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AllGoNoShow 01-27-2013 08:12 PM

Adding Duct Lines for Finished Basement?
Hi guys. I am certainly no expert in HVAC and was hoping you guys could point me in the right direction (websites/books/etc) to gain a little bit of knowledge.

I have a 1750 sq ft. house and am adding about 550 sq ft in the basement below grade. I have forced air/ac and want to add some duct-work and just branch of my existing lines. I have already framed/insulated the basement and even with the temperature outside in the single digits in upstate NY my basement is currently only 2 degrees cooler that the rest of the house (and the basement currently has no heating registers) and during the summer the basement temperature was the same if not cooler.

The basement consists of one big family room of 400 sq ft and one bedroom of 150. Considering my basement already is so close to the whole house temp, could I get by with 2 4" round ductlines to the main room and one 4" duct to the bedroom? Or should I go 6"? Each room with a return equal to a 6" pipe? I know there is alot of math to this all but I am hoping someone can point me in the right direction to figure out if I can branch of my existing line or if I should upgrade that, etc. Thanks for all the tips.


gregzoll 01-27-2013 08:15 PM

You really need to have a load assessment done by a local HVAC company, to see if the current furnace can handle that extra space. If not, you may find you will have to either install another unit, or replace the current system. What that means, is that you need to have the company do a Manual D & J.

Also, most systems are over sized to begin with, but just to make sure if your current unit can or cannot handle that extra space, see the above.

beenthere 01-27-2013 09:07 PM

I would use 6" min. And add returns also.

AllGoNoShow 02-02-2013 06:57 PM

Well I've done a little research and its not very good!

I'd like to add 2 6" pipe runs to heat the main 325+ Sq Ft Family Room and would like to branch them off of the 12x8 trunk. The 12x8 trunk already has 3 6" lines coming from it which would put my CFMS with the two additional runs prolly a little high for the trunk (ive seen a few different ratings but 6" pipe seems to be 100cfms and I would have 5 of em and 12x8 is only good for about 450cfms). Thats not terrible but for the 12x12 room I want to use a 6" pipe that has to branch off the other trunk which is 16x8...unfortunately there are already 7 6" pipe branches off this one. Two of the 7 are for small bathrooms and the adjustment flappers in the pipe are half closed but that still puts me around 600-700 cfms worth of pipe and a 16x8 trunk that is already maxed out on that (and I want to add another 6" to heat the bedroom).

And on top of all that I just realized my 1700 sq ft house in upstate NY only has a 60K BTU is a two stage/variable 97% efficent Trane but still.....I don't know if it is really up to the task of heating another 500+ sq ft in the basement. The basement only really needs to be heating it enough for 3 degrees but I don't know if it could cause other problems trying to tax it to do the basement. House heating is fine now and bills are reasonable.

Maybe I am overthinking this. Any thoughts?

beenthere 02-02-2013 07:48 PM

Basement won't take much to heat it, so should not be a problem for your current furnace.

I'd run a new trunk line for the basement.

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