Exhaust Only Air Changes
Exhaust Only Air Changes
(OP)
Our company does environmental testing in medical settings for a variety of chemicals. A debate has come up I need some guidance on.
If I have a room that has either ONLY supply or ONLY exhaust, can air changes per hour be calculated using the standard N=60Q/Vol? I have an building manager who is arguing that ACH can only be calculated if there is BOTH a supply AND an exhaust/return in the room
So, if I have a room that has a volume of 500 CuFt, Supply of 100 cfm and exhaust of 0 cfm, the ACH = 12. Is that right? Also, if the room had exhaust of 50 cfm, the ACH would still equal 12? And, if the room had exhaust ONLY of 100 cfm, the ACH would still be 12?
Any basic help would be greatly appreciated as we are trying to develop a spreadsheet to use.
If I have a room that has either ONLY supply or ONLY exhaust, can air changes per hour be calculated using the standard N=60Q/Vol? I have an building manager who is arguing that ACH can only be calculated if there is BOTH a supply AND an exhaust/return in the room
So, if I have a room that has a volume of 500 CuFt, Supply of 100 cfm and exhaust of 0 cfm, the ACH = 12. Is that right? Also, if the room had exhaust of 50 cfm, the ACH would still equal 12? And, if the room had exhaust ONLY of 100 cfm, the ACH would still be 12?
Any basic help would be greatly appreciated as we are trying to develop a spreadsheet to use.





RE: Exhaust Only Air Changes
If there's air being moved in or out of the room, then there's sufficient openings to let air out of or suck air into the room.
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RE: Exhaust Only Air Changes
RE: Exhaust Only Air Changes
RE: Exhaust Only Air Changes
RE: Exhaust Only Air Changes
RE: Exhaust Only Air Changes
So in essence your manager is correct, but his explanation doesn't look at the entire picture. The air required for the calculation can come from other places, not just that one room. Looking at the building pressurization is helpful as well (and can help determine if the front doors will be pushed out by the air, or vacuum seal the building).
RE: Exhaust Only Air Changes
Not exactly. What he is trying to say is that in order to correctly calculate air changes for a room, the room must have BOTH a supply and an exhaust (or return) IN THAT ROOM.
This is nonsensical to me because there are plenty of rooms out there (toilet rooms in hospitals, for instance) that are designed with just an exhaust. The reason that room is not a vacuum is that it is drawing offset air from the adjacent corridor, room, etc.
The issue really came about when we were measuring a decontamination room and found that while it had an active supply, the exhaust was not pulling anything. It also measured positive pressure under the door to .0034" wc. Therefore, the room failed on our report and we stated that the room showed 17 ACH from the supply when it was supposed to have 10 ACH supported by the exhaust. THAT opened a long discussion about how to properly calculate ACH that I asked about yesterday. I finally felt our method was vindicated by a few comments in this forum and through a call to ASHRAE.
Hope this helps to clarify. And, if anyone has some more helpful info, please let me know. Great site with good stuff.
RE: Exhaust Only Air Changes
RE: Exhaust Only Air Changes
If there is supposed to be a fan running in the exhaust, you have a chance to make the room negative, but it is the balancing of the supply inlet that will reduce your air changes.
Adding a preferential exit path can only increase the ACH without also choking the inlet down.
RE: Exhaust Only Air Changes
If you have excess supply air (cutting back from 17 to 9 ACH) I would recomend looking into sending it to perimeter area outside of containment. That won't help make air changes, but it does help with maintaining dP. A magnahelic on the wall helps.