Air Changes Per Hour

Cronyd

Thanks Brad, the info help me to better understand air changes.

Cronyd

AIr Changes Per hour

I have a question about ACH. The formula states (CFM*60) / Room Volume. Is CFM just the supply? What about the CFM I'm exhausting with an exhaust fan?

Brad White

In-ee versus Out-ee

Dan, whatever your air source, supply or exhaust, you can only count it once.

Say you are supplying a room with 6 air changes per hour but are not exhausting nor returning it. The room will exfiltrate as well as it can. If the room is tighter, the pressure will build and the supply will drop to a point of equilibrium. If the room is looser, say open to every adjacent room, neutrality is as good as it gets but by definition will be positive for that is what is driving this.

This same tighter room with 6 air changes supply and the same exhausted or returned (mechanically assisted) will be said to be in balance. The exhaust or return is only helping the supply, saving it from having to develop pressure to force the air out.

The room still has 6 air changes either way but when you mechanically assist a tighter room, you are more likely to achieve a consistent air change rate.

Does that make sense?

Some other tips:

To figure cfm needed to achieve a given air change rate:

(Volume in Cubic Feet/60)= CFM required for one air change.

Multiply that by the number of air changes desired and you have your total CFM requirement. Same formula basis you cited but looking at it a different way.

Keep in mind in all of this that the ACH rate, unless stipulated by code, is really an incidental number. It should not be used as a design basis in place of calculated cooling loads for AC or heat gain loads for exhaust ventilation. Your CFM in each case is based on what it takes to absorb the heat you want to remove. An air change number is interesting but for comparison only. Hope this helps!

Brad