What is a Trap Primer?

Mary McMahon

A trap primer is a plumbing fixture that adds water to a trap, which is a section of plumbing that prevents backflow of sewer gases. Trap design is relatively simple, and trap primers keep the plumbing in good working condition so people do not need to check on it. In some cases the building code requires them, and in others, they can be a very good idea. Most plumbing supply stores carry them along with other plumbing accessories for people who need to repair or replace fixtures.

Man with a drill
Man with a drill

In the trap, a layer of water suppresses sewer gases, keeping them out of a building. The water will evaporate over time, making it necessary to periodically refill the trap. With something like a toilet, every flush automatically refills the bend in the toilet designed to prevent sewer gases from seeping out. Floor drains and similar fixtures lack this feature, making it necessary to periodically refill the trap. A trap primer automates this process.

The trap primer can work in several different ways. One design routes a small amount of cold water from the sink every time it is run, keeping the trap full, but not overflowing. Others respond to pressure changes in the system. A plumber may have advice on the best option for a given setting, depending on how often a particular drain is used.

It is important to periodically inspect drains and traps to make sure they are working well. Any clogs should be cleared, and the trap primer should be run to confirm that it is adding water periodically to keep the trap appropriately full. If a strong or unpleasant odor starts to come from around a drain, this can be a sign of leaking sewer gases or another problem, like a clog. To fix the problem, any covering should be removed and some water poured in to see how it behaves; if it drains very slowly, for instance, this is a sign of a clog, and the drain may need to be plunged or snaked to be cleared.

In older homes, plumbers may not have installed trap primers at the time they put in the plumbing system. People should be aware of this when buying or moving into an older home and may want to take the time to check the plumbing to determine if it needs any repairs. If the building code mandates trap primer use for certain settings, residents can discuss the situation with a plumber and get advice on when and how to overhaul the plumbing system to bring it up to code.

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