Trap primer
A trap primer is a plumbing
Plumbing is the system of pipes and drains installed in a building for the distribution of potable drinking water and the removal of waterborne wastes, and the skilled trade of working with pipes, tubing and plumbing fixtures in such systems. A plumber is someone who installs or repairs piping...

 device or valve
A valve is a device that regulates, directs or controls the flow of a fluid by opening, closing, or partially obstructing various passageways. Valves are technically pipe fittings, but are usually discussed as a separate category...

 that adds water to traps
Trap (plumbing)
In plumbing, a trap is a U-, S-, or J-shaped pipe located below or within a plumbing fixture. An S-shaped trap is also known as the S-bend invented by Alexander Cummings in 1775 but became known as the U-bend following the introduction of the U-shaped trap by Thomas Crapper in 1880. The new U-bend...

. The water seal
Water seal
Water seal may refer to:*Liquid trap seal in plumbing traps*Underwater seal in pleural drainage apparatuses*Waterproofing...

s in traps are needed to prevent sewer gas
Sewer gas
Sewer gas is a complex mixture of toxic and non-toxic gases produced and collected in sewage systems by the decomposition of organic household or industrial wastes, typical components of Sewage....

es from entering buildings, but because this water is exposed to the air, it is subject to evaporation
Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....

 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 code
Building code
A building code, or building control, is a set of rules that specify the minimum acceptable level of safety for constructed objects such as buildings and nonbuilding structures. The main purpose of building codes are to protect public health, safety and general welfare as they relate to the...

s 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 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.


There are many types of trap primers. The simplest, typically for a floor drain's trap, is simply a connection from a nearby sink
A sink is a bowl-shaped plumbing fixture used for washing hands, for dishwashing or other purposes. Sinks generally have taps that supply hot and cold water and may include a spray feature to be used for faster rinsing...

'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
Moen (company)
Moen is a product line of faucets and other fixtures started by inventor Alfred M. Moen that is now part of the Fortune Brands Home & Security company. The Moen division is headquartered in North Olmsted, Ohio. Moen was originally part of Ravenna Metal Products of Seattle, Washington...

) produce laundry faucets with a built-in trap seal primer outlet.

Other solutions

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
    Rancidification is the chemical decomposition of fats, oils and other lipids . When these processes occur in food, undesirable odors and flavors can result. In some cases, however, the flavors can be desirable . In processed meats, these flavors are collectively known as "warmed over flavor"...

    , 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.

Poor venting or windy conditions can cause water seals to be siphoned or blown out; for these cases, the drain must be refilled.
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