Intermittent gas smell in AC vents tied to hot water usage - Page 2
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  1. #14
    Quote Originally Posted by bustedknuckles View Post
    Possible causes; 1- gas leak after the gas valve,. 2-dirty sock(or in this case dirty fart syndrome). Or most likely a dry condensate trap possibly connected to the sewer system
    Both condensate traps are not dry. Sewer line from the house to the street has been inspected with a camera and is fine.

  2. #15
    Quote Originally Posted by martyinlincoln View Post
    How did they verify the venting? A combustion analyzer taking multiple readings and a draft gauge is the correct answer. Blowing out a match and sticking it under the draft diverter is the wrong answer.
    Would improperly combusting gas possibly smell more like sulfur than the smell of natural gas straight from the line? When the gas company was there they did a pressure test on the gas line and both outside and inside pressure was fine. In the process I got a good experience of the smell of the natural gas and think the problem smell coming from the vents is stronger and more like sulfur.

  3. #16
    Join Date
    Feb 2011
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    Peoria, IL
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    what about the floor drain next to furnace? shine a light in the top and see if you see water, if the trap is cracked or dry its possible its sucking sewer gas into a loose return at furnace and through the supply vents. (also if you have a sewage ejector pit for basement plumbing, make sure the lid is sealed as it will do the same thing.

  4. #17
    Join Date
    Sep 2009
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    Arnold mo
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    It is a definite possibility that your water heater is spilling flue gas at start up, and then vents properly after it has run for a while. The cause of this could be that your openings to your return ducts are depressurizing the basement. When the flue gas spills from the water heat flu pipe it is sucked into your air handler via the return side and then distributed to your supply vents.
    Get someone out their to do some pressure testing of your basement and testing of the water heater venting.

    The reason why this does not happen in winter can be due to the fact that it is colder outside, so your water heater flue gas vents quicker/easier. Also, your furnace runs in winter and that assist the venting process for the water heaters.
    An answer without a question is meaningless.
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  5. #18
    Quote Originally Posted by tipsrfine View Post
    It is a definite possibility that your water heater is spilling flue gas at start up, and then vents properly after it has run for a while. The cause of this could be that your openings to your return ducts are depressurizing the basement. When the flue gas spills from the water heat flu pipe it is sucked into your air handler via the return side and then distributed to your supply vents.
    Get someone out their to do some pressure testing of your basement and testing of the water heater venting.

    The reason why this does not happen in winter can be due to the fact that it is colder outside, so your water heater flue gas vents quicker/easier. Also, your furnace runs in winter and that assist the venting process for the water heaters.
    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?

  6. #19
    Had the venting scoped. Flue vents, ac vent, wall cavity all look clear. Also smell clean. The smelly air must be getting drawn into the air handlers and not from the ducts themselves.

    Since the problem only occurs related to the master shower hot water I think we will focus our attention on the drain line.

  7. #20
    Join Date
    Sep 2009
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    Arnold mo
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    AJ wrote, "The smell is present in all vents serviced by this AC unit."
    Now AJ writes "...the problem only occurs related to the master shower hot water".

    All I can take from this is that the smell is being distributed through all of the supply vents connected to the air handler in common with these vents. Not sure how you are connecting the smell with the master shower hot water.
    An answer without a question is meaningless.
    Information without understanding is useless.
    You can lead a horse to water............
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  8. #21
    Join Date
    Feb 2011
    Location
    Peoria, IL
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    2,133
    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)

  9. #22
    Join Date
    Feb 2011
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    Peoria, IL
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    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)

  10. #23
    Quote Originally Posted by garychance View Post
    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.

  11. #24
    Quote Originally Posted by garychance View Post
    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?

  12. #25
    Join Date
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    Quote Originally Posted by HomeownerAJ View Post
    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.
    An answer without a question is meaningless.
    Information without understanding is useless.
    You can lead a horse to water............
    http://www.mohomeenergyaudits.com

  13. #26
    Quote Originally Posted by tipsrfine View Post
    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.

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