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Community and Q&A

Vent stack steaming 24/7

user-903081 | Posted in General Questions on

I’m in the cold northeast (about 9 degrees right now) and I noticed that my vent stack is putting out steam even when nothing is running/nobody using sinks or bathrooms.

As I look around I don’t see steam coming from anybody else’s vent stack so I’m wondering if this represents a problem or just an unusual oddity, perhaps because the water in the toilets is being warmed by the temperature in the house (about 65.)


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  1. GBA Editor
    Martin Holladay | | #1

    It's probably just water vapor condensing -- a phenomenon that is most noticeable on cold days.

    It's possible that your plumbing vents have more water vapor because you observed the vent right after someone took a shower or emptied the bathtub. Warm water in your waste pipes will evaporate up the vent pipe.

  2. kevin_in_denver | | #2

    Check if the water heater T&P valve is leaking. Sometimes it can be piped to a drain that you might not see.

  3. GBA Editor
    Martin Holladay | | #3

    Excellent guess.

  4. user-903081 | | #4

    That was my original thought but nobody had used any water for more than 90 minutes when I noticed the steam, and no other stack pipes in the neighborhood seem to be steaming. (Maybe my home is warmer than the others.)

    Kevin- T & P is open to the floor, so that's not the issue, but thanks for the suggestion.

  5. GBA Editor
    Martin Holladay | | #5

    Clothes washer running? Dishwasher?

  6. Riversong | | #6


    9° is hardly cold today. But a plumbing vent serves two purposes: it allows air to enter to break the siphon and prevent water traps from emptying when the toilet is flushed or a bathtub or sink drained, and it allows sewer gases to escape 24/7. Sewer gases will always contain moisture and, on a cold day, will be warmer than outside air - so condensation may be visible.

    It would not require any fixture draining at the time, since draining water will draw air down into the vent. When fixtures are not draining, humid air rises up.

    Why your vent shows this while others don't may be a function of your location on the town sewer pipe or the activity of your septic tank, or perhaps the pipe material (cast iron and copper will condense much of the moisture while better insulating plastic would let it pass upward).

  7. user-903081 | | #7

    Nope, nothing running for at least 90 minutes prior. It may have been residual heat from morning activities though.

    Robert, that (plastic vs. iron) may be the reason, though I've got a combination in my home. Based on people's suggestions here it may also be due to a better insulated/tighter home keeping the residual heat in the pipes longer.

    Thank you all for your thoughts. I always appreciate reading everybody's expert opinions on these pages!

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