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Plumbingaka: floor drain primeraka: automatic trap primer

Trap Primer

In Plain English

A device that automatically keeps water in a floor drain trap so sewer gas can't escape into the building.

Definition

A device that automatically supplies water to a floor drain or other infrequently used trap to maintain the water seal and prevent sewer gas infiltration. Trap primers are required by code for floor drains in mechanical rooms, commercial kitchens, and other locations where drains may not receive regular water flow. Types include automatic valve primers and distribution units.

Why It Matters in Bidding

Trap primers are small-dollar but code-required, and missing them in a plumbing takeoff causes failed inspections and callbacks that destroy margin on an otherwise tight bid. Estimators must scan plans for every floor drain in mechanical rooms, kitchens, and rarely used areas, since each typically needs a primer line, water supply, and sometimes a distribution unit feeding multiple drains.

Example

Counting floor drains during takeoff, an estimator adds trap primers and the supply tubing for each mechanical-room drain, then prices a single distribution unit to serve a cluster of restroom floor drains per the plumbing code.

Related Terms

P-TrapDrain-Waste-Vent (DWV)Valve

Frequently Asked Questions

Codes generally require trap primers on floor drains and other traps that do not receive regular flow, such as those in mechanical rooms, kitchens, and storage areas. The goal is keeping the water seal intact so sewer gas cannot enter. Always confirm the locally adopted plumbing code, since requirements and approved alternatives vary.
Some codes accept barrier-type trap seal protection devices or deep-seal traps in lieu of supply-fed primers, and waste-water-fed primers tap discharge from a nearby fixture. Each has cost and maintenance trade-offs, so estimators should price what the spec and approved submittal allow rather than substituting on their own.
Primers are tiny symbols easily overlooked among larger piping runs, and the supply line often routes from a remote source, hiding the labor involved. Skipping them means an inspection failure and rework. A disciplined floor-drain count cross-checked against the primer schedule prevents this common scope gap.

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