Soil stack venting issues

Question

For a little while now we've been hearing at random a loud sucking noise coming from out kitchen sink. This sink is a 1.5 bowl with a washing machine attached.

The noise happens at random and appears to be when the downstairs neighbors are in and is nothing to do with our appliances or the sink itself. After some looking I could see the sink trap and the surrounding pipes had no air admittance valve, so I have fitted a p-trap with a air valve on it hoping this would prevent the vacuum which appears to be coming from the soil stack from reaching the water in the p-trap. The fitted trap can be seen below

Technical drawing here http://mcalpineplumbing.com/sites/default/files/uploads/drawings/SA10V.pdf

This appears to haven failed as last night during the nightly washing up the sink started making that noise. I was draining no water at the time. I use a spoon handle to press the air valve up and a lot of stinky air came rushing out.

We are in a 20 year old top floor apartment which is using Durgo/Air Admittance Valve on the soil stacks as these cannot be seen from the roof. To me it sounds like the AAV is failing on that soil stack and the pressure from the neighbors is causing the noise. It appears to be strong enough to skip the valve on the p-trap completely and suck the air through the p-trap water.

I wonder if this sounded right to anyone.

Answer 1

I am mostly just guessing here, and please forgive me while I review what you already know.

The purpose of a water trap is to prevent stinky air from wafting into the room through the drain. But low pressure in the drain can suck (siphon) the water out of the trap and down the drain, and high pressure in the drain can blow the water out of the trap back into the sink etc. So, a soil stack vent may be installed.

The purpose of a soil stack vent is to relieve any low or high pressure in the drain, so that water will stay in the traps. But many builders and property managers feel that a soil stack vent is unsightly or inconvenient. So, an air admittance valve may be installed instead.

The purpose of an air admittance valve is to relieve any low pressure in the drain, so that water will stay in the traps. However an AAV cannot relieve any high pressure in the drain.

The fact that you were able to get a lot of stinky air to come out of the AAV demonstrates that part of your problem is momentary high pressure in the drain, which no AAV, no matter how well it is working, can relieve.

This in turn shows that it is not any individual AAVs or traps that are bad, but the entire building drain system is failing in a way that is too subtle and complex for a DIY plumber to diagnose and fix. You will have to hound your building manager until he calls a professional plumber, and it won't be cheap.

This is the kind of job where at the end I expect to see a bill like this:

Site visit door-to-door ................................  $ 122.00
Clean and adjust Veeble valve ..........................     16.00
Knowing that there is such a thing as a Veeble valve ...  1,800.00
                                                          --------
Total                                                     1,938.00

If you want to continue investigating while waiting for the plumber, try sealing the kitchen sink drain by placing a sheet of cling wrap over it. Then when you hear the noise from the drain, you can see which way air is being forced through the trap.

Answer 2

Mr Stephens, you better be careful. It's talk like that, that will send you down the long stinky road of being a full-time plumber!

Jokes aside, I believe, from what you say, that you are correct. You may want to call the building manager and have them check the main vent(s). Each apt should have it's own smaller vent as well, but if this is a 20-year old apt building, it may be a remodel, where there wasn't enough access to vent every apt. The main vent may be failing for whatever reason, or the neighbor's you speak of could be failing, or a combination thereof.

If you smelled stink out of yours, you may be okay.