Trap Primer

A trap primer is a small-diameter water supply line, usually 3/8" or 1/2", that feeds a steady trickle of water into a trap to maintain its seal. The most common type is the automatic trap primer, which uses a float valve or diaphragm mechanism triggered by water flow elsewhere in the building—typically when a toilet flushes or a sink drains. When that flow stops, the valve closes and the primer resets.

They're essential in floor drains, especially in commercial kitchens, loading docks, and equipment rooms where floor drains sit idle for long stretches. Without a primer, the trap seal evaporates in weeks or months depending on humidity and temperature, leaving the drain open to sewer gas backdraft and potential code violations. Many jurisdictions require them by code for floor drains in certain occupancies.

Install the primer supply line upstream of any strainer or backflow device on a cold water line, and run the outlet line directly into the trap weir (the overflow chamber), not the main drain outlet. Size the supply line appropriately—undersized line equals inadequate primer flow, oversized means too much water waste. Test primers annually in commercial settings to confirm they're still functioning; a failed primer is silent and leaves you exposed.