Hello all.
I'm really hoping one of you talented fellows can help me pin point my problem.

Description: We just moved into our first house, and whenever we run the AC (or even just the fan, though it's not as potent) we get a sewer-ish smell. Doesn't smell like mildew, nor decomposing animals, just gassy and awful, like a dried out p-trap but not as intense.

So far we've:
-Replaced the AC coils. A friend is in the HVAC business and thought we had the dreaded "dirty sock syndrome." Hasn't charged us for labor despite spending at least 10 hours spread amongst several days at my house, but we still had to dish out ~$1500 for coils that were dipped in an antimicrobial powder coating of sorts. At least i'll never have to worry about dirty sock syndrome now :/

-Checked for mold/bacteria growth in the air handler. Sprayed bleach and simple green (on separate days) everywhere. No dice.

-Installed a crazy, black, carbon covered filter that's supposed to absorb any odors. Not working.


Setting: Air handler is in the garage. The condensate line that connects to the pan beneath the coils runs straight outside. Not aware of any other condensate line.

Return air is in the hallway, not near any restrooms or anything that could be stinking up the place.

The whole garage will sometimes smell with the same smell that I get through the ac, especially when it's very warm. Garage doesn't seem to have any real source of ventilation, so it gets very hot and muggy if we run the washer and dryer.

Got a wiff of the smell in my bedroom near the ac vent. AC nor fan hadn't been run in weeks.

Inspected the lines in the attic: no tears, not terribly smelly up there, nothing dead.




I'm now thinking it has to been a sewer vent. I've heard of plumbers doing a smoke test to find such leaks, but I don't want to throw more money at the problem until i'm sure. Like I said, we just moved into our first home and already had to dish out $1500 on top of buying furniture, yadda yadda.

Can any of you think of anything else I can try? It's supposed to be in the 100's again next week and we've already been dealing with this intense heat for weeks.

First thing first. Pour a quart of water into each and every single drain in the house. Every sink, washer drain, floor drain, garage drains etc. Is your basement carpeted? I have seen drains under carpet. They can dry out and let sewer gas in as well. Try that and see if the problem stops.
The prime suspect in foul odors when the A/C gets turned on is a dry drain in the basement, one that is away from the furnace.

"...The condensate line that connects to the pan beneath the coils runs straight outside. Not aware of any other condensate line..."

The line that you refer to is probably the secondary condensate drain. You need to determine if there is a primary condensate line and follow it to its terminal. It could very well terminate in a plumbing vent.

Is this a brand new house or your 1st used house?
If the house is not new... how old is it?

Best Wishes

Is the A/C creating a negative pressure in the house? Leaky supply ducts can lead to a situation where there's more air going back to the airhandler than what's being put into the house. That would also be wasting your money. On a calm day, open a window. If you get hit with a blast of warm air, you probably have a negative pressure.

It would only make a difference if there were a secondary condensate drain and it wasn't trapped. I haven't ever seen a residential home with more than a single drain from the A coil. (but it possible I suppose)

"First thing first. Pour a quart of water into each and every single drain in the house. Every sink, washer drain, floor drain, garage drains etc. Is your basement carpeted? I have seen drains under carpet. They can dry out and let sewer gas in as well. Try that and see if the problem stops.
The prime suspect in foul odors when the A/C gets turned on is a dry drain in the basement, one that is away from the furnace."


-Water has been running through every drain for some time. 2 bathrooms, each with their own shower and toilet, used daily. Dishwasher and kitchen sink used daily. Washer outlet used if not daily, frequently. I'll try dumping more water down there to make sure it's not that one. That's in the garage though, so it shouldn't be getting into my vents from the air handler there. Also, I have no basement. Not too many basements in Southern California at all.


"Is this a brand new house or your 1st used house?
If the house is not new... how old is it?"

-My first used house. Built in '84. Sorry, should have included that.

"Is the A/C creating a negative pressure in the house? Leaky supply ducts can lead to a situation where there's more air going back to the airhandler than what's being put into the house. That would also be wasting your money. On a calm day, open a window. If you get hit with a blast of warm air, you probably have a negative pressure."

-That's a possibility, but if so, it's not very extreme. Also, I'm not sure how that could make my ac smell? The supply ducts are letting in sewer gas?


"It would only make a difference if there were a secondary condensate drain and it wasn't trapped. I haven't ever seen a residential home with more than a single drain from the A coil. (but it possible I suppose)"
""...The condensate line that connects to the pan beneath the coils runs straight outside. Not aware of any other condensate line..."

"The line that you refer to is probably the secondary condensate drain. You need to determine if there is a primary condensate line and follow it to its terminal. It could very well terminate in a plumbing vent."


- I'll look for a second drain. I feel like my AC guy would have mentioned that though.




Thank you all for the input! I really appreciate any and all help.

A smoke test would be more conclusive at this point. It is possible that sewer gas is coming in from under a toilet as well. Until you can rule out sewer gas getting into the house, I wouldn't get too crazy with hunting down the smell. It could always be a dead animal in the ductwork as well, but I would check the drains with a smoke test.

We had a problem with a sewer odor in our home that was eventually tracked down to a clogged and corroded stack. Apparently, from what I was told, the drain line had clogged and when water/waste went into the drain, it would back up into the vent eventually clogging that as well and all that crud sitting there caused a hole to rust/corrode through the heavy cast iron pipe and the sewer gas to have free reign in the house. If I hadn't seen the pipe as it was removed, I'm not sure I would have believed it.

Good luck.

House was built in '84, it shouldn't have metal drains.