-to-air energy recovery equipment ........................................................................ 3 

1.3.2 Optimizing run-around coil heat recovery systems .................................................... 5 

1.3.3 Fluid flow rate optimization ....................................................................................... 8 

1.3.4 Liquid flow rate control ............................................................................................ 12 

1.3.5 Variation of properties of ethylene glycol with temperature and concentration ...... 14 

2. Laboratory Unit ................................................................................................................. 17 

2.1 Lab test rig ....................................................................................................................... 17 

2.2 Arrangement of sensors and devices in the Novel control setup ..................................... 19 

2.3 Control methods used in the run-around heat recovery unit ........................................... 23 

2.4 Novel control system ....................................................................................................... 24 

3. Methodology ..................................................................................................................... 32 

3.1 Method of evaluation ...................................................................................................... 32 

3.2 Limitations of the laboratory tests ................................................................................... 33 

3.3 Performance evaluation metrics ...................................................................................... 34 

3.4 Evaluation approach ........................................................................................................ 35 

3.5 Measurements and accuracy ............................................................................................ 37 

4. Results and Analysis ......................................................................................................... 39 

4.1 Results of Xt and Xv regulations .................................................................................... 39 

4.2 Performance evaluation of demand control ventilation (DCV) systems ......................... 49 

4.3 Effect of property selection (different EG%) on Xv regulation ...................................... 66 

5. Discussion ......................................................................................................................... 71 

6. Conclusion and Recommendations ................................................................................... 74 

7. References ......................................................................................................................... 75 

Appendix ...................................................................................................................................... I 

A.1 :  Calibration of temperature sensors ................................................................................. I 

 



iv                                  CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 

List of tables 
Table 1: List of sensors and their functions. ............................................................................. 20 
Table 2: Maximum effectiveness and corresponding liquid flow rates for three air flow rates.45 
Table 3: Lecture hall - DCV schedule for balanced ventilation. ............................................... 49 
Table 4: Lecture hall - DCV schedule for unbalanced ventilation with a 20% reduction in extract 
airflow. ...................................................................................................................................... 49 
Table 5: Comparison of average effectiveness and fluid flow rates for different time periods 
under Xt and Xv control regulations. ........................................................................................ 57 
Table 6: Comparison of average effectiveness and fluid flow rates for different time periods 
under Xt2 and Xv2 control regulations. .................................................................................... 58 
Table 7: Comparison of average effectiveness and liquid flow rates during different time periods 
under Xt, Xt2, Xv, and Xv2 regulation methods in an unbalanced DCV system. .................... 65 
Table 8: Thermophysical properties corresponding to the selected ethylene glycol 
concentrations. .......................................................................................................................... 66 
Table 9: Liquid flow rates corresponding to different Xv ratios for various working fluid 
concentrations. .......................................................................................................................... 67 
Table 10: Overall influence of thermophysical properties on fluid flow rates for different EG 
concentrations. .......................................................................................................................... 68 
Table 11: Comparison of average effectiveness and liquid flow rate at 750 l/s airflow using 
different EG concentration properties in the controller. ............................................................ 70 
 
 

List of figures 
Figure 1: The effectiveness related to the coupling liquid Reynold number with 37% ethylene 
glycol. ........................................................................................................................................ 10 

Figure 2: The UA-value for the coil at different air flow rates and Reynold number with 37% 
ethylene glycol. ......................................................................................................................... 10 

Figure 3: Effectiveness as a function of capacity flow ratio for different ethylene glycol 
concentrations. .......................................................................................................................... 11 

Figure 4: Viscosity of ethylene glycol at three different temperatures as a function of 
concentration. ............................................................................................................................ 14 

Figure 5: Density of ethylene glycol at three different temperatures as a function of 
concentration. ............................................................................................................................ 15 

Figure 6: Specific heat of ethylene glycol at three different temperatures as a function of 
concentration. ............................................................................................................................ 15 

Figure 7: Thermal conductivity of ethylene glycol at three different temperatures as a function 
of concentration. ........................................................................................................................ 16 

Figure 8: General arrangement of the AHU used for lab testing. ............................................. 17 

Figure 9: Test tig photos taken in the laboratory....................................................................... 18 

Figure 10: Pumping system of the RAHRU, photos taken in the laboratory. ........................... 19 

Figure 11: General arrangement of novel control setup. ........................................................... 20 

Figure 12: Control diagram of the Xt-based regulation system. ............................................... 24 

Figure 13: Field photos of the controller and its display unit.................................................... 25 

Figure 14: Dashboard interface of the web application. ........................................................... 25 



 CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30                                                 v 
 

Figure 15: Web application interface for monitoring and control of the run-around heat recovery 
unit. ........................................................................................................................................... 26 

Figure 16: Web application interface for the trend analysis window. ....................................... 26 

Figure 17: Controllable parameters on the web application's monitoring and control interface.
 ................................................................................................................................................... 27 

Figure 18: Heat capacity ratio window on the web application interface. ................................ 27 

Figure 19: Fixed setpoint tab on the web application interface................................................. 28 

Figure 20: Air and fluid properties tab on the web application interface. ................................. 28 

Figure 21: Regulator tab used set the PID value. ...................................................................... 29 

Figure 22: Control diagram for the Xt2 - based regulation system. .......................................... 30 

Figure 23: Control diagram for the Xv - based regulation system. ........................................... 31 

Figure 24: Control diagram for the Xv2 - based regulation system. ......................................... 31 

Figure 25: Variations of liquid flowrates with the pump control signal percentage. ................ 34 

Figure 26: Schematic diagram of the run-around heat recovery test setup. .............................. 37 

Figure 27: Control and monitoring interface of the run-around heat recovery unit. ................. 38 

Figure 28: Effectiveness as a function of liquid flow rate at an airflow rate of 1000 l/s for the 
novel control regulations (Xt and Xv) and the previous lab test results. .................................. 40 

Figure 29: Heat transfer effectiveness as a function of Xt and liquid flow rate for 500 l/s air flow 
rate using 30% EG. ................................................................................................................... 41 

Figure 30: Heat transfer effectiveness as a function of Xt and liquid flow rate for 750 l/s air flow 
rate using 30% EG. ................................................................................................................... 41 

Figure 31: Heat transfer effectiveness as a function of Xt and liquid flow rate for 1000 l/s air 
flow rate using 30% EG. ........................................................................................................... 42 

Figure 32: Comparison of effectiveness and liquid flow rate for 1000 l/s air flow rate. .......... 43 

Figure 33: Comparison of effectiveness and liquid flow rate for 750 l/s air flow rate. ............ 43 

Figure 34: Comparison of effectiveness and liquid flow rate for 500 l/s air flow rate. ............ 44 

Figure 35: Xt regulation, effectiveness as a function of liquid flow rate for different air flow 
rates. .......................................................................................................................................... 45 

Figure 36: Xv regulation, effectiveness as a function of liquid flow rate for different air flow 
rates. .......................................................................................................................................... 46 

Figure 37: Old lab test, effectiveness as a function of liquid flow rate for different air flow rates.
 ................................................................................................................................................... 46 

Figure 38: Comparison of Xt, Xv and Old lab results for 500 l/s air flowrate. ......................... 47 

Figure 39: Comparison of Xt, Xv and Old lab results for 750 l/s air flowrate. ......................... 47 

Figure 40: Comparison of Xt, Xv and Old lab results for 1000 l/s air flowrate. ....................... 48 

Figure 41: Novel control unit utilizing Xt regulation for a demand-controlled ventilation (DCV) 
system in a lecture hall application. .......................................................................................... 50 

Figure 42: Novel control unit utilizing Xv regulation for a demand-controlled ventilation (DCV) 
system in a lecture hall application. .......................................................................................... 51 

Figure 43: Comparison of  Xt and Xv regulations for a demand-controlled ventilation (DCV) 
system in a lecture hall application. .......................................................................................... 52 

Figure 44: Novel control unit utilizing Xt2 regulation for an unbalanced demand-controlled 
ventilation (DCV) system in a lecture hall application. ............................................................ 53 



vi                                  CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 

Figure 45: Novel control unit utilizing Xv2 regulation for an unbalanced demand-controlled 
ventilation (DCV) system in a lecture hall application. ............................................................ 54 

Figure 46: Comparison of Xt and Xt2 regulations for a demand control ventilation (DCV) system 
(both balanced and unbalanced) in a lecture hall application.................................................... 55 

Figure 47: Comparison of Xv and Xv2 regulations for a demand control ventilation (DCV) 
system (both balanced and unbalanced) in a lecture hall application. ...................................... 56 

Figure 48: Xt and Xt2 regulation for balance airflow of 750 l/s and unbalanced airflow of 
750/600  l/s. ............................................................................................................................... 59 

Figure 49: Novel control unit utilizing Xt regulation for an unbalanced demand-controlled 
ventilation (DCV) system in a lecture hall application. ............................................................ 61 

Figure 50: Novel control unit utilizing Xv regulation for an unbalanced demand-controlled 
ventilation (DCV) system in a lecture hall application. ............................................................ 62 

Figure 51: Comparison of Xt and Xt2 regulations for a unbalanced demand control ventilation 
(DCV) system in a lecture hall application. .............................................................................. 63 

Figure 52: Comparison of Xv and Xv2 regulations for a unbalanced demand control ventilation 
(DCV) system in a lecture hall application. .............................................................................. 64 

Figure 53: Estimated and measured liquid flow rates as a function of the Xv ratio for different 
EG concentrations. .................................................................................................................... 68 

Figure 54: Effectiveness and liquid flow rate for DCV airflows ranging from 300 to 750 l/s using 
Xv regulation with varying ethylene glycol (EG) concentrations. ............................................ 69 

Figure 55: Liquid flow rate for DCV airflows ranging from 300 to 750 l/s using Xv regulation 
with varying ethylene glycol (EG) concentrations. ................................................................... 69 

 

 

 

 

 

 

 

 

 

 

 

 

 

 



 CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30                                                 vii 
 

Preface 

In this master’s thesis, the performance of a newly implemented novel control system for run-
around heat recovery system was analysed and evaluated using a laboratory test rig. During this 
period, the  performance of the control unit was thoroughly tested under various conditions. 
Additionally, the associated web application was improved by integrating multiple regulation 
options, enabling the execution of diverse test scenarios for both balanced and unbalanced 
demand-controlled ventilation systems. This project was conducted at the Division of Building 
Services Engineering at Chalmers University of Technology. 

