View Full Version : Heat system return emitting cool air in mid heating season

01-19-2012, 04:18 PM
I'm was really surprised to find out that the main " return" at floor level in my home's family room's heating system is gently blowing cool air into my family room when the heat registers are not blowing heated air.
That register sucks air when the heat comes on and begins blowing warm air into the room at the outlet registers
I've lived here 11 years but never noticed this oddity but was on the floor itself today and was shocked
to feel cold air comming OUT of the intake return register.
Can anyone explain why this should be occuring; It's 28 degrees outside.

Thanks, jemry

01-19-2012, 04:26 PM
A house that age, they may have installed an outside supply to "mix" some fresh air with the return, due to tight construction. Not very energy efficient, and a side effect could easily be a cold draft from the return grill when the blower was not running.

Look at the furnace, and see if there is a duct coming into the return from somewhere outside!

Runs with bison
01-19-2012, 08:32 PM

Is this single story or multi-story? Where is the heating system and where do the returns run in the home (all internal, up through an attic, under the floor along external walls, in between internal walls, etc.) I can think of half a dozen ways for this to happen. If some of the return duct is running through unheated space it could be cooling air on the wall of the duct, or leaking cold air into openings in the duct and sinking into the space. Elsewhere, higher in the system hot air is probably escaping. So in addition to what jimbo said, you might want to look for leaky joints in the return ducts, uninsulated ducts outside the heating envelope, etc.

Up until rather recently much of the duct work installs were shoddy--and many installers/builders still don't seem to understand it. For some reason nobody seemed to care if the work the blower was doing actually moved nearly all of the air from point A to point B, then returned it. So open joints were common, as were uninsulated lines where insulation should be. 30% cirulation losses were typical.

01-20-2012, 03:35 PM
Seal all of the duct seams & joints with duct mastic, poke around see if you can't find other leakage or a broken supply duct feeding that room. Unless there's a balance supply & return system with guaranteed return paths even when doors are closed it's pretty easy for the unbalanced system to end up back-pressuring some room, especially if it's supply leaks like crazy and the house isn't very tight.

Bad duct systems can easily account for a third of your heating bill, if unbalanced and leaking. You can often "fix" supply/return unbalances with "jump ducts" to get around closed doors, or vents designed to be cut into the bottom of doors to equalize pressures. Even if your house is fairly leaky to the outdoors, equalizing room-to-room pressure differences with jump ducts grills and vents can keep the air handler from driving massive air infiltration.

See: http://www.greenbuildingadvisor.com/blogs/dept/qa-spotlight/how-track-down-leaks-forced-air-ductwork