Blocked drain or faulty AAV?

1 Mar 2011
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United Kingdom

In my upstairs bathroom we have a toilet, sink and shower. The shower trap occasionally dries out and I put this down to not being used and hot weather. Today I noticed that when the toilet is flushing the trap level in the shower bobs up and down and drops lower with each flush.

Is worse if the sink is draining at same time.

Sink and shower connect to soil pipe which is internal and has an AAV. Very rarely we can smell sewer gases in attic but it usually goes with a pull of the flush.

AAV or should I be looking for a leak? To me it seems like the flush is making negative airpressure as the toilet water travels down the pipe and this is then pulling on the shower trap. It does not gurgle it just quietly lowers. Sounds like AAV faulty?

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Yes the aav should let the air in. I presume the connections to the stack with the aav are independent. But showers often have very shallow traps so can be pulled easier. If you can remove the aav head to check operation then do that.

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