Hot Water Temperature - Scalding and Legionella

Among heating engineers, this is really going to ruffle some feathers. It's entirely possible to keep hot water permanently at 40°C, literally save 50% of your hot water bill and not worry about legionnaires disease. 

But! This is definitely not advisable for everyone! So stick around while we dive deeper into this subject, and find out what works for you or how or your customer could save energy.

The argument we often hear from engineers is that we need to keep hot water at 60°C to kill legionella. But where does that advice come from? How risky is it and what is legionella?

Legionella is a bacteria that grows in water and if consumed or inhaled can cause a pneumonia like illness known as Legionnaires Disease.

If you look at this table you can see there's around 460 cases of legionnaires disease (or legionellosis) a year on average in the UK. 

Of those cases 2% are from hospitals occupied by those with weaker immune systems, 51% of cases are from people travelling, (hence why there was a quick drop off in 2020 during the pandemic) and 47% are from ‘the community’.

The community being offices, restaurants, gym, pubs, shops or your home. 

The most common form of transmission of Legionella is inhalation of contaminated aerosols produced in conjunction with water sprays, jets or mists of contaminated water sources such as spa pools, humidifiers and irrigation systems.

• ultrasonic humidifiers/foggers;
• misting devices used for humidifying vegetables, meat and other food products;
• spray humidifiers;
• air washers, wet scrubbers, particle and trivial gas scrubbers;
• water softeners;
• emergency showers, eye-baths and face wash fountains;
• sprinkler and hose reel systems;
• spa pools;
• whirlpool baths;
• horticultural misting systems;
• vehicle washers including automatic washers for cars, buses, lorries and railway rolling stock;
• powered dental equipment;
• fountains and decorative water features including those on display for sale;
• non-disposable nebulisers used for respiratory therapy;
• industrial effluent treatment plants;
• irrigation systems;
• fire, dust and odour suppression systems;
• paint spray preparation equipment;
• tunnel pasteurisers and similar equipment.

So of these 216 cases a year, how many are from domestic properties, let alone hot water stores?

Well we've scoured the internet for data and although we can see cases of Legionella growth, we cannot find a single case of legionnaires disease contracted from a domestic property. We can, however, find many, many cases from commercial properties. 

Now because we can’t find the data, (by the way if you do please get in touch or comment and I'll punn all the findings, if any, to the top) let's be hugely overly cautious and say 50% of these 216 cases are from domestic properties. Even though they don't typically have evaporative cooling, spas fountains or spray humidifiers.

That would leave a 1 in 620,000 chance of contracting it from your home each year. It's also worth noting that risk varies with vulnerability, 75–80% of legionnaires disease cases are age 50 plus, if you are in the 0 to 50 age range your risk is more like 1 in 1.3 million, if half of the cases were from domestic sources. 

However, we CAN find cases of the Legionella bacteria existing in domestic properties, in fact it's absolutely everywhere! 

For example, in 2017 there was a study by public health England infection experts that took samples from 99 showers heads in 82 properties across the UK and found nearly a third tested positive for legionella bacteria. But despite this, there are still very few recorded cases of the disease that we could find.

The real risk of legionnaires disease is when the bacteria grows to dangerous levels. 

So The Question Isn't: How Do We Prevent Legionella, it’s How Do We Prevent Legionella Bacteria Growth?

To grow legionella to dangerous levels you need two things. Firstly you need stagnant water, meaning the water is not moving or changing, and secondly for that water to be in the legionella multiplication and growth temperature range of 20°C to 45°C. 

Eliminate either one of these two things and legionella simply cannot grow.

Temperature we know we can control, but what constitutes stagnant water?

What is Stagnant Water?

Stagnant water is hard to define as its relative to both the store volume and time.

For example, water heaters termed as ‘instantaneous water heaters’ or combination boilers will have a very high turnover, a small wash of your hands will typically turn over all the hot water content and aren't really deemed as a high risk. 

Conversely larger store volumes or heaters used more sporadically, such as ones found in commercial premises, are much higher risk. If you imagine an extra large cold water and hot water store that a few people wash their hands from each day, this will have relatively low water turnover and give much more chance for legionella to grow.

