Sewer smell in vents only AFTER furnace has run

10 years ago

Well, the title says it all. I can't figure it out. Every HVAC vent in the house has a sewer gas smell seeping out of it, but only after the furnace has run. The vents smell just fine while the furnace is blowing air through them.

We've owned the house since Feb. 2013 and didn't smell the odor until it turned cold (maybe 2nd or 3rd week of Oct.).

I thought maybe it was the condensate drain being improperly installed in the floor drain in our basement, but I would think that if that's the problem, we would have noticed the odor when the A/C or heat ran and only when they ran, not afterwards.

Anyone have any ideas?

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Comments (4)

klem1
10 years ago
We could guess at the odor until next week but it would still be up to you finding it.
If you have natural gas or LP gas service (even if the furnace is all electric) , I reccomend having your supplier test for a leak. CAVEAT, If it is natural gas and there is a leak,they will turn the service off and tell you to hire a plumber before service is restored. LP depends on your supplier what happens.
SpecialtyAirInc
10 years ago
The source of the sewer gas can be plumbing fixtures whose traps have gone dry or have lost enough water that the water seal within the trap has broken. Cracks in either plumbing drain lines or vents pipes are the other source of sewer gas leaks. If the crack is in a drain line, you often see an associated water leak. But vent pipe cracks are far more elusive.
mike_home
10 years ago
"I thought maybe it was the condensate drain being improperly installed in the floor drain in our basement..."

If the condensate drain is connected to the sewer and there is no drain trap, then it is possible sewer gas is coming up through the condensate and into the furnace. It may be more noticeable when the furnace operates since warm air rises and is carrying the sewer gas up through the supply vents.

You can install the trap in the condensate line. I like the clear EZ Trap product. It is a clear tube with openings at both ends which can be easily inspected and clean as needed.
asharry
3 years ago
Chances are your furnace fresh air intake is drawing in the odor from the vent pipe. The reason you only smell it at the end of the heating cycle is because when the burner is running the air coming through the intake is being used for combustion. But once the burner shuts down, and the blower continues to run for 2-3 minutes, then that air is being sucked in and not burnt off, so the smell continues out through your registers. Solutions include making sure your drain traps are refreshed, filling and priming your furnace trap, and reducing the amount of time the blower is on after combustion ends.