My home is two stories, 5100 sq feet, and was built in 1990. I have a septic system, radon mitigation, and central forced air.

This has gone on for two winters now, but not before that.

The second story will intermittently smell like sewage (my wife thinks it smells like something died) when the heater kicks on. But the first story and basement don't smell.

  • When it does smell, it is almost always in the mornings
  • And it seems to happen only when weather outside is cold and the heater is on.
  • The smell goes away after an hour even if the temperature outside is below freezing.
  • It does not smell during the summer with the air conditioning.
  • Mechanical room doesn't seem to smell when this happens.
  • Attic doesn't smell when this happens.
  • I have checked the blowers for dead rodents and didn't find any.

Does anyone have any ideas as to why this is happening?

Does anyone have any ideas as to how to find/fix the problem?


The issue did turn out to be a dry p-trap located in the mechanical room that we have never used and so I forgot about it.

The reason the smell was only on the second floor is because the p-trap was next the blower that serviced the second floor. The first floor blower was further away and so didn't suck in the sewage smell.

Once I put a cap on the drain pipe, no more smell.

  • Do you have a smart thermostat that bumps the temperature down at night and back up in the morning? Dec 27, 2019 at 16:31
  • No, I don't, it just can be set to a single temperature.
    – Fred
    Dec 27, 2019 at 16:54

1 Answer 1


Drain pipes have P-traps in them that hold water to seal the drain from sewer gasses that would otherwise waft up out of the drain.

Check your drains (sink and floor) to see if they have water in the traps.

(the smell is probably being moved around by the fan in the heater)

  • The P-traps appear to have water in them. But one thing I don't understand, is that the fan is in the basement. But the basement has no smell. Only the second story has a smell. So if the smell is being moved by the heater fan, then wouldn't the basement smell too?
    – Fred
    Dec 31, 2019 at 19:33
  • Maybe! It was just a theory/guess. BTW: The P-traps must have enough water to fill the 'P' and seal the pipe. And heat does go up. Jan 1, 2020 at 20:38

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