How a trap stops smells
A trap is a bend that holds a small amount of water. That water is a seal: it blocks smelly drain air from coming up into the room. If the water goes, the smell comes in. It is the SSS idea with water as the seal.
Common causes
- Rarely used sink or shower — the water evaporates.
- Dirty trap full of grease, hair or soap.
- Trap siphoning: when a large flow drains, it can suck the water out of the trap.
- Loose trap nut or missing washer letting air out.
- Washing machine hose not fitted correctly into the standpipe.
- Blocked or damaged vent pipe on the outside.
Fixes
- Run each tap for a minute to refill traps.
- Clean the waste and trap.
- Check all washers and nuts on the trap.
- If the trap gurgles and empties, fit an anti-siphon trap or an air admittance valve.
- If many drains smell, check outside gullies and manholes.
Pro tip: Holiday homes and spare bathrooms: pour a little water in every few weeks to keep the trap seal full.
