View Full Version : Sewer odor coming from A/C ducts

09-16-2004, 02:05 PM
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.

09-16-2004, 04:11 PM
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.

09-16-2004, 04:12 PM
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?

09-16-2004, 06:26 PM
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.


09-16-2004, 07:11 PM
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.

12-09-2009, 08:34 AM
If the doors in front of your unit(s) are closed and not vented properly, and / or the HVAC unit is not properly vented to the outside of your house then the stale air is not circulating properly. Also, if the condensation drain pipe for your unit is not plumbed to the exterior of your home it might be part of the cause of the smell only compounding your lack of ventilation issue. (Also, make sure you have a condensation trap...)

Think about it this way; if you put a jar of water underneath the sink in the kitchen and then you don't open the cabinet door for a long time, when you finally do, what will that standing water look like and smell like? It will become brown & smelly. It will stink because it doesn't have a chance to drain and the lack of fresh, circulating air will cause the cabinet and water to become moist and stale. P.U.

If the air ducts are not properly ventilated to the exterior then the stale air in your home needs to escape somehow. The only option is for it is to be sucked back in through the vents into the area where the units are located. That air wants to be circulated but cannot. There must be exterior ventilation and drainage for the HVAC systems so that air and water can go somewhere.

Ask the installers about the exterior drainage and ventilation and you'll at least be headed in the right direction. And get a second opinion. If your original installer charges for the service call and then charges for resolving the problem that he caused in the first place you probably will want to have a serious conversation with him.

Good luck!

12-09-2009, 09:48 AM
Relatively few residential HVAC units have fresh air in them, they rely strictly on recirculating what's in the house. But, if you have leaky ducts, you could be sucking in things from areas they pass through on the return side.

As houses are being built tighter, having a source of fresh air to mix in is becoming more important (not that it wasn't before, but if the whole house leaks, you functionlly are getting some fresh air in). Canada is much further ahead on this than the USA.

12-09-2009, 04:52 PM
In dry weather, or wintertime when there is low humidity, there is no moisture to keep the trap sealed, so if the air handler is connected directly to the drain, there is nothing to prevent it from drawing the sewer gas in and distributing it to the entire house. The trap, if there is one, also has to be designed for the "suction" developed in the air handler.

Peter Griffin
12-09-2009, 05:36 PM
This is why I hate to see air handlers piped to a sewer drain. 1/2 the time, the air handler trap dries out and sucks air through the drainage trap and the other 1/2 the drainage trap dries up and stinks the house up. Pipe it outside where it belongs.

Ian Gills
12-09-2009, 06:17 PM
Has a six year old post just been reincarnated?

The O.P. has probably deceased as has his AC unit.

12-09-2009, 06:26 PM
I think its just over 5 years ;)

02-11-2010, 12:54 AM
Thanks for posting this drip doctor! The same thing happens at my house - no sewer odor until I light a fire in the fireplace! Never smell anything any other time. In researching the web, I stumbled across this and it's got me thinking. Some articles mention the same problem, but the answers usually point to the fact that water may not be present in the sink traps/tub traps, etc. BUT, I never smell it any other time, so that doesn't really seem to solve the problem (not sure, but wouldn't it stink all the time if the traps weren't functioning properly?)

Someone mentioned a "smoke test". Perhaps I should have one done in case my pipes are eroding like you mentioned? The house was built in 1940 and has thick, plaster walls. I suppose the stink might be in there all the time and I just don't know it.... Anyone in the Chester, CT area who knows how to fix this?

02-11-2010, 01:14 AM
Also, if I crack a few windows to let in fresh air from the outside, the odor quickly disappears. It seems that the fire's drawing of oxygen is the culprit....the house is insulated very well, so not much extra outside air is available unless I open the windows...which of course, defeats the whole purpose of having the fire for warmth in the first place. Open windows in the middle of winter = no bueno.