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Thread: Another sewer gas mystery

  1. #1

    Default Another sewer gas mystery

    We have an intermittent sewer gas problem in one of the two bathrooms in our new home. We really only notice it after using either of the toilets during cool weather. At those times the smell is strong, though. The other toilet has no problem. I replaced the wax ring clumsily; then tried again with a rubber valve instead. The third time I used 2 wax rings Ė one on the floor and one on the toilet horn. Still no good. I also shoved a hose pipe down the roof vent and flushed it out.

    The house used to have a septic tank but is now connected to the town sewer. Why would cold weather bring out the problem and why would the other toilet not be affected? Could the toilet itself be defective? It is one of those all-in-one units. There is enough water in it. The smell is definitely coming from the toilet and my wife swears it is coming out of the water though Iíve never seen gas bubbling up.

    I would be very grateful for any suggestions.

  2. #2
    Aspiring Old Fart, EE, computer & networking geek Mikey's Avatar
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    Try covering the bowl of the offending toilet tightly with Saran Wrap or similar. Flush the other toilet and watch and smell what happens.

    Since you're now experienced at setting toilets, you could swap the two toilets and see if the problem moves.

    How cool/cold does the weather have to get? Might it be related to the furnace coming on?

  3. #3

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    Hi Mikey,

    Thanks so much for your answer. I will give the saran wrap a try. I did consider switching the toilets but the problem one is a big beast and hard to deal with on my own. Thatís why I wanted to know if the defective toiled theory was in the realm of possibility before I tried that. Itís also possible that my third attempt at replacing the seal was also botched but Iím still in denial about that one. Actually, I messed up one of the 2 wax seals I used and had to remold it into shape. Donít know if thatís a problem and I don't want to face a 4th attempt.

    There was no sewer gas smell for a few days after that until the night temperature dropped to mid 40ís outside. As far as I can remember the furnace never came on but thatís another great suggestion to check out.

    Thanks again.

  4. #4
    Retired Defense Industry Engineer jadnashua's Avatar
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    Wax seals have been around for a long time are cheap and work well. That being said, when I replaced my toilets, I used Fluidmaster waxless seals. Figured, no cleaningoff old wax and no ruining the seal if the toilet is not let down square. Seem to work well.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  5. #5

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    My 2nd attempt was with the Fluidmaster waxless but that didn't stop the smell either. Hard to know if I botched the fitting somehow or if the seal is not the problem at all.

    Life is full of mystery, isn't it

  6. #6
    Plumber Cass's Avatar
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    Are you on a well or city water?

  7. #7

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    Neither, We have a community water system. In fact, the water main runs past the end of our drive and has been leaking for a couple of years. It is eroding the public road but that is downhill from us.

    The weather here warmed up again so I'm not getting the sewer gas smell for now and can't really test any solutions until the cold nights return. But it will be back!

  8. #8
    DIY Senior Member Phil H2's Avatar
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    Are you sure the smell is coming from the toilet and not some another portion of the drain or vent system? If you haven't used the tub or lav in months, run some water through them. Over a long period of time, the water in the traps can evaporate.

  9. #9

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    Hi Phil,

    Thatís a good question. There is a bathtub right beside the toilet but we use it every day so the trap water couldnít possibly be evaporating. However, my wife now says she thinks the smell might be coming from the tub rather than, or in addition to, the toilet. Iím not sure sheís right Ė Iím still waiting for the temperature to go down so I can get a better idea. There is little or no smell except when the outside temperature goes below the mid-40s.

    Could this indicate a venting problem? Maybe the vent is blocked and pressure is forcing gas past the water lock in both the toilet and tub. One of the first things I did was try to clear any blockages in the vent, but maybe I didnít do it well enough. If we only got the smell when temperatures fell below freezing, that might indicate water in the vent freezing up but why would the smell only come when it gets down to 40 degrees? Is it safe to say that a faulty wax ring would give us a consistent gas smell all the time, regardless of temperature? If itís inconsistent, pressure build up must be a factor, right?

  10. #10
    Retired Defense Industry Engineer jadnashua's Avatar
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    Here's a thought...is your furnace a condensing version (assuming you do have a furnace vs say a boiler)? Is is possible that there isn't a trap on it, and when the blower motor is running, it pressurizes the vent? That would account for only noticing it when cold - the heat is on; the furnace blower is moving air through the vents. You could try a smoldering match and see if air is moving out of the overflow of the tub when this happens.

    If you can turn the blower motor on manually, see if it comes and goes with the fan.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  11. #11

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    Getting a little technical for me here, Iím afraid. We have a furnace but I donít know if itís a condensing version or not and I donít know if it has a trap. To the best of my knowledge, the heater didnít come on at all so far. We primarily use a wood fire and it hasnít gotten cold enough indoors for the heater to kick in. So I donít think that is it but I could be wrong. I will try turning on the blower and see what happens.

    There are two vents on the roof; one above the problem toilet and another close to where the furnace would be. When I put a hosepipe down that second vent it came back up with fibers of insulation attached, which canít be a good sign. The problem is I donít know what that vent is for or if it is even really in use. There is an odd little built in furnace that obviously hasnít been used for many years and I though the vent might be for that. I need to do a bit more research this weekend.

  12. #12

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    It turns out the sewer gas is almost certainly coming from the bathtub only. So it is a venting problem but the gas doesn't come through any other tub, sink or toilet in the house. I'm thinking it must be a problem with the installation of this bathtub. If we leave water in the tub overnight with the drain closed, the water drains out by morning. Could be a fault in the water trap.

  13. #13
    Aspiring Old Fart, EE, computer & networking geek Mikey's Avatar
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    The tub draining out overnight only indicates a problem with the stopper seal. A leaky stopper will allow the water out of the tub, then into the waste through a properly functioning trap. If the trap should siphon dry, that will allow sewer gas to get out through the tub, either via the drain or the overflow. You could fix it temporarily by duct-taping over both the drain and the overflow, but that limits the usefulness of the tub. Get a big old rubber stopper for the drain, and leave the duct tape on the overflow (obvious caution here...) for a working fix.

    Still doesn't explain the cold -weather thing. Time to scope out the whole DWV system's precise configuration and go from there.

  14. #14

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    We had a problem with the overflow in our sink. The hole to the drain was plugged which caused bacteria or other 'nasties' to grow in there. But it was a sulfury smell - distinct from the smell of sewer gasses. Have you checked the tub overflow for clogging?

    I have no clue why the problem activates with cold weather, though, so I'm just grabbing at straws for you.

    Also, when you do get the sewer smell, have you tried pouring 2 cups of water down the tub drain? Does that stop the smell? If yes, then you know yr trap isbeing siphoned. If not, then it's something else.
    (important note: I'm not a pro)

  15. #15

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    Quote Originally Posted by Raybies
    It turns out the sewer gas is almost certainly coming from the bathtub only. So it is a venting problem but the gas doesn't come through any other tub, sink or toilet in the house. I'm thinking it must be a problem with the installation of this bathtub. If we leave water in the tub overnight with the drain closed, the water drains out by morning. Could be a fault in the water trap.
    I sense a couple misunderstandings here.
    1) The trap doesn't prevent water from draining. The stopper does. A faulty stopper won't cause the odor you're speaking of.
    2) The fact that the smell comes and goes hints that it's a problem in the draining/venting - NOT the tub installation.
    3) The fact that none of the other fixtures experience the problem does not contradict yr theory that it's a venting prob. Each fixture is individually trapped, and a faulty vent could affect each fixture differently depending on size and distance from any obstruction.
    (important note: I'm not a pro)

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