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.