Smells caused by plumbing faults
Bad smells from a washing machine are most commonly caused by issues inside the machine itself – but plumbing faults can also be responsible, particularly when the smell resembles an outside drain and tends to be worse first thing in the morning. This guide covers the two most common plumbing-related causes.
Is There a U-Bend in the Waste Pipe?
If the bad smell is reminiscent of an outside drain and is particularly noticeable first thing in the morning, the most likely cause is a waste water pipe that lacks a proper U-bend.
A U-bend always retains a small amount of water inside it. This water acts as a seal, preventing smells from the drain system from travelling back through the pipe and into the house. Each time the washing machine pumps out, the water in the U-bend is refreshed. Without it, the drain hose is effectively an open tube connecting the outside drain directly to the interior of the room.
If the drain hose runs over an outside drain without a U-bend, drain smells have a clear path into the house. If the hose pushes into a standpipe at the back of the machine, check that there is a U-bend at the base of that standpipe.
Is the Drain Hose Connected to a Sink U-Bend?
Many washing machines are plumbed into a spigot fitting on the U-bend under the kitchen sink, rather than into a separate standpipe. This is a clean solution but must be set up correctly – otherwise dirty water poured down the sink can travel back down the drain hose and into the washing machine, carrying unpleasant smells with it.
To prevent this, the drain hose must loop upward by at least 3 to 4 inches from the point where it connects to the sink U-bend, before running down to the machine. This upward loop creates a natural barrier against backflow. The hose may need to be secured in place to hold this position. Without the loop, water can siphon directly from the sink into the machine.
For more on correct drain hose routing: 5 things to check after repairing or reconnecting a washing machine.
Frequently Asked Questions
Why does my washing machine smell like an outside drain?
This is typically caused by a missing or inadequate U-bend in the waste water plumbing. Without a U-bend, there is nothing to prevent drain smells from travelling back up the pipe and into the room. The smell is often worse first thing in the morning when the machine has not been used for several hours. Check that the waste pipe has a properly functioning U-bend and that it is not blocked or dry.
What does a U-bend do in washing machine plumbing?
A U-bend retains a small amount of water at its lowest point. This water forms a physical barrier between the drain system and the room, preventing smells from passing back through the pipe. Each time the washing machine drains, the water in the U-bend is replenished. Without this barrier, drain smells have an unobstructed path into the house.
Why is dirty water getting into my washing machine from the sink?
This happens when the drain hose is connected to the sink U-bend but does not loop upward before connecting. Without the upward loop, water poured down the sink can siphon directly into the washing machine via the drain hose. The hose should rise at least 3 to 4 inches from the connection point before dropping down to the machine. Securing the hose in this position with a clip or cable tie is usually sufficient to prevent the problem.
How do I know if my washing machine smell is a plumbing issue or something inside the machine?
Plumbing-related smells typically resemble outside drain or sewer odours, and are often worse when the machine has not been used recently – particularly first thing in the morning. Smells originating inside the machine are more likely to be musty, mouldy, or like stale laundry, and may be worse immediately after a wash cycle. If the smell is the latter, see: what causes washing machine smells?
Hello Diane, the smell could be coming from under or behind the washing machine, potentially caused by a persistent small leak. Have you completely pulled the washing machine out and checked behind it?
Hi there, read through all advice and followed however the rotting wood/damp smell in my washer is getting worse especially since being away on my hols and hasn’t been used for a couple of weeks. There is no gunge and the drum is clean (washer only 6 months old) it is very clean but cant get rid of this smell. drains into u bend under sink. anymore suggestions please? many thanks
Hello Clare, all my thoughts on the subject are in this article and the 4 related ones I link to within it.
Hi
I have a draining hose that goes into a standing pipe with a u bend, however there seems to be a musty smell coming out of the standing pipe which gets worse when the washing machine hasn’t been used for a few days…any ideas?
Thanks!
Clare