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Recently (in the last few days) the drain of my bath has started to 'bubble' occasionally. This mainly seems to happen at night when we are in bed and no one or nothing is moving in the house. The bubbling is a single bubble every 5 - 10 secs for about a minute. Then it stops. Its not happened that we've noticed during the day, though did once happen when I flushed the toilet.

I have 2 bathrooms that share a wall and both baths and both sinks in the bathrooms all have drain pipes that lead to a common 'junction box' drain thing in the corner of one bathroom. I live in a basement flat (if that matters).

I suspected that it might be due to a blocked pipe, and poured drain cleaner down the plug holes of both baths. This, strangely, resulted in a definite blockage in the other bathroom. A go with the snake and some more powerful drain cleaner later and the blockages seem to have been cleared and this bath now drains much more quickly than it did before. However the 'bubbling' was still present.

What else might be the cause of the bubbling?

Is it something I should worry about? (I'm about to rent out my flat for 6 months and don't want my tenants to have a problem whilst I'm away.)

What else can I try? I thought it might be to do with vents, but I can't see anything that looks like a vent...

EDIT
Turns out it was the main drain from the house, which had become blocked and was backing up. Managed to catch it just in time, the 5 foot deep drain hole was only a couple of inches from the top and both the drain at the front and the back of the house were close to overflowing. The water had risen so high that it had covered the entrance hole for the vent pipe into the system. So the bubbling was indeed caused by the lack of vent, it was just 'blocked' by the high water level due to the other blockages.

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up vote 4 down vote accepted

My first guess would be that the vent is blocked preventing air from entering the drain pipes and causing a sort of siphoning action. Usually the vent is a PVC pipe that comes out of the roof. Any chance that might be blocked (by snow possibly?)

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We live in a basement flat and as far as I can tell there are no vents on the pipes. The 4 pipes all meet at a sort of drain 'junction box' and there doesn't seem to be any other pipes in there. would there need to be for there to be a vent? Or could the vent let air in somewhere else? The drains all run towards the back door I think so might it be there? –  Sam Holder Feb 9 '11 at 20:46
    
Could also be wind blowing across the vent stack, causing the water in the trap to slosh about. –  Tester101 Feb 9 '11 at 21:33
    
At least in my part of the world, code requires a vent in all waste plumbing. The P-trap is supposed to stop sewer gases from coming in your home, so the vent allows pressure to equalize. –  Jay Bazuzi Feb 10 '11 at 7:31
    
@Jay Bazuzi, I'm not saying it doesn't have a vent, I just don't know how to identify it and to determine if it is what is causing my problem. I'm in the basement flat of a 4 storey house, so would the vent for everyone come out on the roof? –  Sam Holder Feb 10 '11 at 9:00
    
Edit the question to include the solution. Marking this as the answer as it was the vent, just blocked by the high water level. –  Sam Holder Feb 10 '11 at 21:10
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