I would like to express my sincere gratitude to Professor Jan-Olof Dalenbäck, who served as 
both my supervisor and examiner. His continuous support, guidance, and readiness to answer 
my questions were invaluable throughout this work. I would also like to thank Håkan Larsson 
at the Building Services Laboratory for his assistance with the experimental testing, which was 
essential for the success of this study. Special thanks to Peter Filipsson at CIT Renergy for 
providing the initial introduction that helped initiate this project. 

I am also grateful to all members of the Division of Building Services Engineering at Chalmers 
for their valuable feedback. Finally, I would like to extend my heartfelt thanks to Despoina Teli, 
whose guidance helped me find this thesis opportunity. 

Gothenburg, June 2025 

Sujith Priyantha, Lorensu 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 



viii                                  CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 

Abbreviations 

AHU  Air handling unit 
ASHRAE American society of heating, refrigerating and air-conditioning engineers 
DCV  Demand controlled ventilation 
Eff  Effectiveness 
EG  Ethylene glycol 
ERVs  Energy recovery ventilators  
HRU  Heat recovery unit 
HRVs  Heat recovery ventilators  
HVAC  Heating ventilation and air conditioning 
HX coil  Heat exchange coil 
Max  Maximum 
PID values Proportional, integral and derivative values 
RAC  Run-around coil 
RAHR  Run-around heat recovery  
RAHRS Run-around heat recovery system 
RAHRU Run-around heat recovery unit 
VAV  Variable air volume 
VFD  Variable frequency drive 
 

Nomenclature 

A Area    [m2] 
Cp Specific heat capacity  [kJ/kgK] 
k Thermal conductivity  [W/mK] 
µ Viscosity   [mPas] 
NTU Number of transfer units [ - ] 
ρ Density    [kg/m3] 
T, t Temperature   [℃] 
U Heat transfer coefficient  [W/m2K] 
V  Volumetric flow rate  [m3/s] 
X Heat capacity ratio  [ - ] 

Subscripts 

a air 
c cool 
ea extract air 
eha  exhaust air 
l liquid 
oa  outdoor air 
sa supply air 
t  temperature 
v  flow 
w warm 

Index numbers 

1 before pump 
2 after pump, unbalanced airflow 
 



CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30                                               1 

1. Introduction 

1.1  Background 
Heat recovery technologies, including widely used fixed plate heat exchangers, rotary heat 
exchangers with desiccant wheels and run-around heat recovery (RAHR) systems, significantly 
reduce the energy demand of ventilation systems in buildings. RAHR systems play a crucial role 
in modern ventilation, particularly in applications where cross-contamination between air 
streams is not acceptable due to air quality requirements, in buildings with complex ducting and 
multiple exhaust locations, and in retrofit projects with space limitations. These systems are 
commonly used in hospitals, commercial buildings and industrial facilities to improve energy 
efficiency while meeting strict sustainability and environmental goals. 

Run-around heat recovery systems employ a closed-loop liquid medium, typically a water-glycol 
mixture, to transfer heat between extract and supply air streams, recovering waste heat and 
reducing the overall energy demand of heating, ventilation and air conditioning (HVAC) 
systems. Although RAHR systems offer unique advantages compared to other ventilation heat 
recovery systems, their design and operation are more complex, especially in ventilation systems 
with variable airflow rates which are commonly used in modern building applications. 

Optimizing heat recovery efficiency in RAHR systems requires precise control of liquid flow 
rates. Conventional control strategies regulate the system by adjusting the heat capacity flow 
rate in response to airflow variations, ensuring that the heat capacity rates of both the air and 
liquid media remain equal. However, in real-world applications, fluctuations in temperature, 
pressure and flow conditions make it challenging to maintain optimal heat recovery 
performance. Additionally, demand-controlled ventilation (DCV) systems introduce further 
complexity, as variations in airflow require real-time adjustments to the liquid flow rate to 
achieve optimum heat recovery efficiency. 

To address these challenges, a novel temperature-based control method has been developed to 
optimize the regulation of liquid flow rates and achieve optimal system performance. Unlike 
conventional control strategies, which depend on dynamic thermophysical properties and precise 
flow measurements, this method uses temperature-based indicators to regulate system 
performance. By reducing the dependence on dynamic thermophysical properties and accurate 
flow measurements, this approach enhances system reliability, simplifies operation and lowers 
maintenance requirements. The method’s ability to improve system robustness and accuracy is 
essential for effectively optimizing liquid flow rates in RAHR systems, ensuring reliable 
performance in real-world applications [1]. 

This thesis focuses on evaluating the effectiveness of the temperature-based control method for 
RAHR units through laboratory tests. The study evaluates its ability to enhance heat recovery 
efficiency, system stability, and adaptability under varying ventilation conditions. Considering 
the increasing focus on energy efficiency and sustainability in building operations, this research 
is particularly relevant to organizations managing large-scale HVAC systems. The findings have 
the potential to inform the development of practical guidelines for integrating temperature-based 
control into existing RAHR installations, supporting efforts to optimize energy performance 
while ensuring operational reliability. 

As energy regulations tighten and sustainability goals drive the adoption of advanced HVAC 
technologies, optimizing heat recovery through intelligent control mechanisms is essential for 



2                                  CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 

reducing energy consumption in ventilation systems. The insights from this study can guide 
facility managers and HVAC designers in implementing more efficient and adaptive control 
strategies, ultimately contributing to improved energy performance and operational efficiency 
in buildings. 

1.2 Aim of the thesis 
The objective of this thesis is to evaluate the performance of a novel temperature-based control 
method for run-around heat recovery systems through laboratory experiments. The study aims 
to evaluate its effectiveness in optimizing liquid flow rates and enhancing heat recovery 
efficiency compared to conventional control strategies. 

Additionally, the research analyses the system's stability, responsiveness, and efficiency under 
varying operational conditions and ventilation demands. The findings can contribute to 
understanding the feasibility of temperature-based control for real-world applications and its 
potential integration into existing run-around heat recovery system installations to improve 
energy efficiency and operational reliability. 

1.3 Literature review 
Buildings play a vital role in the global energy system, accounting for 30% of global final energy 
consumption and contributing approximately 26% of energy-related CO₂ emissions in 2022, 
according to the International Energy Agency [2,3]. A significant portion of these emissions 
results from space heating which alone accounts for around 12% of the total global energy 
demand and emissions, primarily due to the thermal and ventilation requirements of indoor 
environments [2,4]. Ensuring thermal comfort and maintaining good indoor air quality is 
essential, particularly in commercial and institutional buildings which require high ventilation 
airflows. 

To reduce energy use while maintaining indoor air quality, buildings are increasingly integrating 
heat recovery systems that capture thermal energy from extract air and transfer it to incoming 
fresh air. Under typical operating conditions, up to 30% of the energy used in buildings can be 
lost through exhaust air, highlighting the importance of recovery systems as an effective energy-
saving measure [2,5]. Commonly used heat recovery technologies in buildings include plate heat 
exchangers, rotary heat exchangers and run-around coil systems. Among these, the run-around 
heat recovery system is widely implemented for complex configurations or retrofit applications. 

A run-around heat recovery system comprises two separate fin-and-tube heat exchangers located 
in the exhaust and supply air streams. These coils are thermally coupled through a working fluid, 
typically a mixture of water and glycol, circulated by a pump. In contrast to rotary or plate 
exchangers, the run-around system enables complete separation of air streams, effectively 
eliminating the risk of cross-contamination. This characteristic makes the system well-suited for 
retrofit projects, hospitals, and laboratories. It is particularly advantageous in buildings with 
space limitations, where supply and exhaust ducts are widely spaced or strict hygiene standards 
must be maintained [2,6]. 

In response to the COVID-19 pandemic, HVAC standards started recommending higher 
ventilation rates and extended operating hours to reduce airborne transmission risks. However, 
these measures unintentionally lead to increase energy consumption in ventilation systems. As 
a result, the integration of efficient heat recovery solutions, including run-around heat recovery 



CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30                                               3 

systems, is increasingly important for balancing indoor air quality with energy performance 
objectives [2,5,6]. 

