What's Wrong With This Picture? Drain Configuration

Answer: Missing P-Trap

Some might think a 'faster' drain is always a better drain. Maybe so, but generally not at the expense of foul smell.

The drain line for this particular sink is missing a couple of key components. The most noticeable is that of a non-existant P-trap. P-traps look just like a "P" lying on it's side and are integral to the design of the drain configuration of most sinks, showers/baths, and floor drains. Their purpose is to retain a small amount of water (within the "P") which blocks the migration of sewer gas back into the home. Without a P-trap, the drain line would be a hollow tube from the sewer directly into the atmosphere of your house.

Secondly, the Air Admittance Valve (AAV) directly behind the sink drain is also a required component, but is installed improperly. At it's current height, it will fail to provide the protection and function for which it was designed. AAV's allow air into the drain line facilitating downward flow, as well as maintaining the presence of pressure in the drain line to prevent backflow. However, it's placement is critical and in this example is not properly positioned. It should actually rise to a point above the drain level of the sink.

Thanks Bob Egan, from our Northern Virginia Team for enlightening all of us!