Sizing HVAC ducting

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Topic by MedicKen posted 01-01-2009 08:44 PM 4469 views 0 times favorited 6 replies Add to Favorites Watch
View MedicKen's profile


13 posts in 5376 days

01-01-2009 08:44 PM

I currently have a wood shop in my attached 2 car garage and want to tie into my homes central furnace for heating and a/c. The garage is a standard 2 car, 20X20X8 with an insulated door. I have also insulated all the exterior walls and will be blowing insulation in the ceiling after HVAC is installed. The HVAC unit is a 5 ton and the home is right at 2000 sq ft. I know I will be taking some efficiency from the home but I will be able to cover the registers in the garage when they are not needed.

My questions are: How do I go about sizing the ducting for adequate air flow? How large of a cold air return do I need to install?

I know this not an easy and straight forward question to answer, there are a lot of varialbles.


-- My job is to give my kids topics to discuss with their therapist

View MRTRIM's profile


743 posts in 5650 days

01-02-2009 05:38 AM

sorry ken i dont know squat about a c . ill keep my eye on your post and if no one comes along that does ill try to get my neighbor to come over and read it and see if he can help. you my be best to talk to one of your local guys though like you said there are a lot of variables

View PaBull's profile


694 posts in 5596 days

01-02-2009 05:38 PM

same here,...... sorry,...... not much help.

View Catspaw's profile


35 posts in 5633 days

01-03-2009 11:29 PM

well….I have a .PDF that I can e-mail you. Should give you more than you want to know. I tried doing something about posting it to Photobucket, but, they only u/l pics. It’s alittle less than 2.5 mB. But, I found it’s a real good primer on HVAC ductwork.

-- arborial reconfiguration specialist

View Sarit's profile


7 posts in 5006 days

05-12-2010 11:35 PM

I’m not sure that would pass code. The danger is that if someone were to ever leave their car idling in the garage, there is a risk that the carbon monoxide could be spread throughout the house killing everyone inside. Most people end up installing a ductless A/C unit and/or heaters specifically for their garage.

View NewPickeringWdWrkr's profile


11 posts in 4915 days

05-18-2010 07:04 PM

You should definately check your local building codes. Where heating flows, a return also needs to run back to the furnace and there would definitely be an issue with CO getting back into the house. I think you would be better off with installing an electric garage heater.

-- Mike - Minimum code = Barely adequate.

View christyh's profile


5 posts in 4828 days

07-03-2010 04:20 PM

Assuming you are now aware of the dangers. . .if it were me, I’d just use a couple of 8” supplies, and if I added a 10” return, I would use a filter grill on it, and I’d keep a good quality allergy filter in it, and keep it clean. When the system is running, it WILL draw air through that return, so all smells, etc. from the shop will circulate through the house. . so keep this in mind also. Simply shutting the louvers on the supplies will not totally close the air off, so if it were me, I’d put a 10” start collar with a good manual damper in it, then run over to the garage with a 10”, using a 10×8x8 wye to split into the ducts, then use a good damper for the return, that way you can turn it off a little more cleaner. IMHO, Christy Hicks

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