How many times the air enters and exits a room in one hour is known as an air change.
In sixty minutes how many times the room will fill up the air from the supply.
Then you can compare the number of room air changes to the required air changes. If it’s in the required range, one can proceed to design or can balance the airflow and get assurance that he is doing a right thing. If it’s out of range, you need to take another look.
Must Read: What is Fresh Air?
It 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.
In other words, it is the amount of air escaped into or out of room in terms of the number of room volume.
To calculate air changes in room, first measure the supply of airflow in room, multiply the CFM (Cubic Feet per Minute) times 60 minute per hour. Then divide the volume of room in cubic feet. In simple word, we are changing CFM into CFH (Cubic Feet per Hour).
It is generally measured per hour. Different rooms depending upon their use need different air change per hour. National Building code of India (NBC) suggests air changes per hour for different rooms as follows:-
Application for Housing
Air Change per Hour
|01.||Living rooms||3 – 6|
|02.||Bedrooms||2 – 4|
|03.||Bathrooms||6 – 10|
|04.||Toilets||6 – 10|
|05.||Shower baths||15 – 20|
|06.||Kitchen||8 – 12|
|07.||Utility rooms||15 – 20|
Air Change per Hour equation in Imperial Units
N = 60Q / V
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 – SI Units
Air change rate can be expressed in SI-units as:
n = 3600 q / V
n = air changes per hour
q = fresh air flow through the room (m3/s)
V = volume of the room (m3)