Traps, Interceptors, and Backflow Prevention
Trap fundamentals
A trap holds a water seal deep enough to block sewer gas but shallow enough to be renewed by each flow: the seal is typically 2 to 4 inches. Prohibited traps include the S-trap (double seal, self-siphonage), bell traps, traps with moving parts, and drum traps except in special permitted cases. Never double-trap a fixture or trap a fixture with an integral trap. Trap seals are maintained by venting, by trap primers on floor drains, and by avoiding long idle periods.
Interceptors
Grease interceptors serve kitchen and food-processing waste; sand and oil interceptors serve garages, car washes, and similar sources. Size them from the fixture load and flow, not by guesswork, and keep them accessible for cleaning. Interceptors do not replace a trap - the fixture still needs one.
Backflow protection
Match the device to the hazard:
- Air gap - highest protection; used for receiving tanks, ice machines, and other severe hazards.
- RPZ (reduced pressure zone) - high hazard, including boiler feed, chemical lines, and irrigation with fertigation.
- PVB (pressure vacuum breaker) - low hazard, no backpressure, commonly irrigation.
- DCVA (double check) - low hazard, non-toxic, such as a dedicated fire line without chemical additives.
- AVB (atmospheric vacuum breaker) - no continuous pressure, downstream of the last valve only.
- Hose bibb vacuum breaker - sillcocks and laundry connections.
Traps the exam sets: putting an AVB on a line that is under pressure for long periods, using a DCVA for a toxic chemical hazard, and forgetting the backflow device at a tankless water heater's combination valve or a boiler fill.
Worked example: A landscape irrigation system with a fertilizer injector is a high hazard; specify an RPZ, not a PVB.