02:50AM | 06/22/04
Member Since: 06/21/04
2 lifetime posts

we have a 70's house which was extended about 15 years ago. since moving in a couple of years ago we have noticed an occasional smell of drains in the kitchen. We noticed that the drainage pipe from the sink to the downpipe[?] was virtually horizontal as it was lengthened after the extension was built. we took it apart and cleaned the inside (which was virtually full of limescale and gunge). we cleaned the ubend for the sink also. However, whenever we use the shower upstairs (which presumably drains into the same downpipe[?] we get the drain smell in the kitchen. This does not appear to be coming from the plug hole so we suspect there may be a problem where the drainage pipe connects to the downpipe[?]. as plumbing pre-novices we are not entirely sure where to start looking. I have heard of a 'no return valve' which a friend pointed out may be responsible but he's no expert. is there something we can do to test what might be wrong that isn't too destructive? the smell also appears when we use the dishwasher which also drains into the drainage pipe from the sink (again the smell does not appear to come out of the plug hole but from behind where the pipe disappears. Any ideas would be greatly appreciated as my wife is starting to get the right hump with me over this,

thanks for any advise


07:46AM | 06/22/04
Lets work on some terminology so we are talking about the same thing:

**downpipe** = waste pipe, horizontal drain pipe.

**plug hole** drain basket.

**no return valve** AAV Air admittance valve ?

OK, the odor does not come from the drain basket, but seems to emerge behind the wall. You mention a no return valve, which I suspect is an AAV. These are used in place of a direct vent to the roof sometimes. The Air Admittance Valve (AAV), like any vent, allows air to enter the plumbing drainage system through a one-way air valve when water flows through the pipes. This is necessary to equalize pressure in the pipes and to prevent siphoning of the water in the trap. It is the water in the trap that prevents foul sewer gas from reaching the sink drain.

When the flow stops, the valve closes by gravity and prevents the escape of sewer gas from the plumbing system. If the valve fails in the open position, sewer gas can enter the space under the sink where the valve is located. Odors are noticible when large flows are going through the waste pipe (dishwasher, upstairs shower). If the valve fails in the closed position, the sink trap may be siphoned out allowing sewer gas to back up to the sink basket drain. Failures can be a matter of cleaning the valve seal, or replacement. These things are sold at all plumbing supply stores and if you take the old one in, they can give you a same-size replacement.

Thats my theory. Hope it helps. If you see something different, post back. Here is a link to a similar question asked earlier this year...same answer.


07:59AM | 06/22/04
Member Since: 07/01/03
549 lifetime posts
Let me give you some terminology to use when you go to the plumbing store.

downpipe = waste drain or horizontal waste drain.

ubend = P-Trap (sometimes S-trap)

plug hole = drain basket or sink drain

no return valve = Air Admittance Valve (AAV)

I suspect your sink is vented using an AAV rather than a conventional pipe rising to the roof. You can recognize it if there is a T fitting after the P-trap with a valve device on top. These valves equalize pressure in the waste pipe system and prevent foul sewer odors from entering the drains or the room. The valves can fail. If it fails in an open position, sewer gas can enter the air by the valve. This would be most noticible when there is flow in the pipe (upstairs shower, dishwasher etc). If it fails in the closed position, the drain will act sluggish and the water seal may be siphoned from the P-trap. That would result in odor at the sink basket.

The valves can fail due to dirt and contamination, in which case cleaning should restore service, or mechanical failure with the seal. The valves are inexpensive and are easily installed. Unscrew the old one (or cut it off if solvent welded), take it to a plumbing supply store, and they should be able to provide you with a replacement, and any connecting parts you need.

Thats my theory of what is causing your problem. If you notice or suspect something different, post back. We answered a similar question earier this year and did not hear back from the original poster at:


12:22AM | 06/23/04
Member Since: 06/21/04
2 lifetime posts
thanks for the advice. we'll take a look at this and will certainly let you know how it goes. thanks again for taking the time to reply.


02:13PM | 09/26/04
Member Since: 09/25/04
1 lifetime posts
Hi there, I am a complete plumbing novice. I recently moved into a Victorian house in London (1870s) that has been renovated in the last 3 years. We recently noticed a drain smell in the kitchen extension near the sink/washing machine/dish washer. There's no smell from the sink, no water leaking from any machine, and the smell seems to appear after we've used the washing machine. Your response to the last post suggested a value failure under the sink letting bad air escape into the kitchen, but could it perhaps be a partial blockage in the outlet pipe from the washing machine causing stagnant water? I'm a little worried about calling in a plumber so all the help can give would be appreciated. Many thanks, Owain

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