Trap primer

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A trap primer is a plumbing device or valve that adds water to traps. The water seals in traps are needed to prevent sewer gases from entering buildings, but because this water is exposed to the air, it is subject to evaporation over time. In infrequently used floor drains, this evaporation can be delayed by pouring a small amount of vegetable oil into the drain to just cover the trap's water surface. The oil floats on the surface of the water and creates a liquid seal preventing further evaporation. Users of this method must flush the drain occasionally as the vegetable oil can go rancid over time. Any environmental impact from its discharge is negligible compared to the contents that can be flushed out of a flooded basement. Also, poor venting or windy conditions can cause water seals to be siphoned or blown out. The trap primer injects water, either directly or indirectly, to the trap to maintain the water seal indefinitely.

There are many types of trap primers. The simplest, typically for a floor drain's trap, is just a connection from a nearby sink's drain so that when the sink is used, some of the water flow is diverted to one or more traps. More common is a primer that is connected to the potable water supply and activates when pressure fluctuations are sensed, such as the flushing of a nearby water closet. Others depend on occupancy sensors or timers, for example.

Building codes may require trap primers for some traps, especially for where experience has shown that there are isolated and/or infrequently used drains such as basement floor drains (which hopefully are only used when the basement is flooded). Where explosion hazards are high, specifying trap primers, whether required or not by code, is wise. Trap primers do require maintenance and replacement, so they should be installed in accessible locations.