Things to check so a sanitary appliance drains properly and keeps its water seal against smells.
A sanitary appliance only stays smell-free and free-flowing if its trap keeps its water seal and its branch pipe is set up correctly. Use this as a revision checklist of what to confirm, then verify the figures against Approved Document H.
- Check the trap matches the appliance and has the correct seal depth, because too shallow a seal is easily lost.
- Keep the branch pipe length within the limit for its diameter; over-long branches are prone to self-siphonage that empties the trap.
- Set the branch gradient within the allowed range so it drains fully without sucking the seal out behind the flow.
- Position the connection into the stack so one appliance discharging does not siphon or back-pressure another appliance’s trap.
- Provide ventilation (a vent pipe or an air admittance valve where permitted) on long or heavily loaded branches to protect the seals.
- Make sure the trap is accessible for cleaning and that there are no sags in flexible waste pipe that could hold water and debris.
The aim is always the same: water flows away and the seal stays put. Confirm seal depths, branch lengths and gradients for each appliance and pipe size in Part H.
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