Air changes per hour

Air changes per hour, or air change rate, abbreviated ACH or ACPH, is a measure of the air volume added to or removed from a space (normally a room or house) divided by the volume of the space. If the air in the space is either uniform or perfectly mixed, air changes per hour is a measure of how many times the air within a defined space is replaced.

In many air distribution arrangements, air is neither uniform nor perfectly mixed. The actual percentage of an enclosure's air which is exchanged in a period depends on the airflow efficiency of the enclosure and the methods used to ventilate it. The actual amount of air changed in a well mixed ventilation scenario will be 63.2% after 1 hour and 1 ACH. In order to achieve pressure, the amount of air leaving the space and entering the space must be the same. ACH equation in Imperial units

${\displaystyle \quad N={\frac {60Q}{Vol}}}$

Where:

Ventilation rates are often expressed as a volume rate per person (CFM per person, L/s per person). The conversion between air changes per hour and ventilation rate per person is as follows:

${\displaystyle \quad Rp={\frac {ACPH*D*h}{60}}}$

Where:

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 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. Hospital design standards use air changes per hour, although this has been criticized.

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