If you store hot water in a cylinder, whether it is heated by a gas boiler or an electric immersion, the temperature you set it to is a balancing act. Too low and you risk bacteria growing in the tank. Too high and you burn energy keeping water hotter than anyone needs, with a real scald risk at the tap on top. The widely accepted answer for a stored cylinder is around 60C, hot enough to keep the water safe, without pushing the temperature so high that you pay to store heat you will never use. Here is why that number matters, what setting it higher actually costs, and how to keep the water safe at the tap.
The short answer. Set a stored hot water cylinder to about 60C. That is hot enough to control legionella bacteria in the tank, and no hotter than it needs to be, so you are not wasting energy or storing scalding water. Then bring the temperature down at the tap, ideally with a blending valve, so what comes out is comfortable at around 45C to 48C. Combi boilers that heat water on demand are a different case and do not store water, so this cylinder guidance does not apply to them in the same way.
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This is about stored water, not combi boilers
First, work out which kind of hot water you have, because the advice differs. A hot water cylinder, usually a tall insulated tank in an airing cupboard or loft, stores a large volume of heated water ready for use. It might be warmed by a gas boiler through a coil, or by an electric immersion heater, or both. Because that water sits around, its temperature matters for safety as well as cost. A combi boiler, by contrast, heats water instantly as you draw it and stores none, so there is no standing tank of warm water for bacteria to grow in. Everything below is about stored cylinders. If you have a combi, the temperature question is really about efficiency and comfort at the tap, not storage safety.
Why 60C is the number to aim for
The reason you cannot simply set a cylinder as low as you like comes down to legionella, the bacteria that can cause a serious lung infection. It thrives in warm, still water, roughly between 20C and 45C, which is exactly the range a lukewarm tank would sit in. Keeping stored water at 60C holds it above the temperature where the bacteria multiply, and periodically reaching that mark kills any that have taken hold. That is why the standard guidance for stored domestic hot water settles on about 60C in the cylinder rather than the merely warm setting you might reach for to save money. It is one place where the cheapest setting is not the right one, and it is worth respecting. If you are unsure how your system is set up, or anyone in the home is elderly or has a weakened immune system, treat the safe temperature as the priority and get advice from a qualified heating engineer.
Why hotter than that just wastes money
If 60C is the floor for safety, it is also close to the ceiling for sense, because heating water beyond it costs more in two ways. The obvious one is the extra energy to lift the whole tank those additional degrees every time it reheats. The quieter one is standing loss: a hotter tank loses heat to the cupboard around it faster, because the bigger the gap between the water and the room, the quicker the warmth leaks away, so you pay to reheat it more often. Neither buys you anything useful, since you only ever mix that stored water back down with cold before it reaches your skin. Setting a cylinder to 65C or 70C in the belief that hotter is better simply means paying to store and lose heat you then throw away at the tap. A good cylinder jacket and lagged pipes cut the standing loss whatever the temperature, and the cylinder jacket and pipe lagging guide covers that cheap fix.
What the difference actually costs
Put some numbers on it. Heating water takes about 0.116 kWh to raise 100 litres by one degree, so a 150-litre cylinder needs roughly 0.17 kWh for each degree across the whole tank. Setting the cylinder 5C hotter than you need, say 65C instead of 60C, adds around 0.87 kWh to every full reheat, plus the extra standing loss that follows all day. At an example electricity rate of 26p per kWh that is roughly 23p added to each full heat-up, and if the immersion reheats the tank most days it stacks into somewhere around £40 to £80 over a year for nothing. On a gas-heated cylinder the unit cost is lower, so the waste is smaller in pounds, but it is still money spent storing heat you never touch. To run the sum for your own cylinder size and rate, the immersion heater cost calculator does the working.
| Cylinder setting | Safe from legionella? | Energy and cost effect |
|---|---|---|
| Below ~50C | No, risk of bacteria in stored water | Cheapest to run, but not safe for a tank |
| Around 60C | Yes | The sensible target: safe, no wasted heat |
| 65C to 70C | Yes, but no added benefit | Extra energy and standing loss, higher scald risk |
Illustrative. Exact costs depend on cylinder size, how often it reheats, insulation and your unit rate.
Safety at the tap: keep the hot, lose the scald
Storing water at 60C solves the bacteria problem but creates another, because water at 60C will scald skin in seconds. The answer is to store hot and deliver cooler. The proper way to do that is a thermostatic mixing or blending valve, which blends the stored hot water with cold before it reaches the tap, so the water you actually touch comes out at a safe and comfortable level, usually around 45C to 48C. Many modern systems already have one fitted. If yours does not, and especially if there are young children or older people in the home, it is worth asking a heating engineer to add one, because it lets you keep the cylinder safely hot without risking a burn at the basin or bath. Do not solve the scald risk by simply turning the cylinder down below 60C; that trades one hazard for another.
Getting the setting right on your system
How you set the temperature depends on what heats the tank. Where a gas boiler warms the cylinder, there is usually a cylinder thermostat strapped to the side of the tank, a small dial you can turn to around 60C; the boiler's own hot water setting may also need to be high enough to reach it. An electric immersion heater has its own thermostat, often behind a cover on the heater itself, adjustable with a screwdriver, again to about 60C. If you are on a night tariff such as Economy 7, a well-lagged cylinder set to 60C and heated on the cheap overnight rate is a sensible way to run an immersion, since the water stays warm enough to last the day. Whatever the system, set it once to 60C, check it holds, and leave it. There is little to gain from fiddling, and dropping it below safe purely to shave pennies is a false economy.
The bottom line
For a stored hot water cylinder, about 60C is the setting to aim for: warm enough to keep legionella at bay, and not so warm that you are paying to store and lose heat you will never use. Then bring it down to a comfortable 45C to 48C at the tap with a blending valve so nobody gets scalded. If you run a combi, none of the storage safety applies, and the temperature is just a matter of comfort and efficiency. The real savings on hot water come not from running a tank dangerously cool but from lagging the cylinder, fixing dripping hot taps, and using less hot water in the first place, which the hot water savings guide sets out. For the shower and bath side of the bill, see bath versus shower cost.
This guide is general information, not a substitute for advice from a qualified heating engineer. Legionella and scald risks are real; if in any doubt about your system, or if anyone in the home is vulnerable, get professional advice.
Common questions
What temperature should a hot water cylinder be set to?
About 60C. That is hot enough to control legionella bacteria in the stored water, without wasting energy keeping it hotter than needed. Then use a thermostatic blending valve to bring the water down to a safe 45C to 48C at the tap.
Is it safe to turn my hot water down to save money?
You can turn a combi boiler's hot water setting down, since it stores no water. But a stored cylinder should not go below about 60C, because bacteria can grow in warm, still water. Save money instead by lagging the cylinder, fixing leaks and using less hot water, not by running the tank dangerously cool.
Does setting hot water hotter than 60C cost more?
Yes. Heating a cylinder to 65C or 70C uses more energy to lift the whole tank, and a hotter tank loses heat to the surrounding cupboard faster, so it reheats more often. Since you mix that water back down with cold anyway, the extra heat is wasted, often tens of pounds a year for no benefit.
Does this apply to a combi boiler?
Not in the same way. A combi heats water on demand and stores none, so there is no standing tank for bacteria to grow in. With a combi the temperature is about efficiency and comfort at the tap rather than storage safety.