I hope you can help me resolve this problem. For about six months a very offensive odor has come in from my bathtub's drain. It seems to occur mostly in the evenings, but not every evening. I have had a sewer man here and have called the city, but neither can figure it out.

I used to take baths in the tub, but now I use my shower stall due to age and arthritis. Could the odor be due to the fact that the drain isn't used daily? I have tried filling the tub and letting the water drain, and have even used Clorox® and Liquid Plumr®, which temporarily solve the problem. Any suggestions would be appreciated.

A few possibilities come to mind that could be contributing to the drain's offensive odor. My first thought is a dry P-trap that allows sewer gases to come up through the drain and enter the room. The clue is when you told me over the phone that the tub is used once a year. Depending on the temperature and humidity, a trap could dry out in about a month.

The plumbing trap is designed to prevent the release of sewer gases into a room by "trapped" water in the U-shaped portion of the pipe. The trapped water forms a water seal.

If you had told me that one of your other existing drains gurgles while the tub drained, then I would have told you the vent pipe may be blocked.

If you had told me that the odor comes from the drain more often in the winter than in summer and especially if the roof vents were galvanized, then I would guess that the vent was plugged with ice. However, tubs generally do not have their own vent; they are mostly tied into the 3-inch or 4-inch main stack. Main stacks rarely get plugged.

The reason you may smell the gases more at night or early in the morning is simple. More people use their bathrooms between 6:30 and 8:00 a.m. and between 5:30 and 9:00 p.m. During these peak times, sewage flows heavily down the sewer pipes, pushing the gases out in front. These gases seek the path of least resistance, such as a dry drainpipe with a branch that has a dry P-trap. Up come the offensive odors to fill the room.

Bathroom cleaners or liquid drain openers temporarily solve the problem because you are filling the dry P-trap with these products, but they, too, will eventually evaporate. However, it's not wise to let these products sit full strength in the plumbing trap as they could damage the seals used in the trap system. I encourage you to flush the drain with water right away to remove these products from the P-trap.

If you have no plans to use this tub for a while, e.g., over a one-month period, then make sure that you run water down the drain at least once a month to make sure the water in the plumbing trap hasn't evaporated. In addition, you can slow down the evaporation by putting 2 tablespoons of mineral oil or pure vegetable oil down the drain. This could help keep the trap wet up to (possibly) three months.

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