Despite the practical advantages, several studies have indicated that RAHRS frequently 
underperforms compared to the theoretical design efficiency, particularly in demand-controlled 
ventilation systems. Factors such as coil fouling, suboptimal liquid quality, malfunctioning 
valves, and inappropriate antifreeze concentrations can significantly affect system performance 
[2,7]. Moreover, design challenges such as unfavourable flow rates, pressure drops and limited 
control optimization further reduce the actual heat transfer effectiveness which typically ranges 
between 45 - 65%, despite a regulatory requirement of at least 68% effectiveness for units placed 
on the EU market after 2018 [2]. 

In comparison, plate heat exchangers typically deliver 50 - 80% efficiency, and rotary heat 
exchangers can exceed 80%, although they carry a higher risk of cross contaminations [1,8]. To 
address the inherent performance limitations of RAHRS and to achieve regulatory compliance, 
optimizing design and control strategies are essential. As suggested by Nelson [9], improving 
system effectiveness may include using large coil surface areas, low-pressure drop designs, 
optimized antifreeze concentrations, variable-speed pumping, and smart control sequences that 
respond dynamically to varying operational conditions. 

With the increasing focus on energy-efficient building retrofits driven by global climate 
objectives and the necessity of ensuring occupant health, there is an evident requirement to 
advance the study and development of innovative control strategies for RAHRS. This thesis 
focuses on evaluating a novel temperature-based control approach for RAHRS to determine its 
effectiveness in achieving optimal thermal performance. The analysis considers varying 
ventilation rates, typically observed in DCV systems, with the goal of maximizing heat recovery 
while minimizing energy consumption. 

1.3.1 Air-to-air energy recovery equipment 

Air-to-air energy recovery systems significantly contribute to enhancing energy efficiency and 
maintaining indoor environmental quality by enabling the exchange of heat and, in some 
systems, moisture between two airstreams with different temperature and humidity levels. These 
systems are particularly important in buildings where substantial energy is used to condition 
ventilation air, in this manner supporting sustainable building design. The integration of these 
systems contributes to global sustainability objectives, particularly the United Nations 
Sustainable Development Goals (SDGs) including goal 7 (affordable and clean energy) and goal 
13 (climate action) [10], as well as the European Green Deal’s goals to improve energy 
performance and reduce greenhouse gas emissions in the building sector [11]. 

These systems can recover energy in both sensible form (related to temperature) and latent form 
(related to moisture). Devices that recover only sensible energy are typically referred to as heat 
recovery ventilators (HRVs), while those that recover both sensible and latent energy are known 
as energy recovery ventilators (ERVs). Both types are widely available for residential, 
commercial, and industrial applications [12]. 

ERVs are particularly advantageous in hot and humid climates, where moisture management is 
a key concern. By transferring moisture from the incoming ventilation air to the outgoing 
exhaust air, ERVs reduce the latent cooling load, improving the dehumidification performance 
of conventional HVAC systems. This capability supports compliance with ventilation 



4                                  CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 

requirements indicated in ASHRAE standards, while maintaining occupant health and comfort 
by reducing indoor humidity and limiting the growth of mold, bacteria and allergens [12]. 

Applications 

Air-to-air energy recovery applications can be broadly categorized into three types that are 
comfort-to-comfort, process-to-comfort and process-to-process. Comfort-to-comfort 
applications include residential buildings, offices, classrooms, healthcare facilities and 
hospitality environments. In these cases, energy recovery devices are used to precondition 
incoming fresh air by recovering energy from exhaust air, reducing heating and cooling demands 
throughout the year. 

In process-to-comfort systems, waste heat from industrial processes, such as furnaces, dryers or 
ovens, is used to preheat ventilation or makeup air for comfort spaces, especially during winter. 
However, in warmer conditions, this recovery must be modulated or bypassed to avoid 
overheating. Process-to-process systems involve transferring energy directly between process 
exhaust and supply air within the same industrial operation. These systems typically recover 
only sensible heat and are designed to handle high-temperature exhaust, avoiding moisture 
transfer that could interfere with process integrity [12]. 

Technical considerations 

The performance of HRVs and ERVs is measured through several key parameters that reflect 
both thermal and operational efficiency. Effectiveness measures the ratio of recovered energy to 
the theoretical maximum possible, indicating how well the system performs under given 
conditions. The recovery efficiency ratio (RER) represents the ratio of useful recovered energy 
output to the energy input required to operate the system. Other important considerations include 
pressure drop which results from airflow resistance and affects fan or pumping energy 
requirements, and cross leakage, the unintended mixing of exhaust and supply air streams that 
may compromise indoor air quality. Additionally, frost control mechanisms are essential in 
colder climates to prevent freezing within the heat exchanger, ensuring reliable operation during 
low-temperature periods [12]. 

From an economic perspective, the feasibility of implementing air-to-air energy recovery 
systems depends on factors such as capital investment, ongoing maintenance requirements, and 
operational cost savings achieved through reduced heating and cooling demands. A 
comprehensive life cycle cost analysis that incorporates these elements is required for evaluating 
the system's cost-effectiveness and determining the expected payback period across various 
building applications. 

Types of air-to-air heat exchangers 

Air-to-air energy recovery systems utilize a variety of heat exchanger configurations, each 
designed to meet specific application requirements and design constraints. These systems are 
integral to improving indoor air quality and energy performance in modern HVAC applications, 
particularly in buildings with high ventilation demands [12]. 

Fixed-plate heat exchangers are among the most widely used configurations in HRVs and 
ERVs. These units consist of alternating layers of air channels that enable heat and, in some 
cases moisture, transfer between supply and exhaust air streams without any direct mixing. Their 
reliability, due to the absence of moving parts, makes them suitable for residential and 
commercial installations where durability and simplicity are priorities. 



CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30                                               5 

Rotary wheels, also known as enthalpy wheels, are porous rotating devices that transfer both 
sensible and latent heat. Due to their high thermal effectiveness and compact design, they are 
particularly well-suited for large-scale commercial HVAC systems. However, their moving 
components require regular maintenance and proper sealing to prevent cross-contamination 
between airflows. 

Heat pipe heat exchangers operate passively, using the phase change and capillary action of a 
working fluid contained within a sealed pipe. They are most effective when the supply and 
exhaust airflows are closely located but not necessarily aligned. These systems offer moderate 
efficiency and are valued for their simplicity and lack of mechanical components. 

Runaround coil loops use a circulating liquid, typically a glycol-water mixture, to transfer heat 
between two separate air-handling units. This configuration allows energy recovery even when 
the supply and exhaust airflows are physically separated, making it a flexible option for complex 
building layouts. Their modularity, design flexibility, and inherent frost resistance make them 
particularly suitable for retrofit applications and cold climates. This configuration is of specific 
relevance to the present thesis, which explores the control optimization of such systems. 

Thermosiphons function similarly to heat pipes but depend on gravity-driven natural convection 
rather than capillary action. These systems are generally used in vertically aligned ductwork 
where gravitational flow can be maintained for effective heat transfer. 

Fixed-bed regenerators store thermal energy in a solid medium that alternates between hot and 
cold air streams. This cyclical operation enables high heat recovery efficiency but may introduce 
complexity in control and system integration. These systems are typically used in industrial or 
process ventilation settings. 

Liquid-desiccant cooling systems are designed primarily for dehumidification, utilizing 
hygroscopic solutions to remove moisture from incoming air. While not primarily heat 
exchangers, they represent a specialized class of air-to-air recovery systems used in climate-
sensitive or humidity-critical environments. 

An ideal air-to-air energy recovery system should aim to maximize the transfer of heat and 
moisture based on differences in temperature and vapor pressure, while minimizing pressure 
drop to reduce fan energy consumption. It should also prevent cross-contamination between air 
streams to maintain indoor air quality and ensure hygiene. Additionally, optimal systems are 
those that achieve a balance between thermal efficiency, physical size, maintenance needs, and 
cost-effectiveness, factors that are especially important when integrating recovery systems into 
existing buildings. 

1.3.2 Optimizing run-around coil heat recovery systems 

Basic system configuration and performance 

A standard run-around coil (RAC) system consists of two or more finned tube heat exchanger 
coils [9]. One coil is positioned in the exhaust airstream and the other in the supply airstream. 
These coils are connected by insulated piping that circulates a working fluid, typically water or 
a water–glycol mixture, using a pump. Since the airflows remain completely separated, the 
system depends entirely on the heat transfer characteristics of the fluid. The system's overall 
performance is influenced by several factors, including fluid flow rate, pump operation, fluid 
properties, and the effectiveness of the control strategy. 



6                                  CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 

One significant advantage of RAC systems is their ability to maintain efficient performance 
under varying load conditions. This is achieved using variable-speed pumps and advanced 
control mechanisms. This adaptability ensures consistent heat recovery performance while 
maintaining indoor air quality and preventing the risk of cross-contamination. Additionally, 
these systems can be installed in air handling units that are not aligned or are located apart from 
each other, which provides practical advantages in retrofit applications or in situations with 
spatial constraints. 

