# Air changes per hour

Air changes per hour is a measure of how many times the air within a defined space (normally a room or house) is replaced.

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.[1] In order to achieve equilibrium pressure, the amount of air leaving the space and entering the space must be the same.

ACH equation in Imperial units

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

Where:

• N = number of air changes per hour
• Q = Volumetric flow rate of air in cubic feet per minute (cfm)
• Vol = Space volume L × W × H, in cubic feet

## Air change rate

 Commercial kitchens & Toilets 15–30 Classrooms 3–4 Laboratories 6–12 Smoking rooms 10–15 Warehousing 1–2 Basement Parking 15–30

The Passive House standard requires a 0.6 ACH.[2] Many if not most uses of ACH are actually referring to results of a standard blower door test in which 50 pascals of pressure are applied (ACH50), rather than the volume of air changed under normal conditions.

## References

1. ^ Bearg, David W. (1993). Indoor Air Quality and HVAC Systems. CRC Press. p. 64. ISBN 0-87371-574-8.
2. ^ "International Passive House Association - Guidelines". Retrieved 23 March 2013.