Whether we realize it or not, plumbing traps are an extremely important component in not only the plumbing system, but the health of the structure as a whole. If traps are installed incorrectly or if they are an older style they can sometimes prove to be troublesome.
Plumbing traps are specially designed to prevent sewer odors from wafting up into house through the plumbing fixtures-that is, sinks, bathtubs and showers. The traps accomplish this by providing a water seal that is created by waste water. Every time the fixture is used, the old water in the trap is forced out and replaced with new water. The trap systems are carefully designed so that they are self-scouring-that is, they don't collect debris-and yet still retain water in order to form a seal.
As with most things, plumbing traps have evolved over the years. Below is a brief explanation of some of the older style traps as well as the new standard "P" trap.
Older Style Traps
If you have ever lived in an older house (the definition of "old" is debatable), chances are you have seen or had to deal with one of the trap styles listed below. Most, if not all, of the older style traps are prone to having issues at one point or another. Here is a brief description of some of the more common types.
"S" Trap - In my area-that is, the Sequim and Port Angeles Washington area-S traps are probably the most common older style of trap I come across. And in most cases, I find that S traps are installed in areas with no plumbing venting. The combination of the S trap and the lack of venting creates a situation where the water seal might be siphoned out of the trap. This is especially true if the trap is located near a larger fixture or if it is on the lower level of two or more floors. If siphoning does occur, then sewer odors will enter the house through the fixture(s) (at least until the fixture is next used).
Crown-Vented Trap - A crown-vented trap is created when the plumbing vent connects to the trap via a "Y" fitting. This condition is prone to clogging, which can lead to the water seal being siphoned out. If siphoning occurs, then sewer odors will enter the house through the fixture (at least until the fixture is next used).
Bell Trap - Depending on the age, condition, and location of the bell trap, it might be unreliable-that is, it could easily get plugged and cause a backup into the fixture. Additionally, if the top of the "bell" gets damaged, sewer odors might enter the house through the fixture.
Drum Trap - Drum traps are probably the second most common older style of trap I find. The potential issues with drum traps are that they are not self-scouring-that is, they tend to collect debris-and might easily get plugged and cause a backup into the fixture.
The New Standard
The newer standard plumbing trap is called a "P" Trap. If installed correctly, a P trap will maintain a water seal and prevent sewer odors from entering a house. An example of a P trap and its associated terminologies is below.
Conclusion / Recommendation
If you have any of the older style traps installed in your house, I suggest closely monitoring the trap(s) for sewer odors or clogging. If either is noted, I strongly recommend contacting a licensed plumber to further evaluate the trap and replace it as needed.
For information on how sewer gasses might affect you, go to: http://www.drweil.com/drw/u/id/QAA309122.
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