The thermal effectiveness of RAC systems typically ranges from 30% to 60%, depending on the 
configuration of the coils, the layout of the piping loop and the implemented control approach. 
Although this is significantly lower than the efficiency achieved by rotary heat exchangers, RAC 
systems provide specific operational advantages. These include improved control adaptability 
and increased freeze protection, particularly when glycol solutions are used in combination with 
well-regulated control strategies [9]. 

Key design parameters influencing the performance of RAC systems 

The performance of RAC systems depends on several key design parameters that influence 
thermal efficiency, energy use and operational reliability. The following eight factors are 
particularly important when optimizing RAC system performance across different applications. 

1. Coil heat transfer characteristics 

The heat transfer ability of a coil is primarily determined by the UA value, which is the product 
of the overall heat transfer coefficient and the coil surface area. High-efficiency coils typically 
demonstrate UA values in the range of 1000 to 3000 W/°C, depending on parameters including 
airflow rate, coil depth and fin density [9]. Increasing fin density or the number of tube rows can 
improve thermal performance. However, it also results in a higher airside pressure drop, leading 
to increased fan energy use. Therefore, an optimal design should achieve a balance between 
effective heat transfer and acceptable pressure losses. 

2. Working fluid selection 

Water is typically the most effective working fluid due to its high specific heat capacity and low 
viscosity, which contribute to excellent heat transfer performance. However, it is subject to 
freezing in colder climates. To mitigate this, glycol mixtures, commonly 20 to 40 percent by 
volume, are added to provide freeze resistance at temperatures as low as -20 °C. Although glycol 
reduces the risk of freezing, it can reduce heat transfer efficiency by 15 to 25 percent and 
increases pumping requirements due to its higher viscosity [1,2,9]. Therefore, the selection of a 
suitable working fluid involves a balance between freeze protection and system efficiency, 
particularly in variable-speed applications. 

3. System fluid flow rate 

The fluid flow rate through the system directly influences the amount of heat exchanged between 
the exhaust and supply coils. Optimal flow rates are typically within the range of 0.05 to 0.1 l/s 
per kilowatt of heat transfer [9]. Flow rates that are too high increase pump energy use without 
a corresponding improvement in thermal performance, while insufficient flow can result in 
underperformance and poor temperature regulation or coil freezing. Using variable-speed pumps 
allows for dynamic flow rate control to maintain optimal operation under varying thermal loads. 



CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30                                               7 

4. Coil surface area 

Larger coil surface areas improve heat recovery by increasing the contact area between the air 
and the coil surfaces. This is typically achieved by using multiple coil rows, generally between 
four and eight, and high fin densities of up to twelve fins per inch [9]. However, increasing the 
coil surface area also introduces challenges including space limitations, higher material costs 
and increased airside pressure drop. Therefore, proper design is required to achieve optimal 
performance within practical constraints. 

5. Coil air pressure drop 

High airside pressure drop across the coil can substantially increase fan energy consumption. It 
is generally recommended to design for pressure drops in the range of 40 to 80 pascals per coil, 
depending on airflow rates and coil size. Strategies to reduce airflow resistance include 
optimizing coil geometry, using smooth airflow paths and applying anti-fouling surface 
treatments. Additionally, appropriate positioning within the AHU contributes to minimizing 
overall pressure losses [9]. 

6. Coil construction and material options 

Specialized coil designs can incorporate features such as optimized fin profiles, corrosion-
resistant coatings, and tailored tube wall thicknesses. Thin tube walls, typically less than 0.5 mm, 
enable efficient heat transfer but are less durable in high-pressure or corrosive environments. To 
enhance resistance in such conditions, materials including stainless steel or epoxy-coated copper 
are typically used, particularly in systems exposed to aggressive indoor or outdoor air conditions 
[9]. 

7. Control sequences 

To prevent frost formation on the exhaust coil in run-around heat recovery systems, appropriate 
control strategies are required when coil surface temperatures approach freezing. Conventional 
methods use fixed fluid temperature setpoints, but more effective strategies regulate the supply 
fluid flow to maintain the coil’s leaving air temperature just above the exhaust air dew point. In 
colder climates, defrost methods such as scheduled flow interruptions can be used. However, 
under typical operating conditions, maintaining a fixed exhaust air temperature provides an 
adequately reliable solution [9]. 

8. Variable speed pump implementation 

Variable-speed pumps are essential for achieving energy-efficient operation in run-around coil 
systems. By adjusting pump speed in response to real-time thermal loads and temperature 
differences, they can significantly reduce auxiliary energy use compared to fixed-speed pumps. 
They also improve control accuracy, enhance part-load performance and contribute to system 
reliability by reducing mechanical stress. These pumps are particularly important in control 
strategies aimed at optimizing performance and reducing operational costs. 

Coil selection 

Coil selection is an important part of RAC system design, requiring a balance between thermal 
performance, physical constraints and long-term reliability. Important considerations include 
airflow rate, supply and exhaust air temperatures, allowable pressure drop and the available 
space within the AHU. A properly selected coil allows the system to achieve the required thermal 



8                                  CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 

effectiveness without excessive fan energy use. To support the selection process, engineers 
generally use coil simulation tools or manufacturer-specific software to align coil performance 
with the specific demands of the application. 

System design 

The design of a RAC system should consider the overall system function. It is important to 
account for how the components operate together. Appropriate system design involves proper 
insulation of fluid pipes, hydraulic loop balancing and the use of bypass lines to support 
maintenance and reduce the risk of frost during low-temperature conditions. Major design 
aspects include pump sizing, coil orientation and AHU zoning. Minimizing piping lengths and 
selecting components with low flow resistance can improve overall system efficiency. 
Integrating temperature sensors and flow meters supports real-time monitoring and diagnostics, 
which enhances reliability and control. 

Energy and economic analysis 

Properly designed RAC systems can recover around 70 percent of the thermal energy from 
exhaust air, with actual performance depending on seasonal outdoor conditions, control methods 
and system configuration [9,20]. From an economic perspective, both capital expenses and 
operating costs must be evaluated. While systems with variable-speed pumps and sensor-based 
controls typically require higher initial investments, they generally offer lower operational 
energy costs, resulting in an acceptable payback period of approximately three to five years. A 
comprehensive life-cycle cost analysis should include considerations of energy savings, 
maintenance requirements, system reliability, and the reduction of risks related to coil freezing 
and air contamination [9]. 

1.3.3 Fluid flow rate optimization 

Optimizing the fluid flow rate in run-around heat recovery systems is necessary to achieve high 
thermal effectiveness while minimizing auxiliary energy use. Earlier research primarily explored 
the relationship between the characteristics of the coupling fluid flow and overall system 
performance. More recently, studies have examined the effects of variable operating conditions, 
including changes in airflow rate, working fluid composition, and coil configuration, on optimal 
flow parameters. The following subsection provides an overview of both foundational studies 
and recent contributions to the understanding of fluid flow rate optimization in run-around heat 
recovery systems. 

Previous studies on Run-around heat recovery systems 

One of the earliest significant contributions was made by Holmberg [2,13], who extended the 
theoretical framework established by Kays and London [2,14]. Holmberg developed an 
analytical expression to determine the optimum capacity flow rate of the coupling liquid, treating 
fluid velocity as an independent variable. His analysis concluded that thermal effectiveness is 
maximized when the capacity flow rate of the coupling fluid equals that of the air. Holmberg 
also emphasized that systems designed for higher thermal effectiveness tend to be more sensitive 
to deviations from this optimal flow rate. 

Further insights were provided by Forsyth and Besant [2,15], who examined the influence of 
various coil parameters on system effectiveness. Their study revealed that liquid flow rate and 



CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30                                               9 

tube diameter have a substantial impact on performance, while fin spacing and fin thickness 
have comparatively minor effects. Particularly, field data demonstrated that the maximum 
thermal effectiveness does not always occur at a capacity flow ratio of one, challenging earlier 
assumptions. In a follow-up study [2,16], the authors validated the theoretical predictions of 
Kays and London by comparing them with experimental data, finding deviations within ±5%. 
They also emphasized the importance of ensuring turbulent flow in the coil's liquid-side passing 
to enhance heat transfer performance. 

One of the studies, Zeng [2,17] introduced the consideration of temperature-dependent fluid 
properties in modelling RAHRS. Their findings indicated that ignoring these variations can 
significantly reduce model accuracy. Conversely, Balen [2,14], through experiments involving 
two coil configurations with different fin pitches and thicknesses, observed that changes in fluid 
temperature did not significantly influence thermal effectiveness. Despite contradicting Zeng’s 
conclusions, their results were consistent with earlier work by Holmberg and Kays and London, 
confirming that effectiveness typically peaks when the capacity flow ratio is close unity. 

However, a frequent limitation in earlier studies is the absence of validated models that consider 
the use of various glycol-water mixtures when determining optimal flow rates. In addition, 
previous research did not examine the effect of variable air volume (VAV) conditions on system 
performance when using water-ethylene glycol solutions, particularly under laminar flow 
conditions on the liquid side. This gap in understanding was recently addressed by Mahmoud 
[2], whose work is outlined in the following subsection. 

