All homes have sinks, many kitchen and wetbar sinks are equipped with a small valve that is attached to a pipe on the drain line under the counter. The purpose of this valve is to allow air to enter, but not to escape the plumbing waste line. Without this equalization of the air pressure in the line the water will not drain properly out of the sink. Think of a soda bottle that is full of water. You turn the bottle upside down and the water has a hard time getting out; but if you tilt the bottle to allow air to enter then the water flows freely. The same principle applies to the waste plumbing system in your home. Air must enter for everything to flow properly.
Now for the problem!
Well, it's not really a major problem but it could cost you a plumbers service call and about $40 worth of repairs. The problem arises when builders or plumbers install what is called a Mechanical Vent in place of an Air Admittance Valve or AAV. Why do you ask? The cost! A Mechanical Vent cost less than $10 and the AAV will cost around $40-$50. Mechanical vents are not allowed in residential homes, only the AAV should be used.
Note: The Mechanical Vent is only allowed in manufactured homes, aka, Doublewides!
Both will do the same job of allowing air to enter the plumbing system, but the difference is that when the Mechanical Vent fails it fails in the open position. This will then allow sewer gas to enter the home. When the AAV fails it does so in the closed position so all that you have is a sink that will not drain very well. They will last on the average about 7-10 years under normal use before they need replacing.
If you have a home with a sewage odor that you can't find the source, this could be the culprit.
The Mechanical Vent is easy to identify. It will be made from black plastic and has two or three rectangle openings on its side and you can see the spring. The AAV will be made from white to tan plastic with no spring large openings. The picture at the top of this page is a mechanical vent.
You just simply replace the Mechanical Vent with an AAV! They screw into the same fitting. It is a very simple repair but as with all plumbing repairs it is always best to call a plumber, just in case!
So look under those sinks and see what you have. If you see a black top on a pipe chances are that you have a Mechanical Vent.