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In our attic is a gas furnace for the 2nd floor. The attic is not insulated (except for the attic floor which is the 2nd floor ceiling). The air return for this furnance is in the 2nd floor ceiling on the hallway (looks like a rectangle, approx. 20ish inches in diameter). Cold air is leaking out of this air return vent. I assume it's not attic air that leaks into the 2nd floor through this return duct, but the hot air rises into this duct, cools down and then sinks into the 2nd floor. I insulated the return duct in the attic with HVAC insulation which made things better. Now I am wondering if a backdraft damper would improve the situation. The flexible hose from the damper has 13 inch diameter and I bought a fitting backdraft damper. It opens easily and shouldn't put too much pressure on the system. Could this damper cause any other issues than putting more load on the fan? The gas heater is 3 years old and a high efficency unit.

Edit - some more information:

The cold air from the return duct is more than 10 degrees cooler than the 2nd floor temperature. Right now it's around 40 degrees in the attic, 70 degrees on the 2nd floor and the air from the vent is around 59 degrees.

You can't feel much of an airflow from the duct. My guess is, that there's no attic air entering the duct, but the hot air from the 2nd floor is rising into the return duct, cooling down and falling into the 2nd floor again.

My hope is, that the backdraft damper keeps the rising air in the duct itself, so it won't travel through the round hose to the furnace. This hopefully keeps the air in the duct quite warm. The hose that goes from the return duct in the attic to the furnace is insulated and is around 12 inches in diameter and maybe 8 feet long.

Since the damper opens easily my hope is that the system won't even notice the additional resistance.

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  • You say that cold air is leaking out of the cold air return duct. How cold is the air - is it cool, but mostly room temp, or is it outside/attic sub-zero temp? Where are you feeling the cold air - in the hallway below the duct or in the attic? What are you attempting to do with the backdraft damper - i.e. where do you plan on installing it in the system and what impact are you expecting it to have?
    – FreeMan
    Commented Oct 14, 2020 at 12:31
  • Please see my edit above. The text was too long to post it as a comment.
    – c3post
    Commented Oct 14, 2020 at 13:06
  • I can't help you with an answer, but the additional info should help those who know HVAC. Also, editing that into the original question is 100% the way to do it. Comments can get deleted, and many people don't read them.
    – FreeMan
    Commented Oct 14, 2020 at 13:10

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This is an old but I have the answer. The cool air flow is caused by the air cooling off inside the return flex duct located in the unconditioned attic. The supply vents are allowing air flow through the furnace/ahu to the return air, it is simply cold air is falling down out of the return air and being replaced with air behind it form the supply side. So yes add dampers to the return side to stop the air from coming out. Question is what type of damper. Electrically operated or not. I have the same problem but single story with ducting in unconditioned attic.

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  • I have been using butterfly backdraft dampers for a few years now and they have made a difference in keeping the warm air inside conditioned space and preventing it from rising into the cold unconditioned attic. I added dampers in the ceiling vents and also a larger one in the hallway air return vent. These butterfly dampers easily open once the heater turns on and then close again.
    – c3post
    Commented Nov 29, 2023 at 0:26

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