Sewer odor coming from A/C ducts

[byndor]

We purchased our home 6 yrs ago and had a new furnace and central air unit installed in the house (this is the first time this home ever had central air), whenever we run our a/c there is a horrible sewage odor coming from the vents, I did see in an earlier post that you suggested that a cold air return could be a culprit if located near a bathroom, none of ours are, we have called the city sewer department out and they were also perplexed. We do not smell it when the furnace runs, only the air. The unit is located near drain but we usually do not smell the odor in the basement only in the house and only when the air is running. Needless to say, it is becoming irritating and I use more aerosol air freshener than any average household. Any ideas would be greatly appreciated. Thanks in advance for any help/advice you may have to offer.

[jadnashua]

There is probably a drain for the condensate of the air conditioner. My guess is that it wasn't installed correctly and is sucking fumes directly from the sewer system. I'm a little surprised that it doesn't also smell when the heat is on, too. OR, it could be mold on the cooling coil or in the drain and you're smelling that. DUring the heating season, no moisture from the condensate, and the mold goes dormant. The second possibility explains your symptoms better, but there may be another reason. Check out the condensate drain and make sure that it does acutually drain and is not plugged. All that water could be going somewhere else and the smell may not be sewer gas, but mold growing somewhere. If that is the case, you'll need to look for rot, as well.

[e-plumber]

Is there a floor drain, or any open drain or clean-out fitting in the same room where the A/C unit is located? If so, it could be drawing sewer odor into the A/C if the trap seal has evaporated, (all plumbing fixtures including floor drains have traps). An infrequently used plumbing fixture can also allow sewer gas to escape.
I have checked for odors similar to this, it turned out to be a dead rodent :eek: in one of the ducts but after a few weeks it dried up.
Has the HVAC company checked the system for proper operation?

[jimbo]

I have seen all too many cases where the condensate drain tube was literally just stuck into a vent stack in the attic. This will possibly cause odor problems. Have this drain connection checked, and also consider having an A/C company service your heat exchanger. Improper condensate, and sometimes just normal operation result in mold problems in the condensate pan. This can be treated.

http://www.health.state.mn.us/divs/eh/indoorair/mold/

[LonnythePlumber]

Last week I was in a crawl space and the two year old air conditioning condensate drain had a hose that ran into an untrapped opening in the house sewer. It was duct taped in tightly.