ACH or Air Changes Per Hour is a metric that tells us how many times an HVAC device can fill up the full volume of a room with air. This is especially useful when comparing different air purifiers or air conditioners.
Example: Let’s take an air purifier with 250 CFM airflow. We put it in a 200 sq ft room with the standard-height ceiling (8 ft). How many air changes per hour does the unit make?
Calculation: 250 CFM is 250 cubic feet per minute. In one hour (60 minutes), we get 60*250 = 15.000 cubic feet per hour. The whole volume of the room is 200 sq ft * 8 ft = 1.600 cubic feet. Such an air purifier is capable of changes the whole volumetric air in room 15.000/1.600 = 9.375 times.
Answer: ACH = 9.375
Here is a neat air change per hour calculator you can freely use. Just put in the area, ceiling height and CFM of an HVAC device in question and you can calculate ACH:
Formula (Calculate ACH Yourself)
The formula of how to calculate air changes per hour from CFM is simple enough. Pretty everybody can calculate it using a digital calculator. All you need to know is the room area, height, and CFM.
This is the formula for ACH (air changes per hour):
ACH = CFM x 60 / (Area x Height)
where ‘Area’ is the area of the space where you intend to have an HVAC device and ‘Height’ is the height of the ceiling.
The formula is basically ‘how many cubic feet of air can an HVAC unit provide every hour’ divided by the volume of the room.
We always get CFM but that is the volume of air per minute. In order to calculate air changes per hour, we have to translate that into hours. Hence the multiplication with 60 in the equation above.
The volume of a room is simply calculated by length*width*height. Obviously, the multiplying length of the room with its width will give us the surface area (‘Area’). To get the volume, we have to multiply the area by height.
If you have any questions regarding the calculation of air changes per hour, you are welcome to ask us in the comments below.