The trap and the vent

Every drain in the building connects, eventually, to the sewer — and the sewer is full of gas you do not want in a kitchen. What keeps it out is not a valve, not a flap, not a filter. It's a puddle.

The seal is just water

The U-bend under every fixture — the P-trap — holds 2 to 4 inches of standing water after each use. Gas rising from the sewer side reaches that water and stops: it cannot bubble through a few inches of head. That standing water is called the trap seal, and it is the only barrier between the room and the drainage system. Every fixture gets one; that's why there's a bend under every sink you've ever opened.

The enemy: siphoning

The seal has a weakness. When a full slug of water rushes down the drain, it acts like a piston — it pulls the air behind it, and that suction can pull the trap's water right along with it. The fixture drains, the trap empties, and the room is open to the sewer. You've heard this failure: it's the gurgle at the end of a drain, which is the trap fighting for its life — air being dragged through the seal.

The vent is the answer

The fix is to give the falling water all the air it wants from somewhere else: a vent — a pipe teed in downstream of the trap and run up through the roof. With the vent open, the drain pulls air from the roof instead of through the trap, the pressure behind the slug stays atmospheric, and the seal stays put. That's the whole reason plumbing is called DWV — drain, waste, and vent. The vents move no water at all; they exist to protect a few inches of it.

Two field notes

• A fixture that gurgles and then smells is telling you its vent is blocked or was never run — the seal is being siphoned on every use.
• A floor drain that smells in an unused room lost its seal to plain evaporation. Nothing is broken: pour a bucket of water in it (that's also why seldom-used drains get trap primers).

Why this matters

Traps and vents are most of what a drainage inspection looks at, and most of what separates a legal rough-in from a stinking one. If you understand that the seal is water and the vent protects it, every venting rule in the book stops being arbitrary.