To visualise this we've created this graphic. You can see with a large daily turn over, legionella in the water essentially stays dormant as it simply doesn't have enough time to grow.

Most domestic properties will use most of their water capacity each day, and are unlikely to stay stagnant longer than a typical holiday.  However, commercial premises are more likely to be at risk as they can go months without being used, or have very little use, then all of a sudden be at full capacity.

This isn't just about your store when it comes to commercial properties, it's the associated pipework too. Again this same graphic can be used.

Commercial spaces have risk as entire floors, wings, or even just rooms aren't used while occupants change. pipework can hold water for a long time at 21°C or above if near heating pipes. Conversely almost all hot water points will be used regularly in a domestic home so a much lower risk if at all.

For that reason we have the L8 legionella regulations or the HSG274 that stipulate all duty holders (that's people in charge of commercial or rental properties to you and me) should keep hot water at least 60°C and fit temperature reducers at all outlets.

Domestic properties don't have this regulation of minimum temperature due to being a much lower risk. In fact the only thing stated in the domestic dwellings regs is the limit to having a maximum temperature delivered to the domestic hot water system of no greater than 60°C in the PART G REGulations.

So What Temperature Should Water Be Stored At?

When it comes to water store temperature, here are the facts.

Approximately….

At 0°C to 20°C legionella is dormant, 20°C to 32°C slow growth. Optimal growth for legionella is 32°C to 42°C with a maximum growth rate at 37°C.  42°C to 45°C is slow growth again, And 45-50˚C its dormant again.

Around this temperature or above is where we can cleans or pasteurise stored hot water to safe levels.

At 50˚C 90% of legionella dies in between 80 and 124 minutes depending on the strain, let’s call it a worse of 2 call it 2 hours.) The rest won't all die after this time but will definitely remain dormant and not multiply, so remain totally safe provided the temperature is kept here continuously or the water is refreshed.

At 55˚C 90% of legionella dies in approximately 20 minutes and 100% is killed over 5 to 6 hours. 

At 60⁰C 90% it dies in 2 minutes and 100% in 30 minutes.

65⁰C = 90% in around 10  seconds and 100% in 2 minutes

70⁰C - it’s instant death for legionella 

Note numbers are approximate and vary with strains.

So How Accurate Are Your Measured Temperatures?

Because cylinder stats are located a third of the way up your cylinder, the temperature at the top will likely be a few degrees above your stat temperature due to stratification. Below the stat will be between 10°C and the set temperature, but for safety assume it's all the same temperature

Cylinder stats also have a hysteresis. So for example, this common stat has an 8k hysteresis or switching differential so if set to 50°C will call for heat at 46°C and turn off when the store is at 54⁰C.

Now you could keep the hot water at 70°C and be incredibly safe from legionella growth, but as the most common place for legionella in domestic homes seems to be shower heads, this won't help as the waters typically blended down to 37°C which is the perfect temperature for Legionella growth. In fact anything over 50°C can cause scalding, and water at 60˚C only takes one second to cause serious burns! Which brings us on to the other problem.

Scalding

Setting hot water to 60°C for a vulnerable person can be extremely dangerous. Look at this table below and you'll see that first degree burns happen in 2 seconds for the elderly at 60°C. 

This is exacerbated by the time it takes for the brain to register the temperature which increases as you get older! An elderly person that only turned on their hot tap to run a bath could be fully in the bath before they realise it!  

For this reason we suggest following the part G regulations and not storing at 60°C or above unless you have scalding protection fitted at the outlets.

Efficiency

The last consideration, outside of safety, is running costs and efficiency. If you have a heat pump or condensing boiler, the efficiency at your heat source can be increased if lower flow temperatures are targeted. 

Additionally a cooler cylinder will lose less energy into the surrounding air. Hot water stored at 50°C will have a 20% lower heat loss than a 60°C cylinder. 

So, you're probably wondering, what bloody temperature should I keep my hot water!

What Temperature Should I Keep My Hot Water?

If you want a simple answer, stick around to the end, however, as we always say, there is no panacea and if you want to know the proper answer for your specific property, first we must do a risk assessment.