Latest studies on Run-around heat recovery systems and fluid flow rate optimization 

Recent research on the modelling and simulation of RAHRS, particularly the work by Mahmoud 
[2], has contributed valuable knowledge on how operating conditions and coupling fluid 
properties affect thermal effectiveness. The developed detailed simulation model incorporates 
variable air volume conditions and accounts for the temperature-dependent properties of the 
coupling liquid, allowing for a more accurate evaluation of system performance under realistic 
scenarios. 

In these simulations, a 37% ethylene glycol–water mixture was used as the coupling fluid. The 
results showed that as the airflow rate increased, the maximum effectiveness of the system 
decreased, primarily due to a decline in the number of thermal transfer units (NTU), as illustrated 
in Figure 1. At lower airflow rates (0.3 and 0.5 m³/s), the coupling liquid remained within the 
laminar flow regime, and maximum effectiveness was also achieved under these conditions [2].  

Importantly, the study found that maximum effectiveness did not always occur at a capacity flow 
ratio of one. Instead, the optimal flow ratio varied depending on the airflow rate, contradicting 
earlier research findings that consistently associate the best performance with a unity capacity 
flow ratio. These results highlight the need for dynamic control strategies that adjust fluid flow 
rates in response to real-time airflow conditions, particularly in systems operating under VAV 
control [2]. 



10                                  CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 

 

Figure 1: The effectiveness related to the coupling liquid Reynold number with 37% ethylene 
glycol. 
(Source, Mohammad Mahmoud (2022) [1]) 

As shown in Figure 2, the overall thermal conductance of the coil (UA-value) increases with the 
Reynolds number of the coupling fluid, with a significant increase occurring around a Reynolds 
number of 2300, which marks the transition from laminar to turbulent flow. However, the 
increase in UA-value was not always sufficient to compensate for the higher heat capacity flow 
rate when it exceeded a ratio of one. At these elevated flow rates, the temperature difference 
between the coil inlet and outlet was reduced, resulting in decreased heat transfer effectiveness 
[2]. 

 

Figure 2: The UA-value for the coil at different air flow rates and Reynold number with 37% 
ethylene glycol. 
(Source, Mohammad Mahmoud (2022) [1]) 



CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30                                               11 

The study also investigated the effect of glycol concentration on system effectiveness. The 
results, presented in Table 1 of the referenced paper [2], indicated that higher glycol 
concentrations tended to reduce the maximum effectiveness. This reduction was more evident 
at certain airflow rates, particularly around  0.5 m³/s. At lower concentrations, such as 10%, the 
variation of effectiveness with respect to the capacity flow ratio became more steady, and the 
optimal flow ratio approached unity. In comparison, increasing the glycol concentration to 30% 
or 40% moved the optimal capacity flow ratio further away from unity. These trends are 
illustrated in Figure 3, which reproduce the results from Appendix A of the study [2]. 

 

 
Figure 3: Effectiveness as a function of capacity flow ratio for different ethylene glycol 
concentrations. 
(Source, Mohammad Mahmoud (2022) [1]) 

Another important variable examined in the study was the coupling liquid velocity, which was 
modified by changing the number of coil circuits. Reducing the number of circuits decreased the 
flow area, leading to an increase in both the velocity and Reynolds number of the coupling fluid. 
This change results in an earlier transition from laminar to turbulent flow, enhancing heat 
transfer. As a result, systems with fewer circuits achieved higher maximum effectiveness, as 
shown in Table 2 and Figures 10 and 11 of the referenced paper [2]. This trend was further 
confirmed by Appendix B, which presents the influence of circuit number on effectiveness 
across different capacity flow ratios [2]. 

Additionally, the study concluded that achieving turbulent flow does not always result in the 
highest effectiveness. Instead, there is an optimal balance between fluid velocity, flow rate and 
the resulting thermal gradients. These findings extend and add to earlier research by Holmberg 
[13], Forsyth and Besant [15,16], and Balen [18], highlighting that maximum effectiveness can 
be achieved within the laminar flow regime under certain conditions, particularly when using 
glycol-water mixtures and operating under VAV systems. 



12                                  CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 

Furthermore, the study concluded that no single capacity flow ratio is universally optimal. The 
ideal ratio depends on the interaction between airflow rate, coupling liquid composition, and coil 
circuit design [2]. These insights make a significant contribution to fluid flow optimization in 
RAHRS and provide a strong foundation for future experimental and control-oriented studies. 

1.3.4 Liquid flow rate control 

Effective control of the liquid flow rate in run-around heat recovery systems is essential for 
optimizing system performance, particularly under varying operational conditions such as 
temperature fluctuations, changes in fluid properties and variations in airflow rates. This section 
reviews prevailing control methods, presents the governing equations used in existing systems, 
discusses their limitations and introduces a novel temperature-based control method proposed 
in recent studies [1]. 

Conventional methods of liquid flow rate control 

Conventionally, liquid flow rate control in RAHRS involves matching the liquid-side heat 
capacity rate with the air-side heat capacity rate, a concept referred to as balanced heat capacity. 
This approach requires measuring various parameters, including the volumetric flow rates of 
both air and liquid, along with their respective densities and specific heat capacities. The liquid-
side flow rate is typically determined by measuring the pressure drop across one of the HX coils 
or an orifice plate and correlating it with the flow rate using calibration curves or models [1]. 

However, this method is subjected to significant uncertainty. The thermophysical properties of 
heat transfer fluids, particularly glycol-water mixtures, vary with temperature and glycol 
concentration. For instance, increasing the glycol concentration or lowering the fluid 
temperature decreases specific heat capacity and increases viscosity. This creates a paradox [1]: 
while the system requires a higher flow rate to maintain thermal performance, the increased 
viscosity raises the pressure drop, which the control system may incorrectly interpret as higher 
flow. As a result, the pump speed is reduced, leading to a lower actual flow rate.  

The selection of glycol type (e.g., ethylene glycol for colder climates due to its lower freezing 
point and propylene glycol for warmer environments due to its non-toxicity) and its 
concentration significantly influence fluid behaviour. For instance, at 20°C, ethylene glycol 
shows a 35% increase in dynamic viscosity when the concentration rises from 30% to 40% [1]. 
This change must be considered in control strategies to prevent performance degradation. 

Another critical factor is the freezing point of the fluid, which depends on the glycol 
concentration. While several guidelines recommend adjusting the glycol concentration based on 
the burst temperature to prevent system damage during stagnation, others emphasize avoiding 
operational issues, such as slush formation, during sub-zero active conditions [1]. 

Equations for control methods 

Prevailing control strategies typically use a pump control approach based on a heat balance 
method. A balanced heat capacity rate is achieved when the heat capacity ratio (denoted as Xv) 
equals one. This ratio is determined by the specific heat and mass flow rate (determined using 
volumetric flow and density) of both the liquid and supply air sides. 

𝑋௏ =
𝑉௔   𝜌௔ 𝐶𝑝௔

𝑉௟  𝜌௟  𝐶𝑝௟
 



CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30                                               13 

Drawbacks of conventional control methods 

Prevailing flow-based control systems are subjected to performance degradation, particularly 
when changes in glycol concentration are not properly considered. The study by P. Filipsson [1] 
demonstrated that increasing the ethylene glycol concentration from 30% to 50% without 
adjusting the control parameters resulted in a series of significant issues. The first issue was a 
reduction in heat transfer efficiency, caused by an increase in viscosity and a decrease in the 
thermal conductivity of the glycol mixture. This hindered the system's ability to transfer heat 
effectively. 

The second issue was an increase in the required liquid flow rate, which resulted from the 
decreased specific heat capacity of the glycol. Despite this, the control system misinterpreted 
the increased pressure drop, caused by higher viscosity, as an indication of higher flow. 
Consequently, the system reduced the pump speed, resulting in a lower actual flow rate. 
Simulation results indicated that failing to account for viscosity increases could lead to 
significant reductions in heat recovery efficiency. While minor decreases in effectiveness may 
be acceptable when the heat capacity balance is not perfectly maintained, the largest losses 
occurred when the system failed to recognize and adjust to changes in fluid properties [1]. 

Proposed control method based on temperature measurements 

In response to the limitations of conventional methods, a novel control strategy was proposed 
by P. Filipsson [1], based exclusively on temperature measurements. This approach determines 
the heat capacity ratio by measuring the terminal temperature differences across one or both 
coils, eliminating the need for pressure or flow measurements and the associated uncertainties. 

When air flow rates are balanced, temperature measurements from only one coil are sufficient 
(denoted as Xt). However, when air flow rates are unbalanced, temperature measurements from 
both the supply and exhaust air coils are required. The average temperature ratio across both 
coils (denoted as Xt,2) offers a more comprehensive indication of the RAHRU thermal balance 
[1]. 

This method offers several advantages: 

 It automatically compensates for changes in fluid properties such as viscosity and 
specific heat capacity. 

 It can adapt to latent heat transfer scenarios, where condensation on the exhaust air coil 
increases heat recovery potential. 

 It enables performance monitoring by detecting deviations indicative of fouling or coil 
imbalance. 

 It identifies unintended air flow imbalances that could degrade system performance. 

