What does a vent do?
How a vent keeps a drain flowing, and what goes wrong without one.
- G13/AS12nd Edition Amendment 102 Nov 2023
A vent gives a drain the air its water needs.
Water in a drain only flows well while air can move with it, and a vent is the pipe that supplies that air. This article goes through it in order: why water needs air, what a vent is, what that keeps working, what goes wrong without one, and where vents go. Traps get their own article next.
Water needs air behind it to flow.
Water running down a pipe pushes the air ahead of it and leaves a gap behind it. Air has to come in to fill that gap, or the water slows, glugs and backs up. The air hole in a bottle's lid is doing the same job.
The vent gives the air a way in and out.
The air the falling water needs comes down the vent, and the air it pushes goes up the vent and out above the roof, where the smell does no harm. Gurgling, slow drains or a smell are the usual signs of a vent that is missing or blocked.
G13/AS1 5.7.1 to 5.7.4
A free-flowing drain keeps the trap full.
Each time a fixture drains, the water in the trap moves on and the last of the flow settles in the bend to replace it. That water is what stops the air in the drains coming back into the room, so anything that disturbs the flow puts it at risk. The next article is about traps and floor waste gullies.
G13/AS1 3.1, 3.2.1
Falling water can pull a trap dry.
A full basin let go, or a toilet flushed upstairs, sends a slug of water down the stack. Behind it the pressure drops, and the nearest thing that will give is the water in a trap. Once the seal is pulled out, the smell comes in until the fixture is used again. The answer is a trap vent, so the air comes down the vent and not through the trap.
It can push air back up too.
It is the same as squeezing a bottle. The air in front of falling water is pushed down and along the drain, and if it cannot get out ahead, it comes back up through the weakest seal: the gurgle in a toilet bowl or a shower waste. The answer is a relief vent at the foot of the stack, which gives that air another way out.
Each problem has a vent where it starts.
Suction starts where water leaves a trap, so a trap vent sits just after it. Pressure builds where falling water turns at the foot of the stack, so a relief vent joins there and rises beside the stack, and G13/AS1 asks for one at ≥ 3 floors. The stack vent at the top lets the air in and out. Where each goes is the document's; which problem each answers is our reading. G13/AS1 calls a trap vent a fixture vent.
G13/AS1 4.7.2, 5.5.2, 5.6.2, 5.6.3, Table 5 · AS/NZS 3500.2 8.5.1.1, 8.5.3.3
G13/AS1 vents nearly every pipe.
AS/NZS 3500.2, the other way to comply, offers four venting systems that use fewer vent pipes where the fixtures are grouped close to a stack.
Key takeaways
- Water only flows while air can move with it, and a vent is the pipe that supplies that air.
- Free flow keeps each trap full. Without a vent, water sucks a trap dry or pushes air back up it.
- Three places, three jobs: a stack vent at the top, a trap vent near the trap, a relief vent at the foot.