occasional air bubble in toilet

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One of the toilets in the house has an intermittent issue with air bubbles.
Occasionally, when just sitting there unused, a single bubble of air will 'blurp' as it reaches the water surface in the bowl. It might happen once or twice in a day, or as many as ten times (possibly more). The toilet has zero problems as far as function go.
I think we first noticed this last year but it went away after a while. It returned a couple months ago.
I've read that a clogged waste vent could cause this. I've looked in them (from the roof) an don't see any blockage. The other toilets in the house don't have this problem either.
ideas?
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ben wrote:

It's a toilet gremlin. The creatures don't usually bother humans, they may eat the occasional dog or cat that might drink from the toilet but any good exterminator can usually get rid of them.
TDD
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ben wrote:

All that could cause that is air being forced up the sewer pipe. Examine your plumbing. That would be gas bubbles from the sewer or septic system that are bringing that gas into the house. Not a good thing.
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Fart return.
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ben wrote:

I think it's charming.
A toilet with a personality! How quaint.
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Can you really see down the vent, maybe run water and a garden hose down it.
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On Sun, 30 Aug 2009 15:41:09 -0700 (PDT), ransley

My thoughts exactly. Tree frogs hang out in the deep shadows of the pipe :-/
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frogs hang out in the deep shadows of the pipe :-/
Frog farts?
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My toilet that is closest to the washing machine does something like that every once in a while when we wash clothes.
Could that be your situation?
Freckles
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Thinking out loud (and I know that's dangerous): there's no pressure in a drain line. They are designed to run at atmospheric pressure.
So there is no way a bubble gets forced backward through a trap unless something temporarily increases the pressure.
My guess would be nothing wrong with that toilet, but something wrong elsewhere in the system. Perhaps your wash machine line, dishwasher, etc. Something else is producing system pressure that doesn't escape through the roof vent and goes to your toilet instead.
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hmmmm... toilet was added later to original plumbing and is not close enough to a soil stack vent? toilet is on basement floor and was added on top of a floor drain with its own trap (double trapped)? clogged traps where leaves block the sewer inlets for roof rainwater?
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I have one which exhibits that sort of behavior, but it is more often than not. I'm not shocked though. It was installed as an after though, above a slab with very little pitch to the drain pipe. It does work surprisingly well though, it flushes solids better than others that seem to have everything going for them, though watching the event it seems unlikely. It farts then swirls then slowly starts to the bottom only to gurgle when it gets there, but somehow in all of that the payload gets delivered.
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ben wrote:

It sounds like an obvious vent problem. The vents should divert pressure differences to the outside.
I suppose it could be related to vent fan usage in a well sealed house.
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Or, the pressure outside of the house could be greater than the inside of the house. If you have a newer 'air tight' house, and you use a kitchen fan for example, to pump air out of the house, then you might create enough of a pressure difference to suck air in through the toilet... Just a thought (do you feel wind every time you open the outside door?)
John
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thanks all.
to answer some questions:
- it is not a well-sealed house. built in 1953. lots of drafts.
- nothing else is happening when the bubbles occur. no other devices connected to water or sewer operating.
- this toilet is in an original toilet location in the house and is on the first floor.
- admittedly i can't see all the way down the vent. perhaps i should try better with a good flashlight. there's certainly nothing within the top 5-10'.
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Since you claim that nothing else is happening in the house at the time, and as VC says, the only thing that can cause this is "air being forced up the sewer pipe", (similar to my explanation) my next question would be:
How are the pipes outside your house connected to the sewer? Where I live, my neighbor's waste pipe and mine are connected to a wye just before the main sewer. From there a single pipe goes into the main sewer. I don't know if it's possible, but maybe a blockage after the wye forces air back in *your* pipe when your neighbor flushes.
Have you discussed this issue with him? If you're friendly enough, you could call them right after the next bubble and ask them if anyone in thier house just took a dump.
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re: "admittedly i can't see all the way down the vent. "
I'm not doubting this as a cause, but I'd like an explanation as to how a clogged vent would cause this symptom.
How would a clogged vent force air back up through a toilet?
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A clogged vent means there is no means for the pressure in the pipes to equalize with the pressure of the house. Gas buildup in the pipes would create added pressure, and eventually it would escape -- say through the toilet.
But, if this were the case, then that bubble would likely be very smelly, and the OP didn't mention smell. Also, it would be more likely to occur when another toilet was being flushed etc, again, inconsistent with the OP's description... so the bubbling toilet is still a mystery.
John
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right, i've noticed no particular smell from these bubbles.
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I don't think you can use smell to confirm anything. If the plumbing is vented properly and in working order any atmospheric pressure in the drain pipe should be vented out the vent at the top. It should not be forcing air back out of the toilet. It's an old house so either improperly installed plumbing or a clog could be the explanation.
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