Mildew/Mold Odor

by Steve
(Winter Haven, Florida)

I spend winters in our double wide in Florida. I've been running the AC off and on for the last 4 weeks. I just started to get a mildew/mold odor from the cold air return when the unit isn't running.


Is there anything I can do? Or do I just need to call a qualified AC firm?

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Jul 10, 2014
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One source of musty smells NEW
by: Anonymous

I discovered that the condensate drip tube that drains into the bathroom lavatory trap is blowing air from the unit down that tube and out the lavatory trap into the house. Keeping the lavatory drain closed proved it was the source of the musty smelling air. Putting bubble soap in the lav and opening the drain also made bubbles come up when the AC was running showing there was air pressure blowing bad odors from the drain into the bathroom and house.

Jun 14, 2014
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Metallic, dusty odor from cold air return . Solution NEW
by: Anonymous

My last post was in 2012 and the same metallic dusty odor was still coming from the bedroom. A week ago, we had a handy man spray mildew/mold killer in all the returns and heating vents. Odor still persisted. Today when the odor was strong, we started smelling around and think the odor was coming from the thin metal liner around the cold air return opening. We covered this liner with plastic and so far, no odor. Is it possible for old sheet metal to give off an obnoxious odor? I always said that the odor was dusty metallic so it makes sense.

Oct 18, 2013
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30 plus years a/c unit in attic NEW
by: jane

my house is 30 plus and has the same unit in the attic, only ac the heat doesnt work, and it smells, My husband said he cannot clean the coils because it is so old and they are fragile that it could do the opposite and not run at all. so i have been putting up with this mildew mold odor for 10 years now. is he wrong help

Apr 15, 2012
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Metallic Musty Odor from Cold Air Return
by: Anonymous

I have the same problem with the cold air return odor. In the master bedroom, a dull, metallic, dusty odor comes from the cold air return. To prove it, I covered it with plastic and the odor is gone, although now it is coming from the air return directly below the bedroom. A heating guy who performed the pressure test in our home said we had to seal the top of the air return shaft since the odor was being pulled from the attic, but this absolutely is not true. My husband caulked the top of the return, and we checked the attic which is odor free and dry. Odor is worse on a warm humid day and more apparent when it seems to ooze into the room without the furnace or AC on. I can also smell this terrible odor which gives us a headache, in other rooms, but not as strongly as the bedroom. Could it be an old or rusty duct system or from adjoining walls? I don't think it is a dead animal or mold problem. It has been there for years and years and nobody can discover the source of the problem.For sure, it is in the cold air return duct which runs between wall studs.

Nov 22, 2010
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Mildew and Mold Smell
by: Dadan

Just to me sure I get this. You only get the mildew and mold odor from the return vent. While the central ac unit is off? You have to get close to the return vent to smells it right?

This odor could be coming from your return vent surround wall or the return duct is touching against ground.

When was the last time the evaporator coil was clean or it pans under neither the evaporator coil? But, I doubt the evaporator smell. If the evaporator coil smells, it should have spread to the entire house, not just the return vent.

You could also check to make sure there is not mold or mildew built near the evaporator drain.

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