We have an issue where smoke from the fireplace is getting sucked back in another unused flue into the basement. After trial an error I discovered that it was being cause by negative pressure, I can fix it by opening a window in the basement 3 to 4 inches. When the window is opening the draft stops coming into the unused flue.

I had an HVAC contractor come out and he gave us a solution to install a 6" fresh air pressure relief duct work with a barometric backdraft damper. He proposed ducting this from the outside directly into the cold air return of the furnace. He then said when we have a fire to turn on the fan to circulate air.

My question is when this relief valve opens and brings cold air directly into the cold air return and the furnace isn't running won't this just circulate cold air throughout the house? Shouldn't it be brought thru some kind of heat exchanger first then dumped into the cold air?

I purposed this question to him and he suggested turning on the furnace fan when having a fire to circulate the warm air from the fire to mix with the cold air that the relief valve is letting in. Just doesn't sound right to me that the heat form the fire is enough to counter act the cold midwest air.

Does this sound correct? Quote of material below.

Install 6" fresh air pressure relief duct work, Includes galvanized wall hood with screen, barometric backdraft damper, 6" pipe and fittings, R-8 insulation, materials and labor.

  • Is sealing the unused flue not a good option for some reason? Jan 19, 2018 at 4:29
  • It doesn’t fix the issue of negative pressure. It would stop the smoke but not the issue of the house becoming negative pressure.
    – tron_jones
    Jan 19, 2018 at 12:13
  • Where is the fireplace obtaining its combustion air? Jan 19, 2018 at 16:37
  • There is a fresh air vent at the front of the fireplace that brings air in but it doesn’t seem to be enough.
    – tron_jones
    Jan 19, 2018 at 16:39
  • 1
    I gather that fresh air vent is not plumbed to outside...My thinking is you're thinking about the wrong problem... You want to relieve pressure in the house generally, but the source of the negative pressure is a specific one, combustion, and it needs supply air specifically. Also you don't care how cold the fireplace's supply air is, because it's about to get really hot and leave Jan 19, 2018 at 16:42

1 Answer 1


When I add a make up air duct I usually do install it in the cold air return this usually filters prior to the air handler and conditions the air when the system is running.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.