Potential health risks from unused hot water tap
A disused hot water tap behind a washing machine can leave stagnant, lukewarm water trapped in the pipework — ideal conditions for Legionella bacteria to grow. The domestic risk is low, but it is easily eliminated by fitting a blanking cap to the unused tap or having the redundant pipework removed by a plumber.
Temperature range in which Legionella bacteria multiply most rapidly
Almost all washing machines sold in the UK today use a single cold-water supply
A blanking cap on the unused hot tap costs very little and eliminates the risk
Why Does an Old Hot Water Tap Create a Health Risk?
Decades ago, washing machines in the UK commonly used both a hot and a cold water supply. Homes were plumbed accordingly, with a dedicated hot tap installed alongside the cold one behind the machine.
Modern washing machines are cold-fill only. They draw cold water and heat it internally, so the hot tap is no longer connected or used. The problem is what happens inside the pipework when that tap sits idle.
Legionella pneumophila is a naturally occurring bacteria found in water. It multiplies rapidly in warm, stagnant water between 20°C and 45°C, and can cause Legionnaires’ disease — a potentially serious form of pneumonia — if contaminated water droplets or aerosols are inhaled. The UK Health Security Agency (UKHSA) and the HSE both identify stagnant pipework as a contributing risk factor.
A disused tap that remains connected to the hot water supply creates what plumbers call a dead leg — a section of pipe with no regular water flow. The water inside stagnates, settles at a lukewarm temperature, and creates the exact conditions in which Legionella can take hold.
How Does the Risk Arise With a Washing Machine Specifically?
The typical scenario is straightforward. A cold-fill washing machine is installed and connected to the cold tap. The old hot tap is turned off and forgotten. No one uses it — but it remains connected to the hot water system through a short section of pipe inside the wall.
That section of pipe never flushes. The water inside it sits at an intermediate temperature for weeks, months, or years. If the tap is ever accidentally opened — during a house move, a repair visit, or a routine check — there is a risk of dispersing bacteria into the air.
| Scenario | Risk Level | Why |
|---|---|---|
| Hot tap in regular use, water flowing freely | Very low | Regular flow prevents stagnation |
| Hot tap turned off but still connected to supply | Low–moderate | Dead-leg pipework can harbour bacteria over time |
| Hot tap capped off with a blanking cap | Very low | No pathway for stagnant water or aerosol dispersal |
| Redundant pipework fully removed by a plumber | Eliminated | No dead-leg pipework remaining |
How Serious Is the Risk in a Domestic Home?
It is important to keep this in perspective. Legionnaires’ disease is predominantly associated with large, complex water systems — cooling towers, hotel plumbing, hospital water supplies, and commercial buildings. The HSE acknowledges that the risk in a typical private home is low.
A single disused domestic tap does not carry the same risk as a commercial water system. However, the risk is not zero, and the fix is simple enough that there is no good reason to leave it unaddressed.
Older adults, smokers, people with weakened immune systems, and those with chronic lung conditions are more susceptible to Legionnaires’ disease. If anyone in the household falls into one of these groups, acting promptly is particularly advisable.
“Water systems in domestic premises present a low Legionella risk, provided basic precautions are taken. Flushing out infrequently used outlets regularly is a straightforward and effective control measure.”
— Health and Safety Executive (HSE), Legionella Guidance for Domestic Premises
What Should You Do About an Old Hot Water Tap?
There are two practical solutions depending on your situation and budget. Both are far preferable to simply leaving the tap turned off and forgotten.
Option 1: Fit a Blanking Cap
A blanking cap screws or pushes onto the tap outlet, sealing it completely. This prevents the tap from being accidentally opened and removes any risk of aerosol dispersal. Blanking caps are inexpensive and the job takes minutes.
Option 2: Remove the Pipework
A qualified plumber can isolate and remove the redundant hot water pipework entirely. This eliminates any dead-leg pipe inside the wall and is the definitive, permanent fix — especially recommended if the pipework runs any significant distance.
If you’re not confident working around your plumbing or water supply, a qualified engineer can help. Book a repair visit or find the parts you need using the links below.
What if My Machine Is Already Connected to Cold Only?
If your washing machine is connected solely to the cold tap — which is the case for almost all modern machines — then the hot tap behind the machine is entirely redundant. Simply having it turned off is not sufficient.
Cap off the old hot tap using a blanking cap, or ask a plumber to remove the pipework. See our guide on converting a hot-and-cold fill machine to cold-only supply for full details on managing this transition.
Does It Matter if the Hot Tap Is Still Being Used?
If your washing machine is actively using the hot water supply — as some older twin-fill machines do — then water flows through the tap with every wash cycle. Regular flow significantly reduces the risk of stagnation and bacterial growth.
That said, very few washing machines sold in the UK today use a hot fill. Check the back of your machine: cold-fill-only models have a single inlet hose connection. Twin-fill machines have two — one marked hot, one marked cold.
Can You Connect a Cold-Fill Machine to the Hot Tap?
Some people wonder whether feeding a cold-fill machine with hot water could improve washing performance or reduce energy consumption. Whitegoods Help does not recommend this.
- Cold-fill machines are calibrated to heat water from cold — feeding them hot water disrupts temperature sensing
- It can cause wash performance problems and damage internal components
- It may invalidate your manufacturer’s warranty
- Connect the machine to the cold water supply only
- Cap off or have a plumber remove the old hot tap
- Let the machine manage water heating as designed
For a full explanation, read our guide: Can you connect a cold-fill washing machine to the hot tap?
