Air Changes per Hour vs. Fresh Air Volume
Air Changes per Hour vs. Fresh Air Volume
(OP)
Today I got into some discussion with a colleague regarding Air Changes and Fresh Air Volume and I would like to get a "referee" to solve this.
Imagine the following situation for the sake of argument:
Room Volume: 100 m3
ACH (Air Change per Hour): 5
My position is that the ACH figures tells me how many times in a hour I have to replace the air, so basically I would need to put in the room 500 m3 of fresh air
My colleague´s position is that ACH means that we will have to circulate the air through the HVAC system in an equivalent volume of 5 times the room´s volume (500m3) and that fresh air should be around 12 l/s per person which give much lower fresh air needs.
I see validity on my colleagues´approach and maybe I am being put off by the semantics, in which for be air changes per hour basically means that I have to replace air inside the room 5 times.
Which approach is the correct one?
Thanks a lot for your help.
Imagine the following situation for the sake of argument:
Room Volume: 100 m3
ACH (Air Change per Hour): 5
My position is that the ACH figures tells me how many times in a hour I have to replace the air, so basically I would need to put in the room 500 m3 of fresh air
My colleague´s position is that ACH means that we will have to circulate the air through the HVAC system in an equivalent volume of 5 times the room´s volume (500m3) and that fresh air should be around 12 l/s per person which give much lower fresh air needs.
I see validity on my colleagues´approach and maybe I am being put off by the semantics, in which for be air changes per hour basically means that I have to replace air inside the room 5 times.
Which approach is the correct one?
Thanks a lot for your help.





RE: Air Changes per Hour vs. Fresh Air Volume
RE: Air Changes per Hour vs. Fresh Air Volume
Thanks a lot for your feedback.
Nevertheless, and since meanwhile I also found some minimum fresh air tables that define a diferent approach for instance Part F UK building regulations 2010 calls for a 10l/s per person of fresh air.
Does thi smeans that if no Air changes are defined, we apply this value, if ACH is defined, then it will overule the minimum fresh air requirements, correct?
RE: Air Changes per Hour vs. Fresh Air Volume
Best to you,
Goober Dave
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RE: Air Changes per Hour vs. Fresh Air Volume
ASHRAE clearly states "Air changes per hour of outdoor air" which is different from "Air changes per hour supplied to room". The latter refers to air movement.
RE: Air Changes per Hour vs. Fresh Air Volume
RE: Air Changes per Hour vs. Fresh Air Volume
RE: Air Changes per Hour vs. Fresh Air Volume
I guess that we can call it a draw then.
The two approaches are correct, depending on the governing code.
RE: Air Changes per Hour vs. Fresh Air Volume
RE: Air Changes per Hour vs. Fresh Air Volume
In a sense you are right, but as someone pointed out above, different standards also have different understandings of the word.
Further to our discussion, today my colleague came with a portion of a HVAC tender that stated: " Air Balance Pressure 12 liters/sec per person" and he claims that this is related with our discussion of the air changes per hour. I have a feeling that it doesn´t but I also couldn´t explain why. Can anybody give a hint on what does this parameter relates with ACH?
Thanks a lot.
RE: Air Changes per Hour vs. Fresh Air Volume
The paper that my colleague showed me was written like this:
"
(...)
Area Air Requirements: 5 ACH
Air Balance Pressure: 12 lts/sec/person.
(...)
"
So that´s why he is claiming that the ACH figure is referring to recirculating air, while the air balance pressure is the fresh air.
Any thoughts on teh line of reasoning?
Thanks a lot
RE: Air Changes per Hour vs. Fresh Air Volume
"Air Balance Pressure" has to do with the pressure in the space relative to another space, typically the outside and expressed as a pressure figure such as inches w.g. or as a percentage. As a percentage it would be calculated as the difference of fresh air intake and exhaust air volumes over fresh air intake: FA-EA/FA x 100%.
When designing HVAC systems, all these factors must be taken into consideration and each impacts the condition of the space differently.
ACH is your primary cooling/heating air removing or adding heat to the space as needed to maintain your thermostat's setpoint.
Fresh air rate maintains the CO2 levels in the space and prevents "stale air" in buildings.
Air balance is used to compensate for the fact that no building or space is ever truly 100% air tight. This is a positive value to keep unwanted moisture and other contaminates out of the building.