Why does my sink gurgle when the washing machine drains?

Waste pipes need air behind the water, like pouring from a bottle. When a washing machine pumps out fast into a shared branch, it fills the pipe, and the only air available is drawn through the nearest trap — which is what you hear.

What you'll need

• A torch
• Access under the sink
• An air admittance valve if required

Step by step

1. Note whether it is new or long-standing.A gurgle that has always happened is a design issue with the pipework. One that has started recently is nearly always a partial blockage.
2. Check the trap water level afterwards.Look in the sink after the machine drains. If the water in the trap has been pulled down or emptied, the trap is being siphoned and drain smells will follow.
3. Clear the obvious restriction first.Take the trap apart and clean it, then clean the appliance waste spigot and the branch beyond as far as you can reach. A partial blockage turns a gentle flow into a plug of water.
4. Check the appliance hose is not pushed too far in.A washing machine hose sealed tightly into a standpipe or pushed deep into the branch makes the whole pipe run full bore, which is what causes the siphoning.
5. Check the high loop on the appliance hose.The hose should rise above the machine's drum before dropping to the waste. Without it, siphoning is far more likely and the machine can also self-drain.
6. Look at how the branch is arranged.A long horizontal run with several appliances joining it, and no vent, is the classic layout that gurgles. Each appliance discharge fills it and pulls air from the sink trap.
7. Fit an air admittance valve if the branch is unvented.A small one-way valve fitted on a vertical tee near the appliance lets air in without letting smells out. It is a straightforward push-fit or compression job under a sink or in a cupboard.
8. Site the valve correctly.It has to sit above the flood level of the highest appliance it serves, upright, and somewhere accessible and ventilated. Buried in a sealed boxing it cannot do its job.
9. Retest with the machine on a drain cycle.Run the machine's spin and listen. A properly vented branch is close to silent at the sink.
10. Escalate if the whole house gurgles.Gurgling from several appliances, or from the toilet, points at the soil stack or the underground drain rather than the branch. That needs a drainage inspection.

Tips

• Run the tap for a few seconds after any gurgling episode to refill the trap seal.
• An air admittance valve is far cheaper and less disruptive than adding a vent pipe, and it is the normal modern solution.
• Gurgling from the toilet when a sink drains is a different and more serious signal — that is the stack, not the branch.

Common mistakes

• Sealing the appliance hose into the standpipe — It creates a closed system with no air path, so the discharge runs full bore and siphons every trap on the branch. The hose should be loose in the standpipe, not sealed.
• Fitting an air admittance valve inside sealed boxing — It needs air on its outside to admit any. Boxed in with no ventilation it simply does nothing, and the gurgling continues.

If it doesn't work

Questions people ask