How do I connect a washing machine and a dishwasher to one waste?

Two appliances sharing a waste is normal and works well, provided each has an anti-siphon loop and the trap actually has open spigots. The two things that go wrong are both easy to get right and both cause dramatic symptoms.

What you'll need

• A washing machine trap with two spigots, or a double standpipe
• Hose clips
• A hacksaw or sharp knife for the spigot caps
• Pipe clips to hold the high loop

Step by step

1. Decide between a trap spigot and a standpipe.A spigotted trap under the sink is compact and common. A standpipe is an open vertical pipe about 600mm tall with its own trap, into which the hose is simply hooked. Both work; the standpipe is more forgiving.
2. Buy a trap rated for two appliances.Standard sink traps often have one spigot. Twin-spigot appliance traps are readily available and no more expensive.
3. Cut the blanking cap off each spigot before connecting.Spigots are moulded closed and have to be cut open with a hacksaw or knife. Connecting a hose to an uncut spigot means the machine pumps against a sealed pipe and floods the floor on its first cycle.
4. Check the cut is clean.Trim off any plastic swarf. A ragged edge catches lint and starts a blockage in an appliance waste that is already narrow.
5. Fit the hose with a clip.Push the hose fully onto the spigot and secure it with a hose clip. Appliance pumps produce real pressure and an unclipped hose comes off.
6. Create the high loop.Route the hose up so its highest point is above the level of the machine's drum, and clip it there. That break in the siphon is what stops the machine draining while it fills.
7. Do not push the hose deep into a standpipe.It should hook over the top with an air gap around it. Pushed down and sealed, it creates a siphon and the same problem.
8. Keep the hose runs short and unkinked.Long hose runs and tight bends slow the pump-out and cause the machine to report a drain fault.
9. Check the trap can take two discharges.A washing machine and a dishwasher rarely drain at once, but if they do, a 40mm trap and branch copes; a 32mm one will not.
10. Test each machine on a drain cycle.Run each one separately, watching the connections and the sink for gurgling, then run both together if you can.
11. Deal with gurgling if it appears.Gurgling at the sink after a drain cycle means the branch needs venting — an air admittance valve near the appliances usually cures it.

Tips

• The uncut spigot cap is the single most common cause of a new appliance flooding a kitchen. Check it before you connect anything.
• Clip the high loop in place. A hose that slips down over time reintroduces the siphon months later.
• Standpipes are more tolerant than trap spigots and worth using where there is room.

Common mistakes

• Connecting to a spigot without cutting the cap off — The moulded cap seals the spigot. The machine pumps hard against a closed pipe, the hose blows off or the pump strains, and the discharge ends up on the floor.
• Leaving the hose lying low between machine and trap — Without a high loop, water siphons out of the machine as fast as it fills, so it never reaches temperature or level, and dirty water from the sink can flow back into it.

If it doesn't work

Questions people ask