Cold Air Return Ducts

chipper · 08-20-2008, 08:16 AM

Is there a rule of thumb or formula to determine the amount of cold air returns required and where they should be placed?

Hube · 08-20-2008, 08:47 AM

A forced warm air duct heating/cooling system should have a Return "area" that is at the LEAST equal to the Supply area.
In other words, if the Main Supply duct is 8x20 (160 sq in ) then the Return size should be at least 160 sq in or even approx 10 % more.
Hopefully, the Supply caculation and the Unit (btu and cooling tonnage) are sized properly in the first place via Manual J and Manual D to determine the correct heat loss/gain.

As for placement of Returns,they are best on the floor opposite and farthest away from the Supplys which are best when placed on the exterior of the rooms (under windows)
Ex; Supplys on the exterior of the room
Returns on the interior of the room
note; for cooling purposes only, Returns can be placed in the interior high-wall location, or for both heating and cooling they can be located in the SAME stud space and activated by installing a damper in the lower grille.

chipper · 08-22-2008, 08:41 AM

Thanks for the info. We just moved into a house that was built in 1869 and I'm trying determine if the duct work is correct. There is no duct work to the 2nd floor which I'm in the process of adding. I have limited space on the 2nd floor so I have added a 6" duct to the master bedroom, 5" to the 2nd largest bedroom, 4" to the smallest bedroom and 5" to the bathroom. I only have enough room for a 6" return duct. Is that enough?
The den on first floor has 2 heating ducts and 2 return ducts. I'm assuming I should remove one of the return ducts?
The kitchen has 2 heating ducts and 12' by 12' sunroom attached. Should I move one of the heating ducts to the sunroom and add a cold air return?
I'm also going to replace the 40 year old furnace with a Trane XV95.

Hube · 08-22-2008, 11:07 AM

In order to get the proper sizes for a heating/cooling unit and a supply and return ductwork one would have to have completed a Manual J and a Manual D calculation sheets. These calculations will take into account the amount of windows,size of rooms, insulation r value, orientation and the climatic temperatures of the area you live in. Doing it any other way would be a foolish guess.
A lot of math goes into figuring how much supply and return is needed.
eg; a 4" diam = approx 12 sq inches.
a 5" diam = approx 20 sq inches
a 6" diam = approx 28" sq inches

That's a total 60 sq inches of area for those 3 ducts you are figuring on using in the upper floor. But since return air should be at least equal to the supply that means the 6" return you are figuring to install will fall far short of doing any real good,if any.
If you are putting supply air in that adds up to 60 sq inches then you should have at LEAST that amount in return fromthat area.

Note; DO NOT install any return in a bathroom or kitchen area.(odor)
In all, you would be wise to have a heat loss/gain and duct sizing done, then you will know for sure that any furnace(btu,s) and duct sizing will be best for you .
if you have more questions feel free to ask.
Hube

chipper · 08-25-2008, 07:36 AM

Thanks for the info. The new furnace will be installed in 2 weeks so I will continue with my ductwork and then ask the installer to make some recommendations.