What Temperature Should a Hot Water Cylinder Be Set At?

The numbers that matter

Three temperatures do almost all the work here. Legionella bacteria multiply in water between roughly 20 and 45 degrees, and they do not survive at 60 degrees and above. Stored hot water held at 60 degrees stays out of the growth range. Water arriving at the taps at 50 degrees or more within a minute tells you the pipework is not holding lukewarm water either.

The problem zone is a cylinder turned down to 45 or 50 degrees to save money, or a large cylinder where only the top gets properly hot and the bottom sits lukewarm for days. Both leave a volume of warm water sitting still, which is the combination the bacteria need. It is also the usual reason hot water smells like rotten eggs, since the bacteria that make that smell like the same lukewarm conditions.

• Stored hot water: 60°C
• At the hot tap, within about a minute of running: 50°C or more
• Legionella growth range: roughly 20 to 45°C
• Bath outlet in a new dwelling under Building Regulations: no more than 48°C

Where the temperature is actually set

It depends on what heats the cylinder, and many homes have two settings that both matter.

On an indirect cylinder heated by a boiler, the cylinder thermostat decides the stored temperature. It is either strapped to the side of the tank or sits in a pocket, usually with a dial marked in degrees. When the water reaches the setting it tells the motorised valve to close and the boiler stops heating the cylinder. The boiler also has its own flow temperature, and it has to be comfortably higher than the cylinder setting, or the cylinder will never reach 60 however long it runs.

On an immersion heater, the rod thermostat inside the element has a small adjuster behind the plastic cap, normally set at 60. On a direct cylinder with two elements, each has its own. Unvented cylinders often have an immersion thermostat with a separate safety cut-out that has to be reset by hand if it ever trips.

A combi boiler has no stored water, so there is no cylinder to set. The hot water temperature is a dial on the boiler, and Legionella in stored water is not the same concern.

The immersion adjuster sits next to live terminals. Isolate the immersion at the switch and fused spur before removing the cap, or leave it to an engineer.

Why turning it down is a false economy

The saving from a lower cylinder setting is small. You heat the water through fewer degrees and lose a little less through the insulation, but it amounts to pennies a day on a typical household. Against that, water sitting at 45 degrees is warm enough for bacteria to multiply and nowhere near hot enough to kill them.

It also tends not to work as intended. A cylinder set lower delivers less usable hot water, because you mix less cold with it at the tap. Households that turn the thermostat down often find they run out of hot water sooner and end up heating it more often. If the running cost is the worry, the guide to immersion heater running cost covers the changes that do save money.

Why turning it right up is not the answer either

In London the main cost of a very hot cylinder is limescale. The higher the water temperature, the faster calcium comes out of hard water onto the hottest surfaces, which are the boiler coil and the immersion element. A cylinder run at 70 degrees in a hard water area furs its coil noticeably faster than the same cylinder at 60, and a furred coil cannot be cleaned in place. We cover that process in hard water and hot water cylinders in London.

The second cost is scalding. Water from a 60 degree cylinder is already hot enough to scald quickly, especially for young children and older people. At 70 degrees and above the risk rises steeply. The third is that you are running closer to the settings of the safety devices, which are there for failures, not for daily use.

What happens when the temperature goes too high by itself

An unvented cylinder has layers of protection above its normal setting. The control thermostat should stop heating at 60 or so. If it fails, a high limit cut-out stops the heat source. If that fails too, the temperature and pressure relief valve opens at around 90 to 95 degrees and discharges water through the tundish, which is the last line of defence against the stored water boiling.

If your hot water is suddenly scalding, the water is steaming at the taps, or hot water is running into the tundish, one of those layers has already failed. Switch off the immersion and the boiler and read our page on hot water cylinder overheating before doing anything else. That is not a thermostat to adjust; it is a fault to find.

Scalding at the taps: blending valves, not lower storage

Keeping the stored water at 60 and delivering it to the outlet cooler are two separate jobs, and a thermostatic mixing valve is what joins them up. It blends hot and cold to a fixed outlet temperature, and a good one shuts off the hot if the cold supply fails. New build baths are required to have one, and they make sense wherever there are young children, older people or anyone with reduced sensation.

Do you need a thermostatic mixing valve? Our guide covers where they are required, TMV2 against TMV3, and what servicing they need.

Heat pumps and the weekly high temperature cycle

Heat pumps heat water efficiently at lower temperatures, often below 60 degrees. To keep the cylinder safe, heat pump systems are normally set to run a periodic pasteurisation cycle, often weekly, that lifts the whole tank to 60 degrees or more, using the heat pump, the immersion or both. If you have a heat pump cylinder, check with whoever commissioned it that the cycle is set, running and timed for when the tank is full rather than empty.

Landlords: the Legionella duty

Landlords of residential property have a legal duty to assess the risk from Legionella in the water system and control it. For most ordinary flats and houses with a cylinder, HSE guidance is that the assessment can be simple and does not usually require water testing. Keeping stored hot water at 60 degrees, avoiding long lengths of little used pipework and making sure a cylinder that has stood empty between tenancies is heated and flushed before anyone moves in are the practical controls.

The wider set of obligations, from repair timescales to annual servicing of unvented cylinders, is covered in landlord hot water responsibilities.

How to check your own cylinder in five minutes

Run the hot tap nearest the cylinder for one minute and hold a kitchen or cooking thermometer in the stream. You want 50 degrees or more. Then do the same at the furthest hot tap in the property. If the near tap is fine and the far one takes much longer, the pipe run is long and poorly insulated, which is a pipework issue rather than a cylinder one.

If the nearest tap will not reach 50 degrees after the cylinder has had its full heating time, something is not holding temperature: a thermostat set low or failed, a boiler flow temperature set too low, a failed element, or a furred coil. Water not getting hot works through those causes in order.

If you want it checked properly, call 020 4652 5111. On an unvented cylinder the temperature and safety controls are checked as part of an unvented cylinder service, and an engineer holding the unvented qualification will set and test them.