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Old 08-11-2011, 11:12 PM   #1
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cfm inquiry


Am doing some preliminary & schematic planning for an research and development design. I'm learning the HVAC requirements and want to get some feedback.

A 12'x16'x9' room = a (V) of 1,720 cu.ft.
4 Air Changes per Hour (N) for a typical residential living/sleeping room

using Q=(NxV)/60 would make (Q) = cubic feet per minute

So with the given room above the room would need 116.42cfm at constant flow. Or 465.68 cfm at 15 min. of operation every hour. Or 232.84 cfm at 15 min. of operation every 30 min....

So, now the inquiry: Given that thermal loading is accomodated outside of this discussion, what would be the ideal air change rate for the environment. Would running a fan for 15 min every hour at higher velocity be beneficial beyond the constant flow? Or would constant flow have better benefits? Essentially, what is the standard cycle rate and duration of air handlers? I don't quite know the "rule-of-thumb" here...

To better explain, this is an analysis into a completely sealed space that will need mechanical ventilation. Just don't know much about actual AHU function and performances.

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Last edited by AGWhitehouse; 08-11-2011 at 11:15 PM.
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Old 08-13-2011, 01:29 AM   #2
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In order to satisfy a rooms conditioning need,it needs a certain amount of air.
Furnaces are designed to deliver a certain amount of air running against certain amount of static pressure.
The next factor would beCFM.You want to keep that to 500 CFM or lower so that when you go to bed at night it doesn't sound like a guided missle aimed at your head.

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Old 08-13-2011, 04:42 AM   #3
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If your talking about fresh air exchanges for IAQ. 4 per hour is too many. One exchange every 5 hours is also a bit much. Unless their are 6 people in that room. Check ASHRAE 62.2
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Old 08-14-2011, 04:38 PM   #4
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Yes, I am just curious about indoor air quality. I found 4 ACH on a random site, so I'm not surprised it's not correct. Somewhere I heard 0.35 ACH for residential, does that seem a reasonable number?
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Old 08-14-2011, 04:41 PM   #5
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That is more then needed for most homes. .2 to .25 is where you want to be.
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Old 08-14-2011, 04:53 PM   #6
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General building ventilation (industrial floor) is usually designed at 3 cfm/ft2 I say this as you had mentioned that this is a requirement for "mechanical ventilation" this would of course have to be balanced with either return or exhaust depending on the possible additions from equipment in the room.

I found this after you peaked by curiousity....http://epb.lbl.gov/publications/lbnl-54331.pdf


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