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AMation · Apr 14, 2013, 02:31 PM

Hello - I am finishing my basement in the Chicago area and doing the new ductwork myself. I was wondering how to determine the size and location of the returns. Total finished area is 1200 sq. ft. With one bedroom (110 sq. ft.) and one bathroom (110 sq. ft.). I installed 5 supplies w/ 6" pipe. One in the bathroom, one in the bedroom, and three in the main living area. Admittedly... I based the size/location of the supplies on other basements of similar size in my area. I am looking at two returns to install, one in the bedroom ( no undercuts allowed in my area) and one in the main room. Problem is, not sure how to calculate the size? I've read a bit about using manual D, but not sure how I would do this without knowing much about the existing ductwork in the house (lengths, elbows etc... ) because it is hidden behind walls. Also, I've read conflicting opinions on whether supplies in a basement should be located near the floor or ceiling?

Any advice would be greatly appreciated!

Grady White · Apr 14, 2013, 06:38 PM

My first question is: Is the basement being supplied with its own HVAC system or are you connecting to the one for the rest of the house?

How high are the basement ceilings? Heat & cool or just 1 (if 1, which)?

AMation · Apr 14, 2013, 06:59 PM

Thanks for the reply. Tapping in to the existing HVAC system. Heat and cool, 8' ceilings for the basement.

Grady White · Apr 16, 2013, 04:59 PM

I suggest you do a manual J to determine how much heating & cooling is needed in the bsmt. There is one, & maybe more, avialable online which also do a manual D. The one I've used & was quite happy with was hvaccomputer.com. To give you a quick answer to your question, I think I would use low supplies with high returns. BTW, I think you are over ducted in the basement. Naturally that presumes the basement is below grade. 5 supplies of 6" is going to give you somewhere around 500-600 cfm & presuming 400 cfm/ton of cooling, you are going to be dumping up to a 1.5 tons (18,000 btu/hr) into the basement. That's a lot of cooling for a basement.

AMation · Apr 17, 2013, 02:50 PM

Thanks Grady - The basement is about 80% below grade. Your answer raises another question I had. Since I am both heating and cooling, and I assume it is harder to heat than cool the basement, my thought was that if it is over cooled I can just close some basement dampers during the cooling months and open them during the heating months. I guess what I'm saying is I'd rather have it over cooled than under heated... does that make sense?

One last question, when asking around about return sizes I've had several people tell me that you just need to make sure that you have the same amount of air returned as supplied. In other words, in the bedroom, one 6" supply needs one 6" return. Is it as simple as that? I've also had people suggest it is not that simple! And if I have 3 6" supplies, what is the equivalent return? Is it based on CFM?

Grady White · Apr 17, 2013, 07:19 PM

Really you should have between 20 & 30% more return than supply. It is based on CFM. The equivalent of 3 6" supplies would be about 4 6" returns. The excess return is used for 2 reasons: You have additional restriction in the return due to filters & in cooling, the return is warmer resulting in lighter air than the supply thus less SCFM (Standard Cubic Feet/Minute).