where: One cubic meter per minute = 16.67 liter/second Air change rates are often used as rules of thumb in ventilation design.
[5] Non-residential ventilation rates are based on floor area and number of occupants, or a calculated dilution of known contaminants.
[5][6][7] Recent research indicates that Air Changes per Hour (ACH) alone may not be a reliable parameter for making ventilation recommendations.
al.[8]), which incorporates both the flow rate and large-scale airflow patterns, could provide a more accurate measure of how efficiently air is supplied and circulated within a room.
The method is designed to more accurately understand the effects of air-flow patterns within a space, to identify or avoid such occurrences as short-circuiting airflow patterns, in which most of the supply air goes directly to the exhaust, and fails to mix with the air already present.
[2][1] There are two ways to apply this method; the most commonly used technique, Tracer Gas Decay (step-down), consists of a short burst of gas injected into the space to establish a constant concentration within the building, then injection is stopped and concentration decay at a certain position is recorded, and second Tracer Gas Step-Up, where tracer gas is injected at a constant rate and the concentration response at a certain position is recorded[10] The most common technique to measure airtightness is the fan pressurization method, also known as the blower door test.
It is measured by the number of air changes per hour (ACH) that occur when there is a differential pressure of 50 pascals between outside and inside the building.
[12] For this reason, the Passive House standard established performance requirements for airtightness requiring be less than 0.6 ACH with a pressure difference between inside and outside of 50 Pa.[13] It is worth noting that ACH measured with the blower door test, at a differential pressure of 50 pascals, mimics the situation of a sustained 20 mph (32 km/h) wind outside.
However, where ACH is already greater than 0.75 a forced ventilation system is unlikely to be of use at controlling condensation and instead insulation or heating are better remedies.
[15] The presence of forced ventilation systems has been shown in some cases to actually increase the humidity rather than lower it.
[15] The use of roof space for heating or cooling was seen as ineffectual with the maximum heating benefits occurring in winter in more southerly regions (being close to the South Pole in these southern hemisphere reports) but being equivalent only to about 0.5 kW or the heating provided by about five 100 W incandescent light bulbs; cooling effects in summer were similarly small and were more pronounced for more northerly homes (being closer to the equator); in all cases the values assumed that the ventilation system automatically disengaged when the infiltrating air was warmer or cooler (as appropriate) than the air already in the dwelling as it would otherwise exacerbate the undesirable conditions in the house.