CSLB C-36 Plumbing ContractorPlanning and EstimatingMedium
A plumbing contractor is performing a final inspection of a newly installed commercial drainage system. The inspector notes that several traps are installed without adequate protection against siphonage. Which of the following is a code-compliant method to protect a fixture trap from siphonage?
- AConnecting a trap primer to the trap.
- BProviding a vent pipe connected near the trap weir.
- CIncreasing the slope of the horizontal drain pipe.
- DInstalling a larger diameter trap.
Show answer & explanationAnswer & explanation
Correct answer: B. Providing a vent pipe connected near the trap weir.
Siphonage occurs when negative pressure in the drainage system pulls water out of a trap. A vent pipe, properly connected near the trap weir, introduces atmospheric pressure into the drainage pipe, preventing the negative pressure from siphoning the trap seal.
Why the other options are wrong
- A. A trap primer replenishes water in a trap seal that has evaporated, it does not prevent siphonage.
- C. Increasing drain slope can affect flow velocity but does not directly protect against siphonage caused by negative pressure.
- D. While a larger trap holds more water, it does not prevent siphonage caused by negative pressure in the drainage system.
Trap Siphonage Prevention
Methods used in plumbing systems to prevent the loss of a trap's water seal due to negative pressure (siphonage), which would allow sewer gases to enter the building.
- Trap seals prevent sewer gases from entering occupied spaces.
- Siphonage is caused by negative pressure (vacuum) in the drain.
- Proper venting is the primary method to prevent siphonage.
Memory trick: Vent pipe is the trap's air-breather.