Heat loss to air change

In May and June 2019 four short articles on this blog discussed heat loss from buildings through air changes, air quality in buildings, and methods of measuring rates of air change.  “A Change of Air” noted that an appreciable proportion of the total energy supplied to a building may be lost through excessive air change rates and outlined methods of measuring the rates through pressure testing, energy balance calculation, and CO2 decay. “Indoor air quality” described the main groups of pollutants which affect indoor air quality, and “CO2 in the Home” discussed indoor CO2 levels, how they might affect the health of occupants, and their measurement.  CO2 decay methods and their limitations were discussed in more detail in “Measuring Air Change Rates by CO2 Decay”. More recent work will be outlined below, referencing papers published from 2021 to 2024.

Nazaroff (2021) presented a review of field studies that measured residential air-change rates. These were mainly in north America, northern Europe, and China, and covered more than 10,000 dwellings. Ventilation simultaneously introduces pollutants into buildings from outside and removes them from inside. It also affects the energy use of buildings. The rate of air change needed to control indoor pollution may conflict with that which would minimise energy use. Nazaroff distinguishes between infiltration, the uncontrolled leakage of air through unintended gaps in the building fabric; natural ventilation through windows and vents; and mechanical ventilation by fans. The first two are substantially influenced by wind and by differences between inside and outside temperature. The air change rate, which is based on the volume flow rate out of the building, can also be influenced by the operation of heating and cooling systems, and the use of flued combustion.

Nazaroff reviewed existing literature with a view to identifying its limitations, such as over-simplification, variations in the use of terminology, and lack of critical assessment. Two main methods of determining the air change rates of residences are described: one uses tracer gasses such as deliberately released perfluorocarbons or metabolically released carbon dioxide, the other uses fan pressurisation of the building.

The numerical values of air change rates measured in the studies follow a distribution pattern described as lognormal. A normal distribution is fully described by its mean and its standard deviation, and a lognormal distribution by its geometric mean and its geometric standard deviation. The logarithms of the values in a lognormal distribution form a normal distribution. The rates of air change in the studies tended to a geometric mean of 0.5 per hour, with a geometric standard deviation of 2.0. In a normal distribution 68% of samples fall within one standard deviation of the mean, and in the lognormal distribution with the values shown above, 68% of the results fell within the range 0.25 to 1 per hour. The 10th, 25th, 50th, 75th and 90th percentiles of the distribution are 0.21, 0.31, 0.5, 0.8 and 1.2 per hour respectively. The extreme values of air change rate found in the studies were 0.05 and 4 per hour.

Measuring methods present two main challenges. The first is the treatment of time-varying air change rates, for example in methods using passive release of a perfluorocarbon tracer and time-averaged sampling. The second ‘concerns the multizone character of residences’ where there may be varying airflows between different zones and intermittent operation of heating and cooling systems.

Frattolillo, Stabile, and Dell’Isola (2021) compared the pressurization test which measures the airtightness of a building, and the tracer gas decay test which measures the actual air exchange rate, which depends on the site and climate. Finding a relationship between the results from these two tests would be very useful but remains challenging. The authors conducted an experimental campaign in a multi-room dwelling performing both air permeability and air exchange rate measurements. Their results showed great variability (from below 0.2 to almost 1 air change per hour) due to the weather conditions. It followed that the conversion factor between air exchange rates measured at 50 Pa, by blower door tests, and the actual air exchange rates obtained by the tracer gas decay tests, ranged from less than 20 to more than 100.

Berquist et al. (2023) investigated the accuracy of CO2 sensors used in measuring air leakage rates in commercial buildings as part of a building automation system (BAS). These sensors can provide a low-cost and environmentally friendly method of continuous monitoring. The authors assessed the accuracy of the sensors compared with SF6 tracer gas and photoacoustic gas monitors. They conclude that with correction for sensor drift, the CO2 method can provide acceptable accuracy. This is significant since the release of SF6 gas into the atmosphere has an environmental impact far greater than that of CO2.

Seddon and Zhong (2023) investigated the accuracy of the pulse method of measuring airtightness which was approved in 2022 under UK building regulations as an alternative to the fan pressurisation method. The pulse method is more convenient in many ways than the fan method, but there was some lack of confidence concerning its use, ‘particularly with very airtight properties’. The authors describe tests on new build dwellings, including two built to Passsivhaus standards, and conclude (with some caveats) that the pulse method is appropriate for testing such buildings.

Fu et al., (2024) investigated the reliability of measurements made using airborne particles to estimate the air exchange rate (AER) in a building. An accurate knowledge of the AER could help in controlling indoor air quality and in estimating the building’s heat loss. The authors note the disadvantages of two other well-known ways of measuring AER, the fan pressurization test, and the tracer gas method (TGM). The pressurization test uses fans in doors to pressurize a building and so test its airtightness, but the measured AER ignores the effect of climate variation, and the method is effective only in simple buildings. TGM estimates the AER of a building by the decay of a tracer gas’s concentration indoors over time and allows AER to be estimated under actual climatic conditions. Gasses used include CO, O3, NO, NO2, SO2, SF6, and occupant generated CO2. Recent work has shown the possibility of using measurements of indoor and outdoor particle levels to estimate the air exchange rate between a building and its environment. Studies on the accuracy of AER estimates achieved through measuring airborne particle concentration indicate that it could provide a useful alternative to the tracer gas method especially in high-rise buildings.

References

 

Berquist, J., et al., 2023, Investigation of the accuracy of BAS-grade CO2 sensors for measuring infiltration rates, Journal of Building Engineering, online, accessed 26 March 2024

https://www.sciencedirect.com/science/article/abs/pii/S2352710223022441

Frattolillo, A., Stabile, L., Dell’Isola, M., 2021, Natural ventilation measurements in a multi-room dwelling, Journal of Building Engineering, online, accessed 26 March 2024

https://www.sciencedirect.com/science/article/abs/pii/S2352710221003351

Fu, N., et al., 2024, Reliability of estimating Real-Time Air Exchange Rates in a Building by Using Airborne Particles, Atmospheric Pollution Research, online, accessed 26 March 2024

https://www.sciencedirect.com/science/article/pii/S1309104223003094

Nazaroff, W., 2021, Residential air-change rates: A critical review, UC Berkeley, online, accessed 26 March 2024

https://escholarship.org/content/qt2cz8v3nv/qt2cz8v3nv.pdf

Seddon, H., and Zhong, H., 2023, An investigation into the efficacy of the pulse method of airtightness testing in new build and Passivhaus properties, Energy and Buildings, online, accessed 28 March 2024

https://www.sciencedirect.com/science/article/pii/S0378778823005005

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