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thebigbbm 09-20-2010 01:12 PM

Cold Air Return next to supply duct
Hello...we have an HVAC pro installing a system in new construction (Iowa). The HVAC company installed a non insulated supply line to a plenum on the third floor using square duct. Their plan was to enclose that supply duct in the wall, and also use the same enclosed space as a cold air return. In other words, the enclosed space will have a supply duct but will also be used as a cold air return (the cold air return will not have a separate duct, it's just using the air space in the wall next to the supply duct). My concern is that during the winter, the supply heat duct will be heating the air flowing through the cold air return...seems inefficient. Is this good for the system?
thank you,

AllanJ 09-20-2010 03:51 PM

The same air is going 'round and 'round, from the furnace via the warm air duct to the room and via the return air duct or space back to the furnace.

I don't think that the duct arrangement is going to hurt. To the extent that the cooler return air takes some of the heat from the warm air going the other way, that cooler air upon going through the furnace again will come out a little warmer for the next trip up.

The return air is not that cold. It came from the room so it is approximately room temperature. Just a few degrees cooler if the return register is near floor level since normally warm air rises.

Now don't put registers or vents in the return air duct close to vents or registers in the warm air duct, otherwise you won't get as much beneficial air circulation in the room proper.

beenthere 09-20-2010 04:27 PM

If the chase isn't sealed right. It will draw air from the attic, or even outside if its an outside wall.
Next, if you decide to put A/C in later. The uninsulated supply will sweat, and have water running down it, and soak and damage anything under it.

hvaclover 09-20-2010 06:26 PM

Who designed the duct system? Is the the heating contractor just working with the the space the plans call? Or was this gent the lowest bidder and you got what you paid for?

concretemasonry 09-20-2010 10:06 PM

Have a supply and intake too close in the "controlled space" is a very bad idea. The supply can be drawn back into the return and will never really get into the room where it is intended to go..


thebigbbm 09-21-2010 01:03 PM

I received three bids and all were close...I picked this guy because he impressed me with the detail in his bid and the other materials, references he provided. That said, there were some constraints with space (as usually the case), but there were other options that could have been pursued. My reason for asking the question was to find out if what he's doing is acceptable, or is the crew just being lazy and taking a short cut.

hvaclover 09-21-2010 05:51 PM

Sounds too me like he is trying real hard to work with what he's got.

One of the missteps when a home is custom designed is that very little consideration is given to the amount of room a mechanical system needs.

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