If you have a smelly kitchen sink drain it can be rather unpleasant, especially if it starts to waft around the house. We take a look at the causes and what you can do to fix it.

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A smelly kitchen sink is unpleasant at the best of times, and worse when it starts to waft through the house.

The good news is that most causes come down to a handful of familiar culprits - bacteria, blocked filters, or a dry trap - and most are fixable without calling anyone out.

This guide covers why kitchen sink drains start to smell, what you can do about it yourself, and when it's worth bringing in a plumber or appliance engineer.

Why does my kitchen sink smell when the washing machine or dishwasher runs?

If the smell seems to appear specifically when an appliance is running, it's usually linked to how your washing machine or dishwasher waste connects to your sink's waste pipe, rather than the sink itself.

Bacteria build-up

The most common cause is bacteria breeding where water has been sitting in the waste pipe.

Since washing machine waste often shares a pipe with the kitchen sink, this bacteria can produce a bad smell that seems to come from the sink even though the real cause is upstream.

Too much detergent

Using more detergent than needed, especially with too little water, can leave residue on clothes and inside the machine. That residue becomes a breeding ground for the bacteria causing the smell.

Water supply issues

An interruption to your mains supply, or unclean water being fed into the machine for any reason, can also contribute to smells further down the line.

Blocked filters

If a washing machine or dishwasher filter hasn't been cleaned in a while, trapped fluff, food and debris will start to smell as it decomposes - and that smell often escapes through the connected sink drain.

If the smell keeps returning after cleaning, it may be worth getting a plumber or appliance repair specialist to check the pipework and appliance separately.

What is a dry P-trap, and why does it cause smells?

One cause that's easy to miss: the P-trap. This is the U-shaped section of pipe under your sink that holds a small amount of water at all times, specifically to stop sewer gases rising back up through the drain.

If a sink isn't used often, that water can evaporate, leaving the trap dry and letting odours through. This is especially common with a second kitchen sink, utility room sink, or one used less frequently than the main tap.

The fix is simple: run the tap for 20-30 seconds to refill the trap. If the smell disappears, that was likely the whole problem. If it returns quickly, there may be a leak or blockage stopping the trap from holding water properly.

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What can you put down a smelly kitchen sink drain?

Before treating the smell, it helps to have a rough idea what's causing it - a blocked appliance filter needs a different fix to a dry trap or a build-up inside the pipe itself.

A simple DIY drain clean:

• Pour a kettle of hot (not boiling, if you have older plastic pipework) water down the drain first, to loosen any grease

• Add around half a cup of baking soda, followed by a cup of white vinegar

• Let the mixture fizz for 10-15 minutes - this helps break down grease and gunk clinging to the pipe walls

• Flush through with more hot water to rinse everything away

Doing this every few weeks, alongside regularly cleaning appliance filters and any visible residue, will keep most smells at bay long-term.

If none of this helps, water may be sitting somewhere it shouldn't - such as an overflow pipe connected to the sink - or there could be a fault with the appliance itself.

Parts worth checking on the appliance side if the smell persists:

• The filter

• Heating elements

• Door seal

• Detergent dispenser

• Hose

• Drum

• Drain

When to call a plumber instead of an appliance engineer

If you've cleaned the drain, checked the appliance, and the smell is still there - or if it's coming from more than one drain in the house - it's usually a sign of a bigger issue with the pipework itself rather than the sink or appliance.

A local plumber can inspect the waste pipe, the P-trap, and the connections under the sink to rule out a partial blockage, a leak, or a missing or damaged trap that's letting smells through.

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Fixing a smelly kitchen sink drain - checklist

Smelly sink checklist

• Run the tap to refill a possibly dry P-trap

• Clean the washing machine or dishwasher filter

• Try a baking soda and hot water flush

• Run a hot maintenance wash on appliances

• Check for smells from more than one drain

• Call a plumber if the smell persists after cleaning

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