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PostPosted: Mon May 17, 2021 6:54 am 
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Location: Baltimore, MD
About a year or so ago, I built a dead vent in my live room to circulate fresh air into my control room. Budget was tight so I spent about $65 on a 6” inline fan putting out 395 CFM. I’ve been looking into it more and have realized that this isn’t nearly enough to exchange the air as much as needed. I've read that ideally you want to exchange the air once per minute but have also read as low as every 3 minutes. Even at 3 minutes though, its slightly under. My room is 16'Dx11'Wx7'H which is about 1,200 cubic feet. I was wondering if I put another 6" 395CFM inline fan on the duct leaving the room, would the CFM overall add up to 790? Not sure if this is a cumulative type of measurement.


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PostPosted: Mon May 17, 2021 11:12 pm 
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diamondj421 wrote:
About a year or so ago, I built a dead vent in my live room to circulate fresh air into my control room. Budget was tight so I spent about $65 on a 6” inline fan putting out 395 CFM. I’ve been looking into it more and have realized that this isn’t nearly enough to exchange the air as much as needed. I've read that ideally you want to exchange the air once per minute but have also read as low as every 3 minutes. Even at 3 minutes though, its slightly under. My room is 16'Dx11'Wx7'H which is about 1,200 cubic feet. I was wondering if I put another 6" 395CFM inline fan on the duct leaving the room, would the CFM overall add up to 790? Not sure if this is a cumulative type of measurement.


That is a massive amount of air for such a small space, and you're doing that with only a 6" fan? That thing must be incredibly loud and almost on fire.

Do you have an air conditioner in that space? If so what type?

How many people will be in your space (max) ?

If it's just you then you only need 20cfm of fresh air.

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Last edited by Paulus87 on Tue May 18, 2021 1:39 am, edited 1 time in total.

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PostPosted: Tue May 18, 2021 1:03 am 
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so there are a couple of ways to gauge this: # of people in the room - so roughly 20cfm per person, or by room volume - 1200ft3 x 6 air changes per hour = 7200ft3/hour = 120cfm - and you should exchange this at about 25-50%. so guess 60cfm of FRESH air via an ERV/HRV unit, simply recirculating the air will not refresh it... so investing in a good quality 100cfm ERV/HRV unit will suffice, be quieter and more reliable. ensure the intake vent and exhaust vents are separated so exhausted air is no sucked back in - usually 6-10 ft apart is plenty.

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PostPosted: Tue May 18, 2021 1:44 am 
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gullfo wrote:
so there are a couple of ways to gauge this: # of people in the room - so roughly 20cfm per person, or by room volume - 1200ft3 x 6 air changes per hour = 7200ft3/hour = 120cfm - and you should exchange this at about 25-50%. so guess 60cfm of FRESH air via an ERV/HRV unit, simply recirculating the air will not refresh it... so investing in a good quality 100cfm ERV/HRV unit will suffice, be quieter and more reliable. ensure the intake vent and exhaust vents are separated so exhausted air is no sucked back in - usually 6-10 ft apart is plenty.



There is something that we may have overlooked, though, (or not... I don't know for sure).

He is pumping air in to the live room first, and then venting it through to the control room. So, I am going to assume the calculations would need to be different in this case, and would need to take in to account the volume of the live room and then some how work out how to guarantee the required amount of air for the control room is being vented through.

Any ideas?

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PostPosted: Tue May 18, 2021 11:03 pm 
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as a general principle, for the HVAC outdoor ventilation i try to get all the rooms included into the volume equation, and if a machine closet with (say) SSL power rack, include that as well (they're usually small but i find oxygen in the closet helps when people need to access it :-) although for fire safety, like in a bathroom, venting it separate from the general returns).

a good number then is 12 people - 6 in CR and 6 in live room - 240cfm using people # - and likely the same in volume. so a 250-300cfm ERV/HRV. e.g. https://www.supplyhouse.com/Fantech-SHR ... on-Defrost

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