# How do I calculate fan size for a single room exhaust system to reach a desired cfm when restricted to specific duct size/route?

I am wanting to exhaust a sealed room (except for air intake and exhaust).

The room is 512 ft3, and due to positioning, orientation, and existing construction, I am restricted to using 6" flexible ducting (I may be able to put rigid sections, but don't want to assume this yet).

The route I have to take to the outside wall is about 15 to 20 feet and will require a number of soft bends, at least 1 x 90 degree bend, and likely 3 x 45 degree bends.

I would like to have the ability to change the air completely once every 3 minutes minimum. My initial simple calculation for my target minimum is 171cfm.

My questions are:

• Given a min requirement of 171 cfm, how can I roughly calculate the fan size I need such that I end up at 171 cfm after accounting for the flexible ducting run?
• My initial search online finds that 6" round duct is capable of handling 100 cfm, is this a different rating, or does that mean that even if I could reach 171 cfm, a 6" duct won't handle it?
• Is there a simple forumla to calculate intake requirements?
• If the required intake size is too large for my application, can I make multiple smaller intakes which add up to the same area as the larger?
• Once I have the target fan size if there is any equipment I then want to place in front of it to pull air through, what do I need to know to calculate the new fan size? For example if it tells me the pressure drop if 42 pascals?
• If that filter equipment has an effective cfm range, do I compare that to my fan's rating before or after calculating the effect of the ducting?

I have tried various calculators such as: https://www.cdicurbs.com/ductcal

The problem is, it asks for things like "Duct Air Flow" in cfm, would this be my desired 171cfm for the room?and in the end, I am not sure I understand how to use these numbers. For example, if I add the numbers in, and it tells me the Friction Loss is .45, would that mean I would convert that to .09 for the 20 foot run (.45 * 20)/100, and add 9% to the cfm of the fan to account for the ducting (this doesn't seem close to right to me)?

The resulting calculation doesn't need to be absolutely exact, but I definitely want to land at or higher than my minimum.

Thank you for any help you can provide, after asking all this I am realizing what a tremendous rabbit hole this is for DIY. I will update as I learn more, or discover new information as I will be continuing to try learn on my own after posting!

Thanks!

Steve

• I've done some poking around the interwebs, and can confirm all of your assumptions. You'll need an 8" duct for that air flow. The first factor I'd look at is the 3 minute complete air change requirement. How did you arrive at that? What's the room used for? If it's a smoking room, then you need to change all the air. If it's a chem lab, or Spice Kitchen, could you get away with a vented fume chamber or fume hood? – Chris Cudmore Aug 29 '18 at 16:35
• Hi @ChrisCudmore, thanks! I am considering creating a dedicated room in my basement for reptiles, fish and tropical plants. The 3 minute ventilation is based on anecdotal evidence which suggested this rate of air change for a thriving ecosystem as well as venting heat build up from supplemental light sources and any pumps or other heating equipment (rocks, pads, etc.). Essentially I am trying to gain some control over every aspect of the environment, and in our cold winters have a lot of trouble maintaining ideal humidity indoors. – Steve Aug 31 '18 at 13:21
• Can you run 2 6" ducts? That will give you 1.125 x the cross sectional area of an 8" duct, which would give you some space for frictional losses. – Chris Cudmore Aug 31 '18 at 13:59
• I thought i would come back to this with what I've learned. If I vent air out at the rate i was thinking, I'm going to throw my HRV's balance off and create negative pressure in my house which could cause all kinds of issues. Luckily I have no combustion based appliances or anything else that might create carbon monoxide issues, but from what I am reading, there is a lot more to this then just dumping the air out a vent. Back to the drawing board for now....not sure how to close this question since it isn't actually "answered" per se and there likely is a scientific calculation? – Steve Sep 14 '18 at 21:10