My husband replaced the bathroom sink faucet and some pipes and now we experience an odor just after we turn on the faucet, which disappears after the water has run for a short time. Thanks for any suggestions you can give us.

Thanks for taking time to discuss your stinky situation and for faxing me detailed drawings of the piping underneath your kitchen and bathroom sinks. When you told me you had a log cabin home, I immediately suspected that your sink drains passed through the floor and you had non-compliance S-traps installed. Your drawings confirmed my suspicions.

I would also have guessed that you hear a gurgling sound from this sink or others in the home and you smell sewer gases that come up through the drain and into the room.

In a typical vented P-trap design (horizontal), a vent extends up through the roof allowing atmospheric pressure on the drain and breaking any vacuum formed by the draining water. This maintains the water seal in the trap, which prevents sewer gases from entering the room.

An S-trap design (vertical) has no vent. Once draining water fills the drainpipe, it can cause a vacuum in the drain line that begins to siphon the water. The end result is that the water seal in the trap can be lost and sewer gases can enter the room freely.

S-traps are no longer acceptable in new construction or in any remodeling projects where plumbing pipes are altered to accommodate the new project.

The solution is to install a vent up through the roof and to change the S-trap to a P-trap design. Unfortunately, this is not as easy as it sounds. The problem normally lies in trying to install a new vent into an already finished wall.

There is an alterative solution but I have to mention up front that the use of the device described here is not approved by the plumbing codes and must be approved by the plumbing inspector prior to installation as an alternate method of construction.

The product is an air admittance valve (AAV), which came to the market in 1974 by Durgo and is also sold under trade names such as Magic Vent (RectorSeal), Ayrlett, FloPlast, Sure-Vent (Oatey), and Studor. This valve allows air to enter the drain system through a one-way air valve once water flows down through the drainpipe. When the flow of water stops, the valve closes by gravity, which prevents the escape of sewer gases.

In order to use the AAV as designed, the drain coming up through the cabinet floor needs to be extended vertically to the maximum height allowed with the valve placed at the top of the pipe. In the vertical pipe, install a sanitary tee at a height that is 18 to 20 inches off the finished floor so a P-trap can be installed and have a minimum of 3 inches between the P-trap and the vertical drain line. However, the AAV is required to be installed a minimum of 4 inches above the trap arm.

The nice thing about working with plastic piping is that you can do a dry trial run to make sure that everything will hook up correctly before applying the cement.

If the drain is too close to the tailpiece, use a 45-degree elbow to move the vertical drain line to the side or back of the sink. Then install a 45-degree elbow to straighten up the drain line. When selecting an AAV, work with your plumbing inspector to verify that the device is acceptable for installation and is listed by the plumbing code in your jurisdiction.

To see how the valve would be used, go to www.ayrlett.com/installation.html; to view the difference between an S-trap and a P-trap, go to www.usinspect.com/car/13ptrap.asp. The visual will help.

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C.R.S., Inc. · Spokane, Washington · USA