Potential health risks from washing machine’s old hot water tap

Hot tap for washing machine

After installing a cold fill only washing machine, most of us no longer use the old hot water supply. Millions of washing machine hot water taps are abandoned. Consequentially, no water has run through them for several years or more. Did you know that this could create a health problem?

This article looks at what the risks are when abandoning a hot water tap. If we fully understand this risk, we can make an informed decision about whether anything needs to be changed, and if so, what. I’m not an expert on health or disease. This article is based on my knowledge of washing machines, plus a lot of research – and thought.

What is the health risk?

All the water inside the pipework branching off from a main pipe, and leading to your unused tap, is trapped. It never flows or gets replaced, and is never heated up, so it is stagnant.

Under certain conditions, it’s possible for several types of harmful bacteria to grow inside the stagnant water. Some of which, could break off and contaminate the rest of the water. One of these bacteria is legionella, which can cause the deadly Legionaries disease.

Dead leg problem

In the plumbing world, this is called a dead leg. There are strict rules and regulations for plumbers regarding capping off unused pipework. This is to prevent Legionaries disease, though other types of bacteria can multiply too.

My diagram shows what happens when you stop using a tap and create a dead-leg. See the thin blue line at the base of the green pipe? That’s where the tap should be capped off. Pipework should be cut back to there, and then capped off.

Dead leg plumbing
Water in the green section (unused tap) is trapped

Don’t panic yet – it may not be an issue for you – but you need to check

It’s important to work out if there is any risk. I think that for most people it is low – especially if you have a combination boiler. But you can’t be sure until you fully understand the issue. There are two issues to consider. First, could bacteria be growing inside the trapped water? Second, if there is, could it cause a health problem at my house?

Here are some quotes that give some basic fundamental facts. I will then go through all risks, and you can decide if you have anything to be concerned about.

Quotes from government website

Legionella bacteria are commonly found in water. The bacteria multiply where temperatures are between 20-45 °C and nutrients are available.

Managing legionella in hot and cold water systems

Ensure water cannot stagnate by ensuring regular movement of water in all sections of the systems, and by keeping pipe lengths as short as possible, and/or removing redundant pipework and dead legs

The first consideration is, how could any bacteria get into my hot water system in the first place? Although we reportedly have one of the safest supplies of water in the world, bacteria can be present in our water, even drinking water. It’s unlikely to be in high enough quantities to cause illness, but there have been cases of bacteria found in our water. Here are just two – E. coli bacteria in tap water, and Sheffield University found bacteria in drinking water (link currently broken).

Though rare, it is possible for bacteria to be present in mains water, so it can’t be ruled out. It’s also possible for bacteria in the air to contaminate water tanks. So hot water systems that store water in water cylinders and water tanks have an increased risk.

If you have a combination boiler, with water heated on demand, then the only route in for new bacteria should be from the mains cold water supply. It is, in effect, a sealed system.

For this reason, my understanding is that combination boiler systems pose the smallest risk. But once water is stored in tanks, either in an airing cupboard or in the loft, there’s an increased risk.

Water temperature

These bacteria multiply at temperatures between 20-45 °C. So could the temperature of trapped water in the dead leg reach 20 degrees at your house? It’s safe to say that the temperature of cooled down water in a hot water system, is typically similar to the temperature outside, or at least in the house.

It’s pretty easy for the trapped water to be at 20 degrees or higher for long periods. Let’s not forget that inside the dead leg, the water is never heated again. So trapped water will always be at ambient temperature.

Could bacteria transfer to someone and make them ill?

As long as your hot water regularly flows in the pipework at 60 degrees, any bacteria falling into the system should be killed off – at some point. Remember that all the water cools down when not in use. So water will not be hot enough to kill bacteria until it’s been running for some time. By the time any water running is hot enough to kill bacteria, it may have already come through the tap (or shower head).

Because we don’t drink the hot water, the risk of diseases caused by drinking infected water is very low. But when it comes to Legionella, these bacteria infect us if breathed into our lungs though a fine spray or water droplets. The danger is from showers, or even running taps, especially (but not exclusively) if taps have fittings to aerate water and create a fine spray. They are the main risk factors.

Bacteria food source

Bacteria also needs a food source to thrive. So the next question is, could there be a food source for bacteria inside your plumbing? Food sources include deposits that can support bacterial growth, such as rust, sludge, scale and organic matter.

Copper isn’t as susceptible to limescale as steel pipes, and our plumbing is typically copper. However, the phrase does not rule it out. Plus limescale could form inside metal taps, or inside water tanks, and get into the system.


When I first learnt about this issue, I was quite concerned. But I realised we’ve had a dead leg spanning 10 feet at our house for 17 years, with no known problems.

And for most of that time our hot water was stored in a cylinder with a large cold water tank in the loft. Now we have a combination boiler, and I’ve regularly flushed out the dead water with hot water, so I probably shouldn’t worry about it.

Most of the UK has also been in the same situation, since cold fill washing machines took over. I’m not aware of any large increase in legionaries disease – at least not associated with UK domestic houses.

But Legionaries is a serious illness. Strict rules and guidelines ensure dead legs are not tolerated in public housing and buildings. There’s clearly a rare but real threat. But as far as I know, there’s no official advice for private households about this. All advice and regulations appear focussed on commercial and public buildings.

If you have any expert knowledge on this subject, please add a comment.

NHS advice currently says “it’s less common to catch Legionaries at home”, but lists taps and showers that are no longer used often as a source of legionnaires disease at home –NHS Legionnaires disease

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2 thoughts on “Potential health risks from washing machine’s old hot water tap”

  1. 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.

  2. Andy Trigg (Whitegoodshelp)

    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.

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