Ventilation Selection

Use the Air Changes Calculation to Determine Room CFM

Engineering room airflow may present a real challenge when balancing an HVAC system. Most calculations only use the heat loss or gain of a room to decide on required airflow and often don’t take into consideration required room ventilation needs. Let’s take a look at how an air change calculation may simplify this step in your air balancing.

What is an Air Change?

An air change is how many times the air enters and exits a room from the HVAC system in one hour. Or, how many times a room would fill up with the air from the supply registers in sixty minutes. You can then compare the number of room air changes to the Required Air Changes Table below. If it’s in the range, you can proceed to design or balance the airflow and have an additional assurance that you’re doing the right thing. If it’s way out of range, you’d better take another look.

The Air Changes Formula

To calculate room air change, measure the supply airflow into a room, multiply the CFM times 60 minutes per hour. Then divide by the volume of the room in cubic feet:

Air Changes / hr = ( CFM X 60 min) / ( Volume of Room )

In plain English, we’re changing CFM into Cubic Feet per Hour (CFH). Then we calculate the volume of the room by multiplying the room height times the width times the length. Then we simply divide the CFH by the volume of the room. Here’s an example of how a formula works:

Air Changes / hr =  (300 cfm x 60min ) / ( 15’x20’x8’)

Air Changes / hr = 18000 / 2400

Air Changes / hr = 7.50

Now compare 7.5 changes per hour to the required air changes for that type of room on the Air Changes per Hour Table below. If it’s a lunch or break room that requires 7-8 air changes per hour, you’re tight on target. If it’s a bar that needs 10-20 air changes per hour, it’s time to reconsider.

Typical Air Changes Per Hour Table