Hot Water Cylinder Temperature: What Should It Be Set To?

What is a cylinder thermostat and how does it work?

A cylinder thermostat is a small device — usually a plastic box with a dial — that clips or straps onto the outside of your hot water cylinder. It sits roughly a quarter to a third of the way up from the bottom of the tank, which is the point at which the water temperature is most representative of the stored volume.

The thermostat monitors the temperature of the water inside the cylinder. When the water drops below your chosen setting, it sends a signal to your boiler (or immersion heater) to switch on and start heating. Once the water reaches that set temperature, the thermostat cuts the heat off again, stopping unnecessary energy use.

One important thing to understand: the thermostat controls when heating starts and stops — it does not control how quickly the water heats up. The speed of heating is determined by the design of your system, such as the size of the boiler, the cylinder coil, or the immersion element rating. Turning the dial to a higher setting will not make your hot water arrive faster; it will simply result in the water being stored at a higher temperature once it eventually gets there.

What temperature should a hot water cylinder be set to?

The recommended hot water cylinder temperature in the UK is 60°C, and you can safely run up to 65°C. This range is endorsed by the Health and Safety Executive (HSE) and is the setting recommended by virtually every heating manufacturer and installer in the country.

The 60°C figure is not arbitrary — it is set specifically to control the growth of Legionella bacteria (more on that below). At 60°C, around 90% of Legionella bacteria die within two minutes. At 70°C, they are killed almost instantly. Below 50°C, the bacteria can survive and begin to multiply; between 32°C and 42°C they thrive.

If the water coming out of your taps feels uncomfortably hot, your thermostat may be set above 65°C. Turning it down to 60°C is the right move — it is safer, cheaper to run and still well within the safe zone for bacteria control.

Important: do not be tempted to drop the thermostat below 55°C to save money on energy bills. Storing water below this threshold creates a genuine legionella risk and is not recommended under any UK guidance.

Combi boiler households: if you have a combi boiler, you do not have a stored hot water cylinder. Combi boilers heat water on demand, so there is no storage tank and no legionella risk from stored water. The hot water dial on a combi is typically set to 50–55°C. The 60°C rule applies only to stored hot water cylinders.

Legionella and the 60°C rule — why it matters

Legionella pneumophila is the bacterium that causes Legionnaires' disease, a potentially fatal form of pneumonia. People contract it by inhaling fine water droplets (aerosols) that carry the bacteria — from showers, taps and other outlets.

Legionella bacteria thrive in stored water at temperatures between 20°C and 45°C, with the most dangerous growth zone sitting between 32°C and 42°C. They survive in lukewarm, stagnant water — exactly the conditions that can develop in a domestic hot water cylinder if the thermostat is set too low or the system is left unused for a period of time.

The HSE's Approved Code of Practice L8 sets out the control measures for legionella in water systems. For domestic hot water cylinders, the key principles are:

• Store hot water at 60°C or above
• Distribute hot water at 50°C or above at outlets (managed using a thermostatic mixing valve to prevent scalding)
• Keep cold water stored and distributed below 20°C
• Flush infrequently used outlets (spare bathrooms, holiday homes) regularly to prevent stagnation
• De-scale and clean showerheads periodically

For landlords, following HSE Legionella guidance is a legal requirement, including monthly temperature checks at sentinel outlets (the furthest and closest outlets from each cylinder). For owner-occupiers in private homes, there is no legal duty, but the same 60°C recommendation applies and is strongly advised.

A thermostatic mixing valve (TMV) is the device that bridges the gap between safe storage and safe delivery. Your cylinder stores water at 60°C to kill bacteria, but 60°C at the tap would cause serious scalds — a TMV blends hot and cold water to bring the delivery temperature down to around 50°C for general outlets, or as low as 44°C for bath outlets in homes with vulnerable occupants. The pipework downstream of any TMV should be kept as short as possible, as blended water sits in the bacteria danger zone by design.

How to find your cylinder thermostat

Before you adjust anything, you need to locate the thermostat. On most vented (gravity-fed) and unvented cylinders, the thermostat is a small plastic box strapped to the outside of the cylinder, positioned roughly one quarter to one third of the way up from the base. It usually has a rotary dial with a temperature scale printed on it.

Some cylinders — particularly older factory-insulated foam models — have the thermostat tucked behind a removable panel or under the foam jacket. If you cannot see it immediately, check around the mid-lower section of the cylinder and look for a wired connection running to a junction box or wiring centre nearby.

Immersion heaters have their own separate thermostat built into the element itself, accessed via a screwed cap on the top or side of the cylinder. This is a different component from the strap-on cylinder thermostat and requires the power to be isolated before you touch it.

If you have an unvented (pressurised) cylinder — common brands include Megaflo, Telford and Range Tribune — be aware that the thermostat on these systems is also a safety control, not just a comfort dial. Work on unvented cylinders falls under Building Regulations Part G3 and should only be carried out by a suitably qualified engineer. Do not remove panels or interfere with the thermostat mechanism on an unvented cylinder unless you are confident the system power is fully isolated and you understand the safety implications.

