What temperature should a hot water cylinder be set at?
A hot water cylinder should store water at 60°C or above. That is the temperature the Health and Safety Executive gives for controlling legionella bacteria, which grow best between 20°C and 45°C. Once the water is flowing, the hot taps should reach at least 50°C within about a minute of running. Going much hotter than 60 to 65°C wastes energy and raises the risk of scalding, so if young children or older people live in the home, the answer is to keep the cylinder at 60°C and fit a thermostatic mixing valve on the bath, not to turn the cylinder down.
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Discuss your plumbing workWhy 60°C is the number
Legionella bacteria live naturally in water at low levels. They become a problem when stored water sits in the range where they multiply, roughly 20°C to 45°C, and are then breathed in as fine droplets, usually from a shower. At 60°C they are killed within a few minutes, and at 50°C they cannot grow. That is why HSE guidance asks for hot water to be stored at 60°C and to reach the outlets at 50°C or more.
The risk is highest at the bottom of a cylinder, where incoming cold water mixes with the hot and the temperature can sit well below the thermostat setting. A cylinder set to 60°C heats the whole volume enough to keep that cooler layer under control, especially if it is heated to full temperature every day.
Recommended temperatures at a glance
These figures cover the common set-ups in London homes. Your cylinder or boiler manufacturer's instructions take priority where they differ.
| Where | Temperature | Why |
|---|---|---|
| Stored water in the cylinder | 60°C or above | Controls legionella in the stored volume |
| Hot taps after about a minute of running | At least 50°C | Shows hot water is reaching the outlets hot enough |
| Bath outlet in new homes and conversions | No more than 48°C | Building Regulations limit to prevent scalding, usually met with a mixing valve |
| Cylinder thermostat on most gas-heated systems | Around 60 to 65°C | Enough margin without wasting heat |
| Combi boiler hot water setting | Follow the manufacturer | No stored water, so the storage rule does not apply in the same way |
Balancing legionella and scalding
Water at 60°C can cause a serious burn in seconds, which is the reason some people turn their cylinder down. That solves one risk by creating another. A cylinder held at 45 to 50°C sits close to the range where legionella grows, and the cooler water at the base of the cylinder can be well below that.
The safer approach is to store hot at 60°C and blend it down at the point of use. A thermostatic mixing valve does this, mixing hot and cold to a fixed safe temperature for the bath or basin and shutting off the hot if the cold fails. Our guide to the thermostatic mixing valve explains which type suits a home and where it is fitted. Thermostatic showers do the same job for the shower.
If anyone in the home is vulnerable to scalding, do not rely on turning the cylinder down. Keep storage at 60°C and have mixing valves fitted to the baths and basins they use.
How to check and adjust your cylinder temperature
The simplest check is at a tap. Run the hot tap nearest the cylinder for one minute and hold a cooking or digital thermometer in the flow. If it reads 50°C or more, the stored water is almost certainly hot enough. If it struggles to reach 50°C, the thermostat may be set too low, the heating may not be running long enough, or a part may be failing.
On a cylinder heated by a gas boiler, the temperature is set on the cylinder thermostat, a dial usually strapped to the side of the cylinder about a third of the way up, marked in degrees. On an electric or immersion-only cylinder, the thermostat sits under the immersion heater cover, next to live electrical terminals. Turn off the power at the isolator before that cover comes off, or leave the adjustment to an electrician or plumber.
Unvented cylinders, such as a Megaflo, have factory-set controls and safety devices that must not be altered. Only someone qualified to work on unvented systems should adjust or replace them, and they should be serviced regularly.
- Run the hot tap for one minute and take a reading
- Check the cylinder thermostat dial reads around 60 to 65°C
- Make sure the hot water timer gives the cylinder long enough to reach temperature at least once a day
- Keep the cylinder insulated, so it holds its heat between heating periods
- Book a check if the water never gets hot enough, or gets scalding hot
Heat pumps, solar and lower-temperature systems
Heat pumps run most efficiently at lower temperatures, so many heat pump cylinders keep water around 45 to 50°C day to day. To manage legionella, they are set to run a regular high-temperature cycle, often weekly, that lifts the whole cylinder to 60°C or above, usually with an immersion heater. That cycle should be left on. Solar thermal cylinders work in a similar way, with a boiler or immersion topping up the heat when the sun has not done enough.
If you are replacing an old cylinder for a heat pump, our guide to heat pump cylinder compatibility covers the coil size and controls that matter.
Signs the temperature control has gone wrong
Hot water that is suddenly scalding, a cylinder too hot to touch, steam, or water running out of the tundish, the small open funnel on the pipework near the cylinder, mean the thermostat has failed and the safety devices have taken over. On an unvented cylinder, a separate high-limit cut-out and a temperature and pressure relief valve that opens at around 90°C are there for exactly this. Switch off the boiler and any immersion heater, leave the cold supply on, and call an engineer.
The opposite problem, water that is lukewarm or runs cold quickly, often points to a thermostat, a zone valve or an immersion element that has failed. Our guides to cylinder thermostat fault signs and hot water running cold cover how to narrow it down.
Getting your cylinder checked
Loyal Plumbers services and repairs vented and unvented cylinders, cylinder thermostats and immersion heaters across London and inside the M25, with 10 vans working 24 hours a day. Gas work on the boiler side is done by Gas Safe registered engineers. Our call-out charge is £60.
When you get in touch, tell us the cylinder make if you can see it, whether it is heated by a boiler, an immersion heater or both, and the temperature you measured at the tap. That is usually enough for us to bring the right parts.
Common questions
- Is 50°C too low for a hot water cylinder?
- For stored water, yes. HSE guidance is to store hot water at 60°C or above. 50°C is the minimum the water should reach at the taps, not the storage temperature.
- Is 70°C too hot for a hot water cylinder?
- It is hotter than needed. It wastes energy, increases limescale and makes scalding more likely. Around 60 to 65°C is enough for most homes, unless the manufacturer says otherwise.
- Does a combi boiler need to be set to 60°C?
- Not in the same way. A combi heats water as it flows and stores little or none, so the storage rule does not apply. Set the hot water as the manufacturer recommends.
- Will turning my cylinder down save money?
- A little, from lower heat loss, but going below 60°C trades that saving for a legionella risk. Better savings come from insulating the cylinder and heating it only when you need hot water.
- Where is the thermostat on a hot water cylinder?
- On a boiler-heated cylinder it is usually a dial strapped to the side of the cylinder. On an immersion-heated cylinder it is under the immersion cover, beside live terminals, so isolate the power first.
Sources and further reading
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