When specifying a fan or ventilation system, one of the most important considerations is the number of air changes per hour (also known as air change rate) needed to sufficiently ventilate the room. It is equally crucial to take into account the volume of outside air (fresh supply air) that the room requires. This can be calculated based on the number of people typically living or working in the space. These requirements can depend on a number of factors but the nature of the room’s usage is a key place to start.
Air changes per hour (ACPH) is a measurement of air volume that is added to (or removed from) a room divided by the total volume of the room. Put simply, it measures how many times the air in the room is replaced. Higher ACPH values mean better ventilation. The formula is as follows:
ACPH = Q / Vol
To find the volume of outside air needed to properly ventilate a space, you’ll need to have an estimate of the typical number of people occupying it and know what the space will be used for. Then, simply use the table below to multiply the number of people by the required metres cubed per hour per person to determine the required fresh air flow.
Air change rates are often used as rules of thumb in ventilation design. However, they are seldom used as the actual basis of design or calculation. For example, laboratory ventilation standards indicate recommended ranges for air change rates, as a guideline for the actual design. Residential ventilation rates are calculated based on the area of the residence and number of occupants. Non-residential ventilation rates are based on floor area and number of occupants or a calculated dilution of known contaminants.
|Room||Minimum Outside Air |
Stream per Person (m³/h)
|Recommended Number of |
Air Changes per Hour
|Toilets||30||4 (private) 10 (public)|
|Showers||60||6 (private) 10 (public)|
|Swimming Pools (private)||-||6-7|
To specify a fan for your application and air change rate requirements, please contact our team of sales engineers who will be able to advise the best solution.