Sewage Smell from Vents

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Snoot Chute

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Starting in mid November of last year, there has been an on and off sewage smell coming from all the air vents in our house. I am certain it's coming from the vents and not a drain or anything like that. The smell fades when we turn heat and air conditioning off. The smell was tolerable up until late January of this year when it started getting stronger. And for the past 3 weeks, one day out of every weekend the smell gets so bad it's unbearable. Then during the rest of the week its off and on but not nearly as bad as the one day on the weekend. I tried turning the air conditioning on instead of heat for about 20 minutes several days ago to see if that would help but the smell kept coming. We had our plumber come check it out last week and I asked him if the A/C condensate drain was piped to outside and he said yes and showed me (I hope that was the right question). He checked under the house and said everything looked great and dry. He poured bleach down most drains and cleared hair out of shower drains, he also opened up the vents beneath our house. The smell came back the next day and continued as it had before he came. One other thing I want to note, is that on the day before the smell started for the first time, our plumber's son (also a plumber) came to reattach the pipe of the right side of our kitchen sink as it was leaking because it had not been properly installed by whoever did it originally. This doesn't seem like it would be related, but I know basically nothing about this sort of thing so I thought I'd mention it. Should I call our plumber back or should I call an HVAC person? If anyone has any insight as to how to fix this problem I would be immensely grateful. Thank you.
 

Jadnashua

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Take a picture of the drains underneath the sink and post it here (use the "upload a file" tab and upload it from your computer). Since things started when that work was done, that would be my first place to look.

If you put the stopper in the kitchen sink, does the smell go away?

It sounds like the trap in the sink is getting sucked dry, leaving a direct opening to the sewer for gasses to escape. Or, the slope in those pipes above the trap are not proper, and some crud is festering in the pipe above the trap. Any horizontal sections should have some slope downwards towards the drain outlet both before and after the trap.
 

Snoot Chute

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Thanks for your reply. I hope the photo is a good angle. I'll have to wait to see if putting the stopper in the sink helps, as the smell is not around at the moment. I guess I'm confused as to how the sink drain would cause the smell to come out the air vents, though, seeing as there is no intake vent near the sink.
 

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Jadnashua

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I don't see anything glaring about that install, but without a level to check the slope, it doesn't shout problem.

If that drain runs by another trap that is not vented properly, the smell could originate elsewhere closer to an air intake.

If the furnace ducts are not balanced well, you can suck in stuff.
 

WorthFlorida

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I agree with Jim's remarks but since it seems to happen when the HVAC is running and all windows are closed. Without a doubt the air pressure in the home is being reduced when the fan is blowing and its pulling in the odor from somewhere. You didn't state where the air handler is and is this the first heating season living in the home or have you been living in the home for more than a year?

Since you mentioned that the odor seems to come from all of the air registers, it's hard to believe that that much gas is coming from the 1.5" p-trap and the odor would be very prevalent in the kitchen. The next time you get the odor, at the kitchen p-trap loosen the lowest compression nut, do not remove it completely . If water comes out then this trap is not the source.

If your system is in the attic, check the vent pipes in the attic that none are cracked or a fitting came apart.

At the thermostat move the switch to "FAN ON" so the heat or AC doesn't need to be running so you can "sniff" out the source. If you get the odors, turn on your clothes dryer. That thing will really pull air out of the home and it when the odor was really bad it might have been running. Another possibility is when the odor came severe, did it happen to be windy outside the home. When steady winds hit a home from a certain direction and depending on the home construction, the wind can suck air out if the home thus dropping it internal air pressure.

A long shot is mold growing on the AC coils and in the drip tray. I've seen plenty of nasty stuff in drip trays but usually very little odor.

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Reach4

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A peppermint test, with the blower off would be a good place to start I think. Then if that is negative, turn on the blower and see if the peppermint smell comes out of the air vents.
 

Dana

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With properly balanced ducts the room-to-room pressure differences in any room will be less than 0.02 water-inches with the air handler running, independently of which doors are open or closed. (An Energy Star house would fail certification with anything more than 0.012".) Both the duct design and duct leakage can affect this.

It's worth inspecting all duct and air handler connections/joints/ seams for leaks, and sealing them with duct-mastic or for hard-ducts that are stil shiny purpose made aluminum tapes. If the ducts (including return ducts) all seem reasonably tight, a $50 box store hand held manometer can usually sleuth out the most offending rooms.

If the ducts & air handler are in a vented attic above the insulation & pressure boundary of th house, duct sealing is even more critical, as is sealing all of the duct boots & penetrations to the ceiling gypsum & wallboard. Air sealing the entire exterior of the house (including the foundation) is would also usually reduce house depressurization, unless the duct design is WAY out of balance.
 
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