Sulfur smell in HVAC system when the toilet flushes

[Sarah Simon]

Hi All,

I'm hoping someone in this community can help me solve this problem. I have 3 story townhouse (4 floors including the basement) and it has 2 bathrooms (2nd & 3rd floors) and a powder room (basement). When the master bathroom toilet flushes (3rd floor), the whole house has a faint but noticeable sulfur smell coming from the HVAC system. To make this even more of a mystery, it only happens about 60% of the time.

I've spoken to several plumbers and nobody has heard of this issue. One plumber said it could be the vent, but the toilet doesn't gurgle; nor does any other plumbing fixture.

Another plumber said the wax ring could be failing, but the smell isn't coming from the bathroom, it's coming from the duct registers.

I'm well aware they are two separate systems but can anyone think of why flushing only 1 of the 3 toilets from the 3rd floor could be causing sulfur smells to infiltrate the house HVAC system?

Please help!

Thank you

 

[Matt30]

Is there floor drains on the upper levels? It’s common for hvac condensate to drain into a floor drain. If the floor drain is being siphoned when the toilet flushes, and if the hvac condensate isn’t protected by its own trap then it could get a smell from there.

It’s the kind of situation where all the stars need to align perfectly to make it happen, but I’ve saw it happen before.

 

[Sarah Simon]

Is there floor drains on the upper levels? It’s common for hvac condensate to drain into a floor drain. If the floor drain is being siphoned when the toilet flushes, and if the hvac condensate isn’t protected by its own trap then it could get a smell from there.

It’s the kind of situation where all the stars need to align perfectly to make it happen, but I’ve saw it happen before.

Thank you for your reply. The only floor drains would be in the showers on both the 2nd & 3rd floors. The condensate line is in the basement and ties into the washing machine drain. I can't see if there's a trap on it though. I read your reply to my boyfriend and he knew exactly what you were talking about and said he'll look when he comes over this week. I hope this solves the mystery!

 

[breplum]

Possibly, look for a direct connection between the HVAC condensate lines (typically white 3/4" PVC) tapping improperly into the plumbing system.
That would explain it for the most part.

 

[Diehard]

Thank you for your reply. The only floor drains would be in the showers on both the 2nd & 3rd floors. The condensate line is in the basement and ties into the washing machine drain. I can't see if there's a trap on it though. I read your reply to my boyfriend and he knew exactly what you were talking about and said he'll look when he comes over this week. I hope this solves the mystery!

he trap would likely be at the other end of that condensate line you see in your basement. In other words, located at the Evaporator unit that's located somewhere in the house. In the attic is a common place for it to be.
And when someone checks out the unit for the presence of a trap on the condensate line, that would be the ideal place to detect for the suspected sewer gas smell.

In fact, if the condensate line dumps in to the washing machine drain line, I assume that is a standpipe that is equipped with a trap as well. To get a smell back from that source, that trap would have to be void of a water seal.

 

[Sarah Simon]

Ok but I don't have an attic. Could it be behind a wall in the 3rd floor or is it something that would have to be easily accessible?

 

[Diehard]

It could be in your Basement.
Do you have forced air heat?
If it is in the Basement, chances are the condensate is coming from a small pump near the floor of the unit.