The Impact of Air Tightness of the Building Envelope on the Efficiency of Ventilation Systems with Heat Recovery
Binamu, A.; Lindberg, R. (2001)
Tampereen teknillinen korkeakoulu
2001
Rakennustekniikan osasto - Department of Civil Engineering
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https://urn.fi/URN:ISBN:978-952-15-3146-0
https://urn.fi/URN:ISBN:978-952-15-3146-0
Tiivistelmä
Ventilation heat recovery is the process by which heat energy is recovered from the exhaust air for re-use to preheat the incoming cold supply air. Among other factors, air infiltration can greatly impair the total performance of a ventilation heat recovery system and in some cases, it can cause the recovery process to expend more energy than that recovered. Moisture problems and severe contamination of the indoor air can also result from air leakage through the building envelope.
This study investigated the impact of envelope air tightness on energy performance of ventilation systems with heat recovery. The research involved various measurements including indoor and outdoor temperatures, the supply and extract air temperatures before and after heat recovery, wind speed and direction, solar radiation, building air tightness, infiltration/exfiltration, and heat energy used to heat the buildings. The tests were carried out in three experimental buildings constructed of different building materials and having different degree of air tightness. The air tightness of these buildings was determined by pressurisation method. The uncontrolled air change rate ( air leakage) was measured by using tracer gas method. Balanced mechanical ventilation systems with air-to-air heat recovery were used to ventilate the buildings.
Two equations were derived in the study. One for theoretical prediction of uncontrolled air change based on the pressurisation test data. The other one is for estimating the annual heating energy that is needed for heating infiltration air caused by various wind speeds. Comparison of the results obtained through the two derived equations correlated very well with those obtained by using other equations that were found in the existing literature.
The results revealed that a significant quantity of heating energy in buildings is lost due to uncontrolled air changes. For example, it was found that the annual quantity of heating energy loss due to infiltration/exfiltration in a building with 11.1 h-1 (ach) degree of air tightness, is approx. 28.076 kWh/m3 annually. On the other hand, the annual quantity of heating energy loss due to infiltration/exfiltration in buildings that had 0.93 and 1.2 h-1 air tightness was 2.324 kWh/m3 and 3 kWh/m3 respectively.
The results also showed that the energy savings through implementation of mechanical ventilation systems with heat recovery systems (MVHR) is significant enough to merit the adaptation of these systems in all Finnish residential buildings. In this case, it should however be emphasised that careful attention must be paid to the degree of envelope air tightness. In addition, it was found that the absolute quantity of energy savings through ventilation heat recovery increased as the outdoor temperature decreased. The heat recovery (HR) efficiencies of the ventilation systems tested were found to vary within the range between 42.5% and 70% depending on the outdoor temperature.
The air tightness of the building envelope was found to have a great impact on the overall performance of a ventilation system including the global energy consumption in a building. An increase in air infiltration was found to be directly proportional to the increase in heating energy demand of a building. It was concluded that for potential energy savings through ventilation heat recovery, the degree of envelope air tightness of buildings that are ventilated by MVHR should strictly not exceed 1.5 h-1 (ach) at 50 Pa pressure difference. Also, it was confirmed that the colder the outdoor climate the greater the necessity for implementing mechanical ventilation systems with heat recovery to ventilate buildings. The research findings also suggest that there is a need for improvements in the quality of building envelopes and, in particular, the degree of air tightness and insulation levels.
This study investigated the impact of envelope air tightness on energy performance of ventilation systems with heat recovery. The research involved various measurements including indoor and outdoor temperatures, the supply and extract air temperatures before and after heat recovery, wind speed and direction, solar radiation, building air tightness, infiltration/exfiltration, and heat energy used to heat the buildings. The tests were carried out in three experimental buildings constructed of different building materials and having different degree of air tightness. The air tightness of these buildings was determined by pressurisation method. The uncontrolled air change rate ( air leakage) was measured by using tracer gas method. Balanced mechanical ventilation systems with air-to-air heat recovery were used to ventilate the buildings.
Two equations were derived in the study. One for theoretical prediction of uncontrolled air change based on the pressurisation test data. The other one is for estimating the annual heating energy that is needed for heating infiltration air caused by various wind speeds. Comparison of the results obtained through the two derived equations correlated very well with those obtained by using other equations that were found in the existing literature.
The results revealed that a significant quantity of heating energy in buildings is lost due to uncontrolled air changes. For example, it was found that the annual quantity of heating energy loss due to infiltration/exfiltration in a building with 11.1 h-1 (ach) degree of air tightness, is approx. 28.076 kWh/m3 annually. On the other hand, the annual quantity of heating energy loss due to infiltration/exfiltration in buildings that had 0.93 and 1.2 h-1 air tightness was 2.324 kWh/m3 and 3 kWh/m3 respectively.
The results also showed that the energy savings through implementation of mechanical ventilation systems with heat recovery systems (MVHR) is significant enough to merit the adaptation of these systems in all Finnish residential buildings. In this case, it should however be emphasised that careful attention must be paid to the degree of envelope air tightness. In addition, it was found that the absolute quantity of energy savings through ventilation heat recovery increased as the outdoor temperature decreased. The heat recovery (HR) efficiencies of the ventilation systems tested were found to vary within the range between 42.5% and 70% depending on the outdoor temperature.
The air tightness of the building envelope was found to have a great impact on the overall performance of a ventilation system including the global energy consumption in a building. An increase in air infiltration was found to be directly proportional to the increase in heating energy demand of a building. It was concluded that for potential energy savings through ventilation heat recovery, the degree of envelope air tightness of buildings that are ventilated by MVHR should strictly not exceed 1.5 h-1 (ach) at 50 Pa pressure difference. Also, it was confirmed that the colder the outdoor climate the greater the necessity for implementing mechanical ventilation systems with heat recovery to ventilate buildings. The research findings also suggest that there is a need for improvements in the quality of building envelopes and, in particular, the degree of air tightness and insulation levels.