Traps, Interceptors, Backflow Prevention and Cross-Connection Control
Traps and interceptors
A trap holds a water seal of 2 to 4 in., deeper for special conditions. Trap-arm length, cleanout access and vent connection all limit where a trap can go. Interceptors - grease, oil, sand, lint, plaster - go downstream of the fixture and upstream of the building drain, sized to the fixture load. Grease interceptors need pumping access and often an external flow-control fitting.
Cross-connection control
The only absolute protection is an air gap (typically twice the effective opening, minimum 1 in.). Mechanical protection: reduced-pressure principle (RPZ) for high hazard, double check valve for low hazard, pressure vacuum breaker for hose-connected lines not under continuous pressure, atmospheric vacuum breaker for non-continuous, non-pressurized lines, and hose-bibb vacuum breakers at sillcocks. RPZ and DCVA assemblies require annual testing by a certified tester.
Traps
- Choosing a DCVA for a high-hazard connection such as a boiler feed or chemical line.
- Installing a PVB below grade or under continuous pressure - both prohibited.
- Installing an AVB downstream of a valve that stays closed under pressure.
- Forgetting a trap primer on a rarely used floor drain; an evaporated seal is no seal.
- Bypassing an interceptor with a cleanout or pump-out connection.
Rule of thumb
Rank the hazard, then pick the device: air gap, then RPZ, then PVB, then AVB, then dual check. If a question mentions chemicals, boilers, hospitals or irrigation, suspect high hazard and specify an RPZ or air gap.
Example: a boiler makeup line requires an RPZ tested annually; a sillcock requires only a hose-bibb vacuum breaker.