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What Size Fan?


What size exhaust fan should I use in my new finishing room? It is 12 x 12' with 10' ceilings.

I have a filter area through the wall from the shop into the finish room. I want good flow out but don't want to suck out all my heat and ac from the shop? Currently I am using a squirrel cage fan from an old furnace to vent the entire shop but I lose all my heat in about 20 minutes and can't keep in warm enough. Works great to vent the shop.

3/8/16       #2: What Size Fan? ...
Mitch Suber  Member


My spray room is about the same as yours, and I place priority on getting the fumes and overspray out, versus trying to keep heat in. Makes for some cold days in the spray room, but I do have it separated by a wall and door. My main work area stays warm, and provides a thaw out environment for really cold days. If your current fan vents the spray room well, I would be cautious about downsizing it. Is your spray room separate, or a part of your work area? I currently have a 30" fan, with a sealed motor on mine. I did have around an 18" that ran at a very high speed, that almost did as well.

3/8/16       #3: What Size Fan? ...
Leo G

Proper ventilation should be 100 feet of air per minute.

So 12'w x 10t is 120sq ft times 100 so 12,000 CFM

3/9/16       #4: What Size Fan? ...
Bob C


Please note that your biggest concern should be safety.
And note that the code is different for a spray booth verse,s a spray room.
As stated you should be moving 100 fpm when spraying. Add a VFD so that you can turn it down when not spraying will help with the heat some.
Do your research and make sure you are safe. You will live longer.

3/9/16       #5: What Size Fan? ...
Paul Snyder  Member


Dave - do a search at the top right of the page for previous discussions on this topic. You'll find additional information that may be helpful.

If you are running a legitimate business, you'll want to make sure you meet the fire code (local and national), OSHA safety requirements, and EPA air quality requirements. I have relevant documentation on the regulations and standards from these organizations if you'd like a copy.

3/9/16       #6: What Size Fan? ...

I have a tubeaxial fan from a commercial kitchen. 21" aluminum blades, 3/4 hp, 2 speed motor. I can not find any cfm rating on the fan. The fan is huge! It must weigh over 200 pounds and about 4 foot long. I was thinking this would be way too big?

3/9/16       #7: What Size Fan? ...
Leo G

Is the fan 21" or 42"? Either way, with a 3/4HP motor you're not going to get 12,000CFM out of it.

I have a 36" fan with a 3HP motor and it does about 17,000CFM

You'll need at least a 2HP 30" fan to be close to proper.

But what you have is absolutely better then a box fan in a window.

According to a fan at

21" diameter, 3/4HP fan at 0 static pressure is 4700CFM

21" 3/4HP Tubeaxial fan

3/9/16       #8: What Size Fan? ...

So how on earth will the room stay warm? With that kind of air movement any heat will be gone in 60 seconds. If you pull in filtered air from the shop the shop will get cold in hurry too???

3/9/16       #9: What Size Fan? ...
Leo G Member

Million BTU makeup air unit.

3/10/16       #10: What Size Fan? ...

how big are the pieces you are spraying?
you might be better off installing a small open face booth.
face size x100 is CFM needed.
i have an 8ft wide by 7ft tall booth. need only 5600CFM.
getting a spray area set up right is not cheap.
Im in NY state and each winter i use about 1800 gallons of propane for heated air make up at about $2.50 per gallon.
i have a 30" exhaust fan with a 1HP motor.

3/10/16       #11: What Size Fan? ...
Leo G

Curious at what the makeup temp is set for. If it were me it would be set for 55-60 with a separate heated drying room.

3/10/16       #12: What Size Fan? ...

Thanks for the info. This is helping.

Thank you.

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