This article looks at bad smells coming from a washing machine that can be caused by problems in the plumbing. The most common causes of bad smells are covered extensively in my previous article (what causes washing machine smells?) But some smells can be caused by the plumbing.
Is there a u-bend fitted?
If the smell tends to be worse first thing in a morning, and smells a bit like an outside drain, one cause could be that your waste water pipe (on the plumbing) doesn't have a proper U bend fitted.
A u-bend always holds some water which acts as a barrier to stop smells getting from the drains back into the house. This water is constantly replaced each time the washer is used. Make sure your washing machine doesn’t pump into a drain system that has a direct run to a drain without some sort of u-bend.
Without it, if the plastic waste water piping hangs over an outside drain, then you have, in effect, an open tube leading from over the outside drain into your house. If the drain hose pushes into a stand pipe at the back, make sure there is a u-bend at the bottom of it.
If yours pumps out under the sink
Most washing machines are plumbed directly into a fitting on the u-bend under the sink these days instead of being pushed into a standpipe (as above). Unless fitted properly though, some of the dirty smelly water poured down the sink can run down the drain hose into the washing machine.
A drain hose connected to a sink u-bend should first go up 3 or 4 inches high (it may need tying up) before running down to the washing machine as shown in the photo. More information – including Make sure that the drain hose is connected correctly – 5 things to check after repairing or reconnecting a washing machine