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.
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.
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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