Supplies and return placement (new diagram)

Old 05-09-17, 03:22 PM
Thread Starter
Join Date: Sep 2008
Location: MA
Posts: 654
Received 0 Upvotes on 0 Posts
Question Supplies and return placement (new diagram)

I posted before but didn't get any replies. Since then I did a little test to get a feel for if my system had enough return air as my supplies appeared to have lost a ton of air pressure after blocking off just one return (temporarily).

With the blower on, I took the trunk/blower access panel off and sure enough my air pressure was back up to par. This made me think if blocking one return showed a noticeable difference than I was most likely already starving the system of return air so I should probably add more than one back.

This is the new diagram, I have concerns that I will be sucking already heated air back through the returns as the returns have to come directly off the duct that runs next to the supplies but I dont think there is much I can do about that.

My thought as shown on the diagram is to have one return at each end of the soffit to take the static air off the walls and keep things moving. I could also place a return in the center but i'm not sure if I would benefit from this or not.

I'd really like to hear some professional opinions on this.
Views: 1396
Size:  36.8 KB
Old 05-10-17, 12:03 PM
Join Date: Oct 2013
Location: United States
Posts: 2,768
Received 141 Upvotes on 133 Posts
The simplest thing I can recommend is you must have at least as much return air as supply air. As you found out you will only get as much supply air out as you have return air coming in.

If you have a 12 x 8 supply trunk you want at least the same size return trunk. To place return vents use common sense. Obviously do not put them right next to supply registers where the air will just get sucked up back to the furnace.

In a perfect world they would like to see 1 return vent for every supply per room but most times that is not practical. As long as the returns are unobstructed and large enough they will work. The system does not care where you get it as long as it is enough. In the old days they used to have 1 large return vent in a common area and those systems are still operating today.
Old 05-10-17, 12:22 PM
Group Moderator
Join Date: Oct 2004
Location: WI/MN
Posts: 19,220
Received 1,290 Upvotes on 1,228 Posts
The rule of thumb I've always heard is up to 150% the volume of return air as supplied, since the latter will have more force/pressure.
Old 05-11-17, 05:00 AM
airman.1994's Avatar
Join Date: Dec 2005
Location: VA
Posts: 5,491
Upvotes: 0
Received 8 Upvotes on 8 Posts
How many CFM are you moving and what size is your return duct?

Thread Tools
Search this Thread
Ask a Question
Question Title:
Your question will be posted in: