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Question DetailsAsked on 6/19/2014

How to find the source of a foul smell coming from cold air return only in master bedroom?

We pulled the grate off and cannot see anything. And it smells at different times. We are trying to find a pattern, but it smells when the air & heat are on and when they are off, but not consistently

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1 Answer


My first move would be check both positive and return vent in all rooms for smell, and around furnace and furnace room for smell.

If you cannot see anything by holding a hand mirror down in the ducts (with grates removed) - both inflow and return ducts, and doing same thing at furnace (with furnace off, of course), then I would recommond finding an HVAC contractor or Duct Cleaner with a video camera that can be run full length to see if you have pooling of condensate somewhere that is going stagnant, wet mold buildup, or a dead animal in there.

To inspect with a hand mirror, use size that will fit in the duct, maybe tape to a stick or bent coathanger to manipulate it is dcut is small, then use it in conjunction with a LARGE flashlight. The 6V lantern type like these is best for this - hold the mirror at about 45 degrere angel in the duct and shine the light at the mirror to illuminate the duct, and then look in the mirror from above and move about slightly to see down the duct - works REALLY well (until the next bend of course) -

I would check from every room nearby because you might see up into the duct leading to your MBR too.

Also - a neighbor found this after hours of trying to track it down - make sure a teenager is not hanging sweat socks or gym clothes or wet towel (which has gone mildewy) in front of the vent in their room to dry them - that is actually a fairly common cause of HVAC smells. Another possibilitiy is kid (or bounce-back kid) using the duct in their room as marijuana or junk food pantry, or burning incense or weed in their room.

Ditto to lost pets - hamsters and guinea pigs and rats and such that have gotton lost inthe system and died.

Alos, if tied to one vent, try removing the cover and then duct tape tightly over the metal or plastic duct but NOT over the surrounding air gap for a day or two and see if resolves the issue - if so, problem is in duct or somewhere it leads to. If no difference, then likely rot or mold or such small coming up from the floor joist area - maybe from a leaking pipe or drain in adjacent bathroom say.

If you can describe the smell more, using the Answer This Question button right below your question, we might be able to help more - is it rancid food, stinky gym clothes, mildewy towel, dead animal, plasticy, sewer gas, natural gas odorant, exhaust fume, rotting wood, etc type smell ? That might help us give you a better clue.

Answered 6 years ago by LCD

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