What to Check Behind Your Washing Machine
Not sure what you have? Follow these steps to assess the situation quickly.
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Pull out the machine slightly and look at the water connections at the rear. Count the number of inlet hoses — one means cold-fill only, two means hot-and-cold fill.
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Locate all taps on the wall. If there are two taps (one cold, one hot) but only one hose connected to the machine, the hot tap is redundant.
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Check whether the hot tap is open or closed. If open but unconnected, close it immediately. Either way, it should be capped off.
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Fit a blanking cap to the hot tap outlet, or contact a plumber to remove the dead-leg pipework entirely. See our guide to blanking off the hot tap for full instructions.
Any work involving your home’s hot water pipework should be carried out by a qualified plumber. Do not attempt to cut or modify pipework yourself unless you are fully competent to do so. If you have broader concerns about Legionella risk in your water system — particularly in larger or older properties — contact a water hygiene specialist. The HSE provides free Legionella guidance for domestic properties, including landlord responsibilities. Landlords have a legal duty of care under the Health and Safety at Work Act 1974 to assess and control Legionella risks.
Related Guides
Frequently Asked Questions
Can a washing machine cause Legionnaires’ disease?
A washing machine itself is not a source of Legionella. The risk comes from stagnant water sitting in a disused hot water tap or dead-leg pipe that remains connected to the hot supply behind the machine. If that water is disturbed and aerosolised, there is a theoretical inhalation risk. In practice, domestic risk is considered low, but capping off unused hot taps removes it entirely.
What is a dead leg in plumbing?
A dead leg is a section of pipe connected to the water supply with no regular flow through it. Water stagnates inside, and if the pipe is part of the hot water system, it can settle at temperatures between 20°C and 45°C — the ideal range for Legionella bacteria to multiply. Dead legs are a recognised risk factor in both commercial and domestic water systems.
How do I know if my washing machine uses hot and cold fill?
Check the back of your machine. A cold-fill-only machine has a single inlet hose connection, usually marked with a blue label or the word “cold”. A hot-and-cold-fill machine has two connections — one for hot, one for cold. The vast majority of washing machines sold in the UK since the early 1990s are cold-fill only.
Is it safe to just turn off the old hot tap and leave it?
Turning the tap off is better than leaving it open, but it does not fully resolve the issue. The section of pipe between the closed tap and the main hot water supply can still hold stagnant water. Fitting a blanking cap to the tap outlet — or having the pipework removed — is the proper fix.
Can I connect my cold-fill machine to the hot tap to save energy?
This is not recommended. Cold-fill machines are designed and calibrated to heat water from cold. Feeding them a hot water supply can interfere with temperature sensing, compromise wash performance, damage internal components, and may invalidate the manufacturer’s warranty. Connect cold-fill machines to the cold supply only.
Who is responsible for Legionella risk in a rented property?
Landlords have a legal duty of care under the Health and Safety at Work Act 1974 and the associated Approved Code of Practice (L8) to assess and control Legionella risks in rental properties. This includes identifying and addressing dead-leg pipework. Tenants who suspect a risk should report it to their landlord in writing. The HSE’s guidance covers landlord responsibilities in full.
Thanks for your comment, Riccardo. It’s very useful to get an opinion from someone who works in plumbing. I thought long and hard about how to put across this advice, it’s very hard to get the tone right because it’s potentially serious (enough for legislation to be made) but also extremely rare.
The whole issue is complicated as there are so many scenarios, but as we both say, regarding Legionnaires disease you need to breathe in droplets of infected water, which is difficult to do at a tap. There are aerator devices fitted to some taps, though, that force air into the water stream. These devices are specifically mentioned in the research that I did. However, I still suspect that it would be difficult to get infected.
Since writing this article, I’ve replaced my dishwasher of 15 years, which was initially installed by someone else. When I pulled it out, I found another dead leg of around 4 feet that once supplied hot water. I had no idea it was there, but obviously it’s never caused any problems.
Hi Andy,
As a plumber, I thought I’d share my ha’p’orth. You are dead right that an unused tap is, in effect a dead-leg and the redundant pipe would ideally be cut out.
I’d say in this case that a setup with this kind of dead-leg on a distributing pipe coming off a cylinder is probably safer than hot water supplied via a combi. The reason is that water leaving a cylinder is likely to be at 60°C and would pasteurise the main drag, so even if bacteria breeds in the dead-leg, the main drag should remain relatively sterile; whereas since a combi often feeds water to taps at only 40 degrees or so, the whole pipe is the right temperature for bacteria to breed in.
Realistically, little-used showers are an issue because the water in them is stagnant and then discharged into a room and in a way that encourages droplets and aerosols to form in the air and be breathed in and in quantity. The small amount of water in a washing machine dead-leg is unlikely to get into the air in any quantity, unless, clutching at straws, a plumber drains the system down to repair the shower and then fires the shower up and gets a lungful. As you say, typically, the water in the pipe is changed several times a day.
The most familiar legislation relating to plumbing design in domestic buildings would be the 1999 Water Supply (Water Fittings) Act and the WRAS Water Regulations Guide that incorporates various guidance and recommendations. There is a note with the title “redundant fittings and dead legs” which does not explore the issue of a tap that is still available to use, but, in practice, is no longer in service which suggests that such dead legs should not be left connected for more than a 60-day temporary period (to allow for work to be carried out etc).
FWIW I’d agree that an appliance tap that is not being used because there is no appliance that can connect to it, is effectively the same thing and should be removed. That said, I found a 10′ dead leg on my own mains supply under the ground floor that had been there at least 25 years.