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A trap primer (or trap seal 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, leading to the release of sewer gas into the environment. The trap primer mitigates this problem by injecting water, either directly or indirectly, into the trap to maintain the water seal indefinitely.
Building codes may require trap primers for traps in certain locations or situations, usually those in which industry experience has shown that they are likely to dry out. The most common requirement is for basement floor drains, which are only used when the basement floods. Sometimes they are also installed in locations where the plumbing code does not require a primer because of a high probability of the trap drying out, but because of a nearby ignition source and the explosion hazard in the event that the trap did leak sewer gas. Most designs require installation in an accessible location where periodic maintenance can be performed.
There are many types of trap primers. The simplest, typically for a floor drain's trap, is simply a connection from a nearby sink's drain so that when the sink is used, some of the water flow is diverted into 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. Several manufacturers (e.g., Moen) produce laundry faucets with a built-in trap seal primer outlet.
In infrequently used floor drains where such a primer does not exist, evaporation can be mitigated a few different ways:
- Routinely adding water to the floor drain
- Pouring a small amount of vegetable or mineral 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. Mineral oil tends to be a better option, as it does not go rancid, where vegetable oil does. Any environmental impact from its discharge is negligible compared to the contents that can be flushed out of a flooded basement.
- Using a propylene glycol solution instead of water in a trap. Raising the boiling point of a solution is equivalent to reducing its vapor pressure, and consequently its evaporation rate. This also protects the trap against freeze damage. The glycol attracts water, and so maintains a minimum concentration which depends on the humidity or water vapor pressure in the air. Unlike ethylene glycol, common automotive antifreze, propylene glycol is non-toxic.
- The evaporation rate of a liquid is directly proportional to its vapor pressure. At 25 °C the vapor pressures for water, pure propylene glycol and pure mineral oil are, consecutively, 24 torr, 0.1 torr and <0.01 torr. Be aware though that the properties of "RV antifreeze" which has propylene glycol in it will be different than pure propylene glycol because alcohol, water and other ingredients are in it. One manufacturer of RV Antifreeze gives the vapor pressure of their product as 17 torr at 20 °C. This material, if placed in a drain, will evaporate almost as fast as water. Some makers of RV Antifreeze will list the vapor pressure of their product as 0.1 or <0.1 torr. This however is only for the propylene glycol component, not the mixture that comprises their product which can be mostly water.
- Check valves designed as "inline trap sealers" are permitted by many local codes. The SureSeal  is a gravity-mechanical type; the Trap Guard  is a duckbill type.
Poor plumbing vent design or extra windy conditions affecting vent pipes can also cause water seals to be siphoned or blown out of traps.
- "Trap Seal Primer Selection Guide" (PDF). www.mifab.com. MIFAB. 2014. Retrieved 1 March 2016.
- Vapour pressure of water
- Duramax 50 RV Antifreeze Material Safety Data Sheet