SANITARY DRAINAGE

Drainage venting rules for Victorian plumbers

Venting is the part of a sanitary drainage system that keeps the trap seals in place. Without it, the pressure swings inside the pipes pull or push the water out of the traps, and the smell comes back into the building.

Why a trap needs venting

A trap holds a small body of water that blocks drain gases from coming back into the building. That seal only holds while the air on both sides of it stays at roughly the same pressure. The job of the vent system is to keep the inside of the drain at atmospheric pressure, so the seal sits still instead of getting pushed or pulled out.

Induced and self siphonage in plain terms

When a fixture discharges, the water moving down the pipe drags air with it and drops the pressure behind it. If that fixture pulls down its own seal as it empties, that is self siphonage. If the moving water lowers the pressure somewhere else and sucks the seal out of another fixture's trap, that is induced siphonage. Either way the seal is gone and the trap is open. A correctly placed vent feeds air in to break the suction before it can empty the trap.

Common fail points an auditor looks for

• A trap with no vent, or a vent that was capped or never connected.
• A vent that is too far from the trap it is meant to protect, so the suction wins before the air arrives.
• A vent connected below the level it should be, where it can flood and stop carrying air.
• Fixtures sharing a drain in a way that lets one pull the seal out of another with no vent in between.

What to photograph

Capture the vent connections and how they relate to the traps they protect, the run up to where the vent terminates, and any point where a fixture connects to a shared drain. A clear set of photos showing the vent is actually there and connected is what backs up the work years later, and Elemetric keeps that with the job.

Common questions

What does a vent actually do?

A vent lets air into the drain so the pressure inside the pipes stays close to atmospheric. When a fixture discharges and water rushes down a pipe, it drags air with it. The vent feeds that air in, so the moving water does not suck the seal out of a nearby trap.

What is the difference between self siphonage and induced siphonage?

Self siphonage is when a fixture empties and the slug of water leaving it pulls its own trap seal down behind it. Induced siphonage is when water moving in another part of the system drops the pressure and pulls the seal out of a trap that was just sitting there. Venting protects against both.

General information for licensed tradespeople, not legal or regulatory advice. The licensed plumber remains solely responsible for compliance. Refer to the current AS/NZS 3500 standards and the Building and Plumbing Commission (formerly the VBA) for authoritative requirements.