Risk Assessment 

Age 

If you're above 50 years of age or have an autoimmune disease or regularly have elderly guests then your risk is higher and more caution is needed, yes it may cost more, but it's also safer. 

Conversely if you don't have temperature limiters at your outlets, scalding is potentially a higher risk also, particularly if you have younger children.

Turnover 

If you have stored hot water, but you don't turn it over more than say 50% a day, for example 2 of you only have 1 shower a week then you are at high risk of legionnaires disease…  and potentially lots of things if you don't shower anywhere else.

Unused Outlet Risk

In domestic dwellings all hot water outlets will probably be used regularly, so your pipework is unlikely to need sterilising with 60°C water. 

In the vast majority of domestic situations storing water at 50°C would be perfectly adequate.  Infact 50°C is the temperature most hot water controls from Honeywell, Viessmann and Vaillant and Worcester come pre-set at. 

What is The Minimum We Can Set Our Hot Water?

If you deem yourself as particularly low risk with high water turnover, the absolute minimum temperature you would theoretically set hot water to is 40°C. 

This is a little hotter than average showering temperatures but may be too cool for washing dishes if you don't have a dishwasher. It also may need bumping up a few degrees if it causes issues with mixer taps.  

Sterilisation / Pasteurisation or Anti-legionella Cycles

If you do store hot water as low as 40 to 49°C we would suggest at least being aware of anti-legionella cycles (ALC). 

This is essentially where hot water temperatures are temporarily increased to kill off any bacteria that might have formed. 

If your water is fully turned over once every day or so on average this won't be needed. But if you are particularly concerned, for peace of mind, you could do a one off sterilisation after a holiday or once a year.

Simply turn up your hot water to at least 60°C and run all outlets for 5 mins, making sure that you have turned up temperatures past any safety limiters.

Heat pump controllers often have weekly or by weekly sterilisation built in. You don't need to heat to 60°C though necessarily, any temperature above 50°C will kill legionella, just depending on how long you've run it and how much you want to kill.

Using anti-legionella cycles may persuade you to keep your cylinder even cooler during the week. Be aware if you find you run out of hot water you may need to turn it back up to give more effective volume.

Guide

SoOOOOOooo for YOUR specific situation.

The name of the game is to get the store temperature as low as possible while keeping safe. The vast majority of homes will even be fine at 50°C, but if you want to increase the efficiency of your gas boiler, or the SCOP of your heat pump, AND reduce the amount of heat lost from the cylinder by keeping it cooler, take another look at this graphic. 

Here you can see that once your average daily turnover is above 100% we don't see too much issue with a MINIMUM temperature of 45°C. 

If you follow the black line on this graph, you will see the minimum temperatures that we suggest. 

If you look at the dotted line, you will see the minimum temperatures we suggest, with anti-legionella cycles. The orange line represents suggested store temperatures for the vulnerable.

• If you have a combination boiler turning down from 60°C hot water to 40°C could make your boiler 10% more efficient on hot water production
  
• By turning down your hot water cylinder from 60°C to 45°C your cylinder will lose 40% less heat from both your store and pipework. 

• Your heat source will cycle less as the cylinder cool down is slower and effective cylinder coil size is larger. 

• The larger effective coil output and lower temperatures also mean your condensing gas boiler could be 10% more efficient

• If you have a heat pump it will be up to 25% more efficient on hot water! If you have a slightly older heat pump that still requires an immersion for legionella cycles that will also stop that pesky immersion turning on with a cop of 1 saving even more energy.

• All of this will directly reduce your monthly bills.

Extra tips for energy saving and keeping legionella at bay

• Insulate your loft tank if you have one, to maintain a lower temperature. 
• If you do have a cold water store in the loft, make sure the cold feed to the tank and the draw from the tank are from opposite ends to encourage full water turnover.
• Reduce stagnation by flushing unused traps
• Insulate secondary circulation pipework if you have it

For my family home personally, as I turnover around 100% of my stored water each day, I keep my stored water at 45°C and don't do anti legionella cycles.

For more advice on energy saving, watch our video: 11 Easy Ways To Reduce Your Energy Bills | SAVE UP TO 51%! On YouTube.

References

Legionella monthly reports January to October 2020 aggregate report

Legionella - Wikipedia