Every plumbing fixture is provided with a trap to prevent sewer gas from coming through the drain pipes. A trap is a very simple device that allows drainage but effectively keeps out gas. Most often, these traps consist of an S-shaped piece of pipe connected to the drain directly under the fixture. A small amount of water remains in the low portion of the S, and this water forms a barrier to the passage of sewer gas.
So long as there is sufficient water in the trap, no gas can enter ; but water in the trap of a fixture seldom used will evaporate, breaking the seal and allowing gas to escape through the fixture drain. Pour water occasionally down the drain of fixtures not in constant use, so that the trap will always be effective.
A trap can go dry for other reasons besides evaporation. An improperly installed plumbing system can cause water to be siphoned out of the trap. If a trap goes dry for no apparent reason, a plumber should be consulted immediately. Examine the washer around the clean-out plug of the trap, however. If it is worn, it can cause the trap to go dry, and replacing it is a task for the home mechanic.
There are many kinds of traps, some more effective than others. The less complicated ones are satisfactory until the sewer gas behind them builds up so much pressure that it can force its way out in spite of the seal of water. This condition can be due to the vent stacks of the sewer lines not working correctly.
Other than the traps located under each fixture, there is generally a main trap located in the sewer line in the basement.
Bathtubs, showers and other plumbing fixtures located close to or at floor level are usually provided with a drum type trap. This trap is located a short distance from the fixture and the trap body is below floor level. A removable lid is exposed, flush with the bathroom floor, and can be unscrewed with a monkey wrench.
When the lid is replaced, after the trap has been cleaned or a stoppage in the pipe removed, it is a good idea to put some grease on the threads of the lid to make a tight seal and prevent gas from working up around the fitting.