Intermittent gas smell in AC vents tied to hot water usage

[garychance]

if the sewer gas not from flue, but from plumbing is getting sucked into the air handler/furnace and the nearest supply is close upstairs to this, plus if this is the bathroom you use mostly for showers you spend enough time in there to notice it. (just a thought)

[garychance]

again look at the sewage pit, not the sump pit, and verify the lid is sealed, and the cord ports (usually we put the pit in the mech room next to furnace)

[HomeownerAJ]

if the sewer gas not from flue, but from plumbing is getting sucked into the air handler/furnace and the nearest supply is close upstairs to this, plus if this is the bathroom you use mostly for showers you spend enough time in there to notice it. (just a thought)

Yes, I think this is about the only thing left. There is a sewage smell that is getting sucked into the air handlers. Both AC units share the same return duct work so the smell is distributed through the vents on both floors.

The smell is triggered by the operation of the master hot shower. It also seems to only happen once. We reproduced it yesterday for the ductwork camera contractor but then about an hour later ran the shower hot water and no smell.

Today i will try running hot water in the tub, but according to the previous owners, this will not cause the smell.

So something causes a sewer smell when hot water is run. Bacteria in the pipes? Something on the pipes reacting to the heat then burning off until it rebuilds or recollects?

If it is anything related to the supply side of the hot water that will be replaced when we replace the polybutylene (another problem not disclosed until we did the inspection).

Anything else to do diagnostically? I'm told you can't really scope an interior drain line. Might be something outside the drain line anyway reacting then the drain pipes heat up.

[HomeownerAJ]

again look at the sewage pit, not the sump pit, and verify the lid is sealed, and the cord ports (usually we put the pit in the mech room next to furnace)

Last time we reproduced the smell, i went to the basement to see if I could smell it there. The smell was present in the mech space, but not nearly as strong as it was coming from the vents. Seemed like I was just smelling the air running through the units.

I put my nose right above the drain where the ac drain lines go and didn't smell the sewer smell. Is this what you are referring to?

[tipsrfine]

Flue gas at startup sounds like a plausible cause - especially with two water heaters firing at the same time. Do you think the venting relationship to temperature would explain why this happens in the summer afternoons when it is over 85 degrees outside vs. in the morning when it may only be 60 or 70?

Have you tried shutting off the gas valve to the water heaters and then running the shower (or putting them on pilot)? The smell could be from the additive they put in your natural gas so you can smell a leak. When it is over 85 degrees outside and your water heaters spill flu gas at start up, the flu gas could contain some of the methanethiol (the additive) that wasn't burnt off in the combustion process. methanethiol has been described as smelling like rotten eggs.

[HomeownerAJ]

Have you tried shutting off the gas valve to the water heaters and then running the shower (or putting them on pilot)? The smell could be from the additive they put in your natural gas so you can smell a leak. When it is over 85 degrees outside and your water heaters spill flu gas at start up, the flu gas could contain some of the methanethiol (the additive) that wasn't burnt off in the combustion process. methanethiol has been described as smelling like rotten eggs.

That would be a good test to help eliminate possibilities. Another check we haven't done is to see if perhaps the wax seal in the master toilet has failed. That would also allow sewer gas to escape, but I think it would be more constant rather than tied to master shower hot water usage. (Although in another test, this problem does seem to occur with hot water usage in the master tub as well)

Also for reference of other people researching this topic, other possible causes we explored was that perhaps a P-trap had dried out from less used plumbing fixtures in the basement. Or that the cleanout plug in the main floor drain in the utility room was allowing sewer gas to escape and get pulled in to the AC.

If we can't determine the source of the smell or the cause our next test will be a smoke test of the drain line service the master shower and bath. The leading theory now is that there is a crack in the ABS drain pipe somewhere that is expanding enough when hot water runs through it that sewer gas escapes and is pulled in by the return air handlers. Since we are not seeing any signs of water leakage and the problem has persisted for over 6 years, the crack is probably on the top of a mostly horizontal section of drain pipe leaving the master bathroom.