Common cylinder thermostat problems and what to look for

A failing cylinder thermostat typically shows one of three patterns:

• Water that scalds at the tap: the thermostat is stuck in the closed position, meaning the heating never switches off. Water overheats significantly above the dial setting.
• No hot water at all: the thermostat is stuck open, so it never calls for heat regardless of how cold the water gets.
• Inconsistent hot water: the thermostat trips and resets unpredictably, giving you hot water sometimes but not others.

Other signs of a thermostat issue include the cylinder continuing to heat long after it should have reached temperature, or stored water consistently running cooler than the dial setting suggests.

However, it is worth noting that the thermostat is not always the culprit. A significant proportion of thermostat replacements turn out to be unnecessary — the real fault was a stuck motorised valve, a seized immersion heater, a faulty programmer, or sludge and limescale build-up in the cylinder coil. In hard-water areas across the East of England, the Midlands and the South East, limescale on heating elements is particularly common and can cause rumbling or kettling sounds from the cylinder.

Rattling or banging from the cylinder can also indicate trapped air or sediment build-up — not necessarily a thermostat problem at all. If your cylinder is making unusual noises alongside temperature problems, have a heating engineer investigate before assuming the thermostat needs replacing.

For unvented cylinders: if you notice the pressure relief or temperature and pressure relief (TPR) valve discharging, or hear unusual sounds repeatedly, stop using the cylinder and call a qualified engineer. This is a safety issue, not a DIY fix.

Energy saving: insulation and the thermostat working together

The cylinder thermostat and your cylinder's insulation work as a team. A well-insulated cylinder loses heat slowly, which means the boiler or immersion heater needs to switch on less often to maintain 60°C. A poorly insulated cylinder haemorrhages heat continuously, costing you money around the clock.

If your cylinder has a loose foam or old fibreglass jacket, check its thickness. UK guidance recommends at least 80mm of insulation. A British Standard cylinder jacket — look for the Kitemark — costs around £20–£30 and could reduce heat loss by more than 75%. According to figures based on October 2025 energy prices, a properly insulated cylinder can save up to £194 per year compared to an uninsulated one. Even adding an 80mm jacket to a cylinder that already has 25mm of factory foam can save around £45 a year.

Combined with basic pipe insulation on the hot water pipes closest to the cylinder, most households can reduce their hot water energy use by 15–20%, typically saving £50–£60 per year.

The key energy-saving principle: keep the thermostat at 60°C (not lower, for legionella safety reasons) and focus your effort on insulation quality, cylinder scheduling and avoiding unnecessary reheat cycles. If your programmer allows it, set the hot water heating period to match when you actually use hot water — most households only need one or two heating periods per day rather than continuous heating.

When to call a Gas Safe registered engineer

There are clear boundaries between what a homeowner can safely do and what requires a professional. Adjusting the thermostat dial, checking the temperature setting, improving insulation and flushing infrequently used outlets are all homeowner tasks.

You should call a Gas Safe registered engineer (or a suitably qualified unvented cylinder engineer for G3 work) if:

• The thermostat dial has no effect on water temperature (stuck thermostat or deeper fault)
• Water at the tap is scalding hot despite a correct thermostat setting
• The cylinder makes persistent banging, rumbling or kettling sounds
• The TPR (temperature and pressure relief) valve on an unvented cylinder is discharging
• You need the thermostat itself replaced or rewired
• Your cylinder is unvented and you need any internal work carried out
• The motorised valve, programmer or wiring centre needs attention
• There is any sign of water leaking from the cylinder or its fittings

Never attempt to open the boiler casing, replace wired components without isolating power, or work on the gas components of any heating system yourself. For anything beyond adjusting an accessible dial, book a professional.

Step by step

1. Turn off the heating and hot water

   Before touching anything, switch off your boiler and hot water timer at the programmer or room thermostat. You do not need to isolate the electricity supply just to read or adjust an external strap-on cylinder thermostat dial, but it is good practice to ensure the system is not actively calling for heat while you work. For immersion heater thermostats, always isolate the power at the consumer unit first.

2. Locate the cylinder thermostat

   Find your hot water cylinder — usually in an airing cupboard, utility room or loft. Look for a small plastic box strapped to the lower third of the cylinder body, with a wired connection and a rotary dial. If the cylinder has a foam jacket or removable panel, check underneath or behind it. The dial will normally show a temperature scale from around 40°C to 80°C.

3. Check the current temperature setting

   Read the position of the dial pointer against the scale. Common factory or installer settings range from 55°C to 70°C. Note what it is currently set to. If it is below 60°C, it needs to be turned up. If it is significantly above 65°C, you are wasting energy and creating a scalding risk.

4. Adjust the dial to 60°C

   Turn the dial slowly until the pointer aligns with the 60°C mark. Most cylinder thermostat dials rotate with gentle finger pressure — you do not need tools. If the dial is stiff, do not force it. If it will not move at all, the thermostat may be faulty and you should call a heating engineer rather than trying to force the mechanism.

5. Switch the heating and hot water back on

   Restore power and turn the hot water programme back on at your programmer or timer. The boiler (or immersion heater) will now begin heating the cylