Plumbing drain traps are an important configuration in a plumbing waste and drain system that are often overlooked. While most homeowners only perform maintenance on these when one becomes clogged under a sink, drain traps help to prevent sewer gases and pests from being able to come back up from the sewer system and into the home. While there are a variety of different styles of traps that have been used in the past, some types are no longer used due to the problems they present.
Drain traps are curved configurations of the waste pipes under or downstream of plumbing fixtures such as sinks, showers, bathtubs, and washing machines. These traps work by trapping a small amount of water in the curved portion of the pipe and keeping it there after water has been run through the drain. This small amount of water that remains (and is refilled every time water is run through the drain) prevents sewer gasses from coming back up through the drain system and into the home. Sewer gas is typically composed of hydrogen sulfide, ammonia, and methane from decaying organic matter. These gasses can be smelly, harmful to your health, and potentially explosive. While it is very rare, methane is a flammable gas and can ignite under the right conditions.
Homes that have been unoccupied for a long period of time can occasionally have this unpleasant smell as the water in the drain traps has dried up and not been refilled by regular plumbing use. While rare, the lack of water in the drain traps could also potentially allow pests to travel from the sewer up, through the plumbing system, and into the home.
While most traps function without a problem, some outdated types can cause issues and allow water to drain out of the trap. S traps are an outdated type of trap that has the appearance of a sideways S. Unlike the common modern P trap (which looks like a sideways P), the S trap is unvented can allow water to be siphoned out of the trap and drawn down into the drain, leaving the drain dry and an open pathway for sewer gas.
If you have an S trap in your home, consider having this trap converted to a modern P trap, installing an air admittance valve, or a installing a loop vent. These vent systems help to equalize pressure in the drain system and prevent too much positive or negative pressure.