A trap primer is a small automatic valve that feeds a little water to a floor drain trap now and then. That keeps the trap seal full so sewer gas cannot rise into the room. Primers are common on seldom-used floor drains in commercial restrooms, mechanical rooms, and restaurants.
Every drain has a trap. The bend in the pipe holds a little water. That water is the trap seal, and it blocks sewer gas from rising into the room. A trap only works while it stays full. A floor drain that never gets used has a problem. The water in its trap slowly evaporates. Once the trap runs dry, sewer gas comes straight up through the drain. A trap primer is the fix.
What a trap primer does
A trap primer is a small valve tied into the water or drain system. Every so often it sends a few ounces of water down to the floor drain trap. That top-off replaces what evaporates. The seal stays full, and the gas stays out. The device is automatic. Once it is set up, it works on its own with no attention from you.
Where you find one
Trap primers show up on drains that sit unused for long stretches. Think of a floor drain in a mechanical room, a rarely used commercial restroom, or a back corner of a restaurant kitchen. Arizona's dry air makes a trap evaporate even faster than it would in a humid climate. That makes primers especially useful on Valley commercial jobs. If a floor drain keeps giving off a sewer smell, a dried-out trap with no primer is a common cause. See why a commercial building smells like sewer gas and what a P-trap is.
The main types
Manufacturers build primers to trigger in a few different ways:
- Pressure-drop activated: the valve opens when water pressure in the line dips as other fixtures run, releasing a little water to the trap.
- Flow activated: the primer taps a nearby fixture and doses the trap whenever that fixture is used.
- Electronic: a timer opens the valve on a set schedule, day or night.
- Manual: someone opens the valve by hand to refill the trap.
What code requires
Plumbing code treats trap seal protection as a real requirement, not an option. Under UPC Section 1007, floor drains that can lose their seal must be protected, and a potable-water-supplied primer valve must meet the ASSE 1018 standard. For the full rule, see our UPC 1007 trap seal protection page.