The simulation study validated the robustness of this approach under varying air flow rates and 
glycol concentrations, demonstrating its ability to maintain near-optimal heat transfer 
effectiveness, where conventional methods could be less effective [1]. 

The new control system 

Developing on the temperature-based approach, the new control system incorporates real-time 
temperature monitoring at the coil inlets and outlets, using primary heat transfer equations to 
guide pump control. This reduces dependence on fluid-specific measurements and minimizes 
the impact of sensor uncertainty and calibration drift. 



14                                  CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 

This system represents a significant advancement in RAHRS control by shifting from rigid, 
flow-based methods that depend on multiple parameters to a more adaptive, temperature-based 
approach with fewer data requirements. This transition enables the integration of fault detection, 
automatic balancing, and energy efficiency improvements through intelligent control algorithms 
based on directly measured thermal performance instead of indirectly inferred fluid behaviour 
[1]. 

1.3.5 Variation of properties of ethylene glycol with temperature and concentration 

As discussed in the previous subsection, conventional liquid flow rate control methods are 
significantly influenced by the thermophysical properties of the working fluid, typically an 
ethylene glycol and water solution, which vary with both glycol concentration and temperature. 
The most relevant properties for optimizing run-around heat recovery systems include density, 
viscosity, thermal conductivity, and specific heat capacity. Among these, thermal conductivity 
and viscosity directly affect the effectiveness of heat transfer and the regulation of liquid flow 
rate, respectively. In addition, density and specific heat capacity determine the liquid-side heat 
capacity rate, which must be appropriately matched to the air-side in order to optimize overall 
heat recovery performance. 

The relevant thermophysical properties of ethylene glycol, a commonly used working fluid in 
cold climates, are presented in Figures 4 to 7 for three different average temperatures 
representative of this environmental context. The property data used to generate these figures 
were obtained from the ASHRAE Handbook (2021) [19]. 

 

Figure 4: Viscosity of ethylene glycol at three different temperatures as a function of 
concentration. 

According to the Figure 4, the viscosity of the ethylene glycol solution increases with higher 
glycol concentration, and this effect becomes more pronounced at lower temperatures. 
Conversely, as temperature increases, the viscosity of the solution decreases. 



CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30                                               15 

 

Figure 5: Density of ethylene glycol at three different temperatures as a function of 
concentration. 

According to Figure 5, the density of the ethylene glycol solution increases with higher glycol 
concentration. However, unlike viscosity, this increase is relatively moderate. Conversely, as 
temperature rises, the density of the solution decreases. 

 

Figure 6: Specific heat of ethylene glycol at three different temperatures as a function of 
concentration. 

According to Figure 6, the specific heat capacity of the ethylene glycol solution decreases 
significantly with increasing glycol concentration. In contrast, as temperature rises, the specific 
heat capacity increases moderately. 



16                                  CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 

 

Figure 7: Thermal conductivity of ethylene glycol at three different temperatures as a function 
of concentration. 

According to Figure 7, the thermal conductivity of the ethylene glycol solution decreases 
significantly with increasing glycol concentration. Meanwhile, as temperature increases, the 
thermal conductivity demonstrates a slight increase. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 



CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30                                               17 

2. Laboratory Unit 

2.1 Lab test rig 

The ventilation test rig used to evaluate the novel temperature-based control system of the run-
around heat recovery unit is a double-deck (two-tier) AHU. The rated airflow capacity of each 
air stream is 1.0 m³/s (1000 l/s), with static pressure drops of 580 Pa in the supply stream and 
520 Pa in the extract stream, in accordance with the standard components. 

The general arrangement of the AHU, indicating its main components, is presented in Figure 8 
below. 

 

 
 

AD : air damper    E-HC : electric heater coil 
B-AD : bypass air damper   FF : exhaust air fan 
BF : bag filter (air filter)   HR-C : heat recovery coil 
CC : cooling coil    TF : supply air fan 
 

Figure 8: General arrangement of the AHU used for lab testing. 

(Source, Product data sheet of AHU eQ, FläktGroup, 2020 [20]) 

The supply air stream deck of the AHU consists of the following components in sequence; 
outdoor air intake, outdoor air damper, air filter assembly, cooling coil, heat recovery coil, 
supply air fan, and supply air damper, with a flange connection at the supply air delivery end. 
Similarly, the extract air stream deck includes the following components; extract air intake, air 
filter assembly, air damper, electric heater coil, heat recovery coil, extract air fan, and a flange 
connection at the exhaust air delivery end. Additionally, a horizontal bypass air damper, labelled 
as B-AD in Figure 8, is installed between the supply and extract air decks. This damper allows 
the supply air to bypass directly into the extract air stream when required for experimental 
purposes, ensuring that test air temperatures do not negatively impact the laboratory indoor 
environment. 

The overall dimensions of the AHU are 5.8 m in length, 1.2 m in width, and 1.5 m in height. 
Photos taken in the laboratory AHU are presented in Figure 9 below. 

exhaust 
air 

outdoor 
air 

extract
air

supply air 

FF 

CC BF 

AD 

HR-C BF AD 

B-AD 

E-HC 

TF AD HR-C 



18                                  CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 

  

Figure 9: Test tig photos taken in the laboratory. 

Supply and extract fans: 

Both the supply air fan and extract air fan are equipped with variable frequency drive (VFD) 
electric motors, allowing for adjustable airflow rates in both streams based on experimental 
requirements. The rated airflow capacity of each fan is 1000 l/s, in accordance with the standard 
components mentioned above.  

Additionally, the fan assembly is designed for high electrical efficiency, with a specific fan 
power (SFP) of 1.49 kW/(m³/s) at the rated airflow rate. 

However, the airflow rate of both the supply and extract fans can be adjusted between 0 l/s and 
1500 l/s using the VFD unit, allowing for greater flexibility in experimental conditions. 

Cooling coil: 

A cooling coil with a capacity of 16.5 kW and a four-pass configuration is installed at the outdoor 
air intake to cool the incoming air before it passes through the heat recovery unit (HRU), as 
required for experimental purposes. 

However, during the evaluation of the novel control system, the cooling coil was not utilized, as 
the outdoor air temperatures during the test period reflected average winter conditions, making 
its operation unnecessary. 

Run-around heat recovery unit: 

As shown in Figure 8, both air streams are equipped with heat exchange (HX) coils. The heat 
transfer capacity of the unit is 26 kW with a fluid flow rate of 0.36 l/s (1296 l/h) using a 30% 
ethylene glycol solution. The pressure drop across the HX coil at this rated fluid flow is 
approximately 125 kPa. Under these flow conditions, the rated efficiency of the RAHR unit is 
67.1%. 

Each HX coil consists of 64 fluid paths, constructed from 25 mm copper tubes with aluminium 
fins spaced at a 2 mm pitch. The liquid volume of each coil is 26 liters. 

 



CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30                                               19 

Electric air heater coil: 

A step-controlled electric heater with a capacity of 24 kW is installed in the extract air stream to 
increase the extract air temperature. It ensures that the air reaches the required temperature range 
before entering the HX coil, in accordance with the set values. This allows for the adjustment of 
the extract air temperature according to indoor conditions, ensuring accurate simulation of 
different test conditions. 

Pumping system of the RAHR unit: 

The pumping system used to circulate the 30% ethylene glycol solution between the two heat 
exchangers consists of insulated copper pipes, a VFD driven vertical inline circulation pump, a 
pressure vessel, pressure gauges, thermometers, an automatic 3-way valve, a manual 3-way 
valve, a smart flow meter, and valves for purging, draining, and refilling. Figure 10 illustrates 
the pumping system installed in the laboratory. 

  

Figure 10: Pumping system of the RAHRU, photos taken in the laboratory. 

The circulation pump has a rated capacity of 3.0 m³/h (0.83 l/s) at a head of 38.3 m, with an 
efficiency of 57.3%. Equipped with a variable frequency drive, the pump allows for precise 
regulation of the liquid flow rate based on system requirements and control signals. 

However, the liquid flow rate in the novel control unit is automatically adjusted based on the 
updated tuning parameters. The flow rate varies between a minimum of approximately 0.1 l/s 
and a maximum of 0.5 l/s, depending on the control signal from the new controller. 

2.2 Arrangement of sensors and devices in the Novel control 
setup 

Figure 11 illustrates the general arrangement of the sensors used for measurement and control, 
along with their locations within the RAHR system for the novel control setup. 



20                                  CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 

 

Figure 11: General arrangement of novel control setup. 

Table 1 below provides a detailed list of sensors, including their labels, locations, functions, and 
relevant remarks regarding the novel control system of the RAHR unit. 

Table 1: List of sensors and their functions. 

