A Change of Air

Energy saving, ventilation and health.

In the UK an appreciable proportion of total energy consumption can be ascribed to heating buildings, and of this energy a significant amount is used to heat air. If the number of air changes per hour (ACH) is unnecessarily high, energy is wasted. This post concentrates on domestic buildings, in which a reduction in ACH can often be achieved by cheap and simple means, giving a rapid return on expenditure through lowered energy costs. However, the ACH figure for a house should not be reduced below the minimum needed for the health of its occupants and the preservation of its fabric. In order to know whether the ACH for a dwelling should be reduced, the actual ACH value is needed, and to know by how much it can safely be cut, the minimum figure is required.

Pressurisation is one method of measuring the actual ACH figure, and is described in ATTMA Technical Standard L1 Measuring Air Permeability in the Envelopes of Buildings, 2016 (open access). Pressure within the building is raised by a known amount above atmospheric pressure, and the rate of air flow into the building through the pressurising device is measured, and equated to the total air leakage. The pressure difference used is typically 50 Pa, and the measured permeability, expressed as rate of volume change per unit of surface area, is adjusted by a standard factor to give the leakage value for the unpressurised building. The result is clearly dependant on this factor, and its validity is disputed: see The Origin and Application of Leakage-Infiltration Ratios by Jones, Persily, and Sherman (available at Researchgate).

A second approach to estimating ACH depends on an energy balance calculation. Here the energy input to the building must be known, as well as the total energy lost through the fabric. Given these two figures, the difference is taken to represent the energy lost to air changes. From the volume of the building, and the appropriate air parameters and temperatures, the rate of change of air in the building can be calculated. However, in many cases it may only be possible to reach an approximation to the total energy loss, due to lack of thermal data on the building structure, and in these cases the ACH figure derived must also be approximate. See: Estimating the air change rates in dwellings using a heat balance approach by Cosar-Jorda and Buswell, 2015 (open access).

By either method, the ACH figure reached may be regarded in many cases as having a wide margin of error, so that even if a minimum desirable value for ACH is known, a cautious approach to reducing the air change rate will seem advisable, to avoid inadvertently falling below the minimum level.

One source of information on minimum ACH figures is a standard of the American Society of Heating, Refrigerating and Air Conditioning Engineers, ASHRAE Standard 62.2. Ventilation and Acceptable Indoor Air Quality in Low Rise Residential Buildings. This publication may not be freely available, but a 2004 paper by M. H. Sherman entitled ASHRAE’s First Residential Ventilation Standard has open access. It provides data on recommended minimum ventilation levels based on individual rooms within a house, and the number of occupants.

More direct methods of assessing air quality such as measurement of CO₂ levels, may be needed to check that reductions in the ACH level of a building remain within safe limits. This issue is discussed in Smart homes and the control of indoor air quality by Alexandra Schiewecka et al., 2018, (open access). Though the data in this paper are based on Central Europe, the methods described are widely applicable. Indoor CO₂ levels above 2000ppm are regarded as “unacceptable”, whereas levels below 1000ppm are taken as “harmless”. Data are provided on the ventilation level needed in a room of given volume with a specified number of occupants in order to meet these two CO₂ levels, and these suggest that one or both levels may be exceeded at ACH rates considerable above those met in passive house environments.