Label Type / description Location Function and remarks 

FF021 extract air fan inside extract air deck, 
between heat exchange (HX) 
coil and exhaust air damper 

flow rate can be varied 
from 0 l/s to 1500 l/s 

TF011 
(TF1) 

supply air fan inside supply air deck, 
between HX coil and supply 
air damper 

flow rate can be varied 
from 0 l/s to 1500 l/s 

FO1 VFD drive 
supply air fan 

inside the control panel regulate the frequency 
based on the control signal 

FO2 VFD drive 
extract air fan 

inside the control panel regulate the frequency 
based on the control signal 

SK011 electric air heater on extract air deck, between 
extract air damper and HX coil 

regulate extract air 
temperature before 
entering to HX coil 

P40 circulation pump 
VFD 
 

on hydronic pipe circuit in 
warm water side, between the 
extract HX coil and supply 
HX coil 

circulated the liquid flow 
and flowrate varies based 
on control signal 

TA smart 2-way control valve 
with integrated 
ultrasonic flow meter 

on liquid circulation pipe, in 
between extract heat 
exchanger and circulation 
pump 

measure liquid flow rate 
and recovered energy  
(flow rate can be regulated)  

GT018 
(GT001)  

temperature sensor 
(supply air) 

inside supply air deck, 
between supply air damper 
and supply air fan 

measure supply air 
temperature after fan 

GT003 temperature sensor 
(outdoor air) 

inside supply air deck, 
between filter assembly and 
HX coil 

measure outdoor air 
temperature 

TF011 

SK011 



CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30                                               21 

Label Type / description Location Function and remarks 

GT011 temperature sensor 
(extract air after heater) 

inside extract air deck, after 
heater coil 

measure extract air 
temperature after heater 
coil 

GT012 temperature sensor 
(extract air before HX 
coil) 

inside extract air deck, 
between electric heater coil 
and HX coil closer to HX coil 

measure extract air 
temperature before HX coil 

GT019 temperature sensor 
(exhaust air after fan) 

inside extract air deck, after  
extract air fan (between extract 
fan and exhaust air damper) 

measure exhaust air 
temperature after fan 

GT40 liquid temperature 
sensor (warm side) 

on hydronic pipe after 
circulation pump (P40) 

measure warm water 
temperature before 
entering to the supply air 
HX coil 

GT41 liquid temperature 
sensor (cool side) 

on hydronic pipe after supply 
air HX coil 

measure cool water 
temperature after supply 
air HX coil 

GT42 liquid temperature 
sensor (warm side) 

on hydronic pipe after extract 
air HX coil 

measure warm water 
temperature after extract 
air HX coil 

GF011 differential pressure 
sensor (supply fan) 

in supply air deck, across the 
supply air fan (TF011) 

measure diff. air pressure 
across the supply air fan 
and control the supply  air 
flow rate 

GF021 differential pressure 
sensor (extract fan) 

in extract air deck, across the 
extract air fan (FF021) 

measure diff. air pressure 
across the extract air fan 
and control the extract air 
flow rate 

GP41 differential pressure 
sensor (liquid) 

on hydronic pipe system, 
across the supply air HX coil 

measure diff. air pressure 
across the supply air HX 
coil 

GP081 differential pressure 
sensor (air filter) 

in supply air deck, across the 
supply air filter (F7 bag filter) 

measure diff. air pressure 
across the supply air filter 

GP082 differential pressure 
sensor (air filter) 

in extract air deck, across the 
extract air filter (F7 bag filter) 

measure diff. air pressure 
across the extract air filter 

ST011 on-off damper actuator  
(supply air) 

in  supply air deck, on supply 
air damper 

open-close control: supply 
air damper 

ST021 on-off damper actuator  
(extract air) 

in  extract air deck, on extract 
air damper 

open-close control: extract 
air damper 

ST022 manual damper 
(exhaust air) 

in  extract air deck, at exhaust 
air end 

manually open-close: 
exhaust air damper 

ST051 on-off damper actuator  
(outdoor air) 

in  supply air deck, on outdoor 
air damper 

open-close control: outdoor  
air damper 

SV031 automatic 3-way valve 
(bypass water-glycol 
from warm side to cool 
side) 

on hydronic pipe system as 
indicated, from warm side to 
cool side 

in this experiment not used 
bypass (0% bypass) 

 

The control system of the RAHR unit integrates various sensors and control devices to monitor 
and regulate airflow, temperature, pressure, and fluid flows, ensuring optimal system 
performance.  



22                                  CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 

Fan control: 

The airflow control is managed through the supply air fan (TF011) and extract air fan (FF021), 
both of which have variable flow rate capabilities ranging from 0 l/s to 1500 l/s. The operation 
of these fans is controlled by variable frequency drives (FO1 and FO2), which regulate their 
speed based on the system’s airflow requirements. To ensure precise control of airflow, 
differential pressure sensors (GF011 and GF021) are installed across the supply and extract fans, 
respectively, providing real-time feedback for maintaining the desired air flow rates. 

Hydronic circulation system: 

The hydronic system is responsible for circulating a 30% ethylene glycol solution between the 
heat exchangers to facilitate heat recovery. A variable frequency-driven circulation pump (P40) 
regulates the liquid flow rate based on control signals. The system includes liquid temperature 
sensors (GT40, GT41, and GT42) to monitor the water-glycol temperature at critical points: 
GT40 measures the warm-side temperature before entering the supply air heat exchanger, GT2 
measures the temperature after the extract air heat exchanger, and GT41 measures the cooled 
liquid temperature after passing through the supply air heat exchanger. Additionally, a 
differential pressure sensor (GP41) is positioned across the supply air heat exchanger to evaluate 
hydronic performance, while a smart flow meter (TA smart) continuously measures the water-
glycol flow rate within the circuit. 

TA Smart flow measuring device: 

The TA smart is an ultrasonic flow measurement device that provides accurate flow 
measurements, even at low flow rates. Accurate monitoring of the ethylene glycol solution's 
flow rate is essential for evaluating heat recovery performance, and the TA Smart ensures 
reliable data collection for effective system assessment. This precision ensures the RAHR unit 
operates efficiently across varying ventilation rates. 

In addition to flow measurement, the TA Smart continuously monitors valve position, 
temperature differences using sensors installed around the extract heat exchanger unit, and the 
recovered heating power and energy across the extract heat exchanger. This comprehensive data 
collection offers valuable insights into system performance. 

Further, the TA smart allows for manual adjustments to the flow rate when necessary, providing 
flexibility under various testing conditions.  

Temperature measurement and control: 

Air temperature regulation is a key aspect of the RAHR system’s control strategy. Several 
temperature sensors are installed at different locations to monitor air temperatures throughout 
the process. GT018 (GT001) measures the supply air temperature before it enters the room or 
extract stream (measures the supply air temperature immediately after the fan). Similarly, the 
GT019 sensor tracks the exhaust air temperature after the extract fan. To monitor the outdoor 
air conditions before entering the system, GT003 is installed at the inlet of the supply air deck, 
just before the heat exchanger coil. 

Extract air temperature is monitored by GT011 and GT012, both of which are positioned 
downstream of the electrical heater (SK011). GT011 is installed immediately after the heater to 
measure the temperature increase due to heating, while GT012 is placed closer to the heat 
exchanger coil to track the extract air temperature before it enters the heat recovery unit. These 



CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30                                               23 

sensors ensure that the extract air temperature is maintained within the desired range according 
to the control setpoints. 

This comprehensive sensor integration, as detailed in Table 1 and depicted in Figure 11, ensures 
precise control of airflow rates, temperature regulation, and fluid circulation, enabling an 
effective evaluation of the novel control strategy under varying experimental conditions. 

2.3 Control methods used in the run-around heat recovery 
unit 
As indicated in section 1.2 (Literature review), the conventional method for controlling the 
liquid flow rate in a run-around heat recovery unit is based on the thermophysical properties and 
flow rates of both the air and liquid media, along with the pressure drop across the liquid side. 
The control strategy aims to balance the heat capacity flow rates of the air and liquid streams to 
optimize the heat recovery effectiveness of the system. This is typically achieved by adjusting 
the pump speed through a variable frequency drive (VFD) based on the following heat capacity 
equation [1] . 

Equation 1 

𝑉௟ =
𝑉௔   𝜌௔  𝐶𝑝௔

𝜌௟   𝐶𝑝௟

 

Based on equation 1, the liquid flow rate Vl is determined to regulate the pump speed. The 
parameters in this equation are defined as follows: Va is airflow rate, ρa is air density, Cpa is 
specific heat capacity of air, ρl  is liquid media density and Cpl is specific heat capacity of the 
liquid media. 

Equation 1 assumes a balanced heat capacity rate, with a heat capacity ratio equal to one. 
Accordingly, the heat capacity ratio based on flow rates, denoted as Xv, is expressed in Equation 
2 below. 

Equation 2 

𝑋௏ =
𝑉௔   𝜌௔  𝐶𝑝௔

𝑉௟   𝜌௟   𝐶𝑝௟

 

For the supply air heat exchange coil, the heat balance equation can be written as equation 3 
below. 

Equation 3 

𝑉௔   𝜌௔  𝐶𝑝௔   (𝑡௦௔ − 𝑡௢௔) = 𝑉௟   𝜌௟   𝐶𝑝௟   (𝑡௟,௪,ଶ − 𝑡௟,௖) 

The parameters in above equation are defined as follows:  tsa is supply air temperature, toa is 
outdoor air temperature and tl,w,2 is warm side liquid temperature after the pump, and tl,c is cold 
side liquid temperature. 

Considering Equations 2 and 3, for a balanced ventilation system, the heat capacity ratio based 
on temperature, Xt can be expressed as Equation 4 below. 

Equation 4 

𝑋௧ =
𝑡௦௔ − 𝑡௢௔

𝑡௟,௪,ଶ − 𝑡௟,௖

 



24                                  CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 

Accordingly, Xv and Xt represent the same heat capacity ratio, however, their calculation 
approaches differ. Xv is determined using the flow rates and thermophysical properties of both 
air and liquid media, while Xt is derived entirely from temperature measurements. Based on 
Equation 4, Xt can be calculated without the need for flow rate or thermophysical property data. 
In dynamic systems, accurately capturing these properties is challenging. Additionally, reliable 
flow measurement requires precision flow meters, which are expensive [1]. 

Using Xt for control provides a more practical and robust approach, as it avoids dependence on 
variable fluid properties and eliminates the need for a precision flow meter. Considering these 
advantages, the implemented novel control unit regulates the RAHR system based on Xt, using 
temperature-based heat capacity ratio regulation. 

2.4 Novel control system 

The proposed novel control system is designed based on temperature measurements in air 
streams and liquid media, in contrast to the previous control approach, which depend on 
thermophysical properties and flow rates. 

In the new control system, the flow rate of the working fluid (30% ethylene glycol) is adjusted 
to achieve optimal system effectiveness based on the heat capacity ratio, Xt, which is calculated 
from temperature measurements. This ratio is derived from the measured temperature difference 
between the air stream and the liquid medium across the supply air heat exchanger coil. The 
control method incorporates predefined set values that consider airflow rates. A single-line 
closed-loop control diagram representing the Xt-based regulation is shown in Figure 12 below. 

 
Figure 12: Control diagram of the Xt-based regulation system. 

The heat capacity ratio, Xt, is calculated based on the temperatures measured around the supply 
air heat exchanger, as defined in Equation 4. If Xt exceeds the setpoint value of 1 (Xt = 1), the 
controller adjusts the system to reduce the liquid flow rate. Conversely, if Xt is below the 
setpoint, the controller increases the liquid flow rate. 

This novel control system continuously monitors and regulates parameters based on 
experimental requirements. The system is managed through a web-based application developed 
specifically for this control approach by Sustainable Intelligence (SI). 

New control unit: 

A new control unit, together with the necessary devices and accessories, was installed within the 
existing control panel and integrated with the AHU and RAHR system panels and the field 
devices. 



CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30                                               25 

The installed controller is the REGIN EXOcompact Ardo, model XCA283W-4. This compact 
controller features 28 I/O points, an external display unit, and supports multiple communication 
protocols, including EXOline, Modbus, BACnet, and EFX. It also provides web server access, 
enabling remote monitoring and control. Field photos of the controller and its display unit are 
shown in Figure 13 below. 

  

Figure 13: Field photos of the controller and its display unit. 

Web application for remote access, monitoring and controlling: 

The web application for remote access, monitoring, and control of the novel control unit is 
provided by Sustainable Intelligence (SI). It integrates with the ARRIGO BMS to enable the 
operation of the run-around heat recovery unit through a web-based interface. 

The following Figures 14 to 16 present screenshots of the Windows web application, including 
the dashboard, the RAHR unit monitoring and control panel, and the trend analysis interface. 

 

Figure 14: Dashboard interface of the web application. 

 



26                                  CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 

.

 

Figure 15: Web application interface for monitoring and control of the run-around heat 
recovery unit. 

 

Figure 16: Web application interface for the trend analysis window. 

Controllable parameters:  

Figure 17 below shows the controllable parameters, highlighted in light blue, on the web 
application's monitoring and control interface. These parameters can be adjusted according to 
different test requirements. 

The primary controllable parameters used in the testing include the supply and extract air flow 
rates, extract air temperature (adjusted using electrical heater coil), and the parameters within 
the heat capacity ratio window. Further details regarding heat capacity ratio window are 
discussed below, along with the relevant figures. 



CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30                                               27 

 

Figure 17: Controllable parameters on the web application's monitoring and control interface. 

Additionally, the output of the TA valve, specifically the valve opening position (expressed as a 
percentage), can be manually adjusted using set points. During the tests conducted in this thesis, 
the valve opening was set to 100%, and the fluid flow rate in the circulation system was entirely 
controlled by the circulation pump, based on control signals from the selected regulation method. 

Heat capacity ratio window: 

Figure 18 illustrates the heat capacity ratio window, which includes the main parameter selection 
tabs required for regulating the novel control unit. 

 

Figure 18: Heat capacity ratio window on the web application interface. 

The heat capacity ratio window includes several key tabs for configuring the control system. 
These tabs consist of the heat capacity ratio selection tab, the setpoint input tab, and the air and 
fluid settings tab, where the thermophysical properties of the air and liquid are entered, 



28                                  CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 

particularly important for flow-rate-based regulation (Xv). Additionally, the regulator tab allows 
the input of appropriate PID values, based on the selected regulation method and airflow rates. 
These tabs are illustrated in Figures 19 to 21 below. 

 

Figure 19: Fixed setpoint tab on the web application interface. 

The fixed setpoint tab is used to define the desired value for the selected regulation option. This 
setpoint represents the heat capacity ratio and is typically set to 1 to achieve maximum system 
effectiveness. However, it can be adjusted to other positive values depending on the specific 
requirements of the test. 

 

Figure 20: Air and fluid properties tab on the web application interface. 

The air and fluid properties setting tab enables the input of the average thermophysical properties 
of the air and working fluid media. These properties are determined based on the average 
temperature observed during the testing phase. This information is essential when selecting 
flow-rate-based regulation methods, Xv and Xv2, as it ensures accurate control of the system in 
relation to the fluid media properties. 



CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30                                               29 

 

Figure 21: Regulator tab used set the PID value. 

The regulator tab allows for the input of PID values, which are essential for optimal system 
performance. As the derivative (D) value is always set to 0 due to inherent signal noise, it is 
crucial to select appropriate proportional (P) and integral (I) values. These PI values vary for 
each test, depending on the selected regulation method and air flow rates. Additionally, the PI 
values must be manually entered for different test scenarios. 

Regulation options: 

As illustrated in Figure 18 above, the heat capacity ratio window presents four different 
regulation options, each designed to control the operation of the novel control unit under 
different system conditions. The primary regulation option, referred to as Xt, is based on 
temperature measurements obtained from the supply HX coil. This regulation method is 
specifically designed for systems with balanced air flow rates. It aims to optimize the 
performance of the RAHR system by maintaining an optimal heat capacity ratio, based on the 
measured temperature differences. 

In addition to Xt, the system includes Xt2, which is designed for conditions with unbalanced 
airflows. As Xt, Xt2 is also based on temperature measurements, but it accounts for differences 
in airflow rates between the supply and exhaust air streams. Xt2 is developed by considering the 
average heat recovery effectiveness of both the supply and exhaust heat exchanger coils. 

The third and fourth regulation options, Xv and Xv2, are developed based on flow-rate methods, 
focusing on adjusting the liquid flow rate to maintain a balanced heat capacity ratio. Xv is used 
for systems with balanced ventilation systems. In contrast, Xv2 is specifically designed for 
systems with unbalanced ventilation. 

Each of these regulation options is governed by specific equations, which are essential for 
calculating the heat capacity ratio and determining the corresponding control actions. The 
following section presents the detailed equations used for each regulation method.  

Equation used in each regulation method: 

The equation used to calculate Xt based on temperature measurements is presented above as 
Equation 4. This calculation requires four temperature measurements across the supply air heat 
exchanger coil. The method is specifically designed for balanced ventilation systems, where the 
supply and exhaust airflow rates are equal. The corresponding control diagram is illustrated in 
Figure 12 above. 



30                                  CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 

For Xt2 regulation, the equation used to calculate Xt2 based on temperature measurements is 
presented below as Equation 6. This regulation method is specifically designed for unbalanced 
ventilation systems. Accordingly, the Xt2 calculation requires seven temperature measurements 
across both the supply and extract air heat exchanger coils. The corresponding control diagram 
for the Xt2 regulation method is illustrated in Figure 22 below. 

Equation 6 

𝑋௧ଶ =
(𝑡௘௔ − 𝑡௘௛௔) + (𝑡௦௔ − 𝑡௢௔) 

𝑡௟,௪,ଵ + 𝑡௟,௪,ଶ − 2𝑡௟,௖
 

The parameters in above Equation 6 are defined as follows:  tsa is supply air temperature, toa is 
outdoor air temperature, tea is extract air temperature, teha is exhaust air temperature,  tl,w,1 is warm 
side liquid temperature before the pump, tl,w,2 is warm side liquid temperature after the pump, 
and tl,c is cold side liquid temperature. 

 

Figure 22: Control diagram for the Xt2 - based regulation system. 

For Xv regulation, the equation used to calculate Xv based on flow rate measurements is 
presented below as Equation 7. This calculation requires two flow rate measurements across the 
supply air heat exchanger coil: the liquid flow rate and the supply air flow rate, as this regulation 
method is designed for balanced ventilation systems. Additionally, the thermophysical 
properties of the air and liquid media, determined based on the average operating temperature 
of the run-around heat recovery unit, are required. These properties are manually entered into 
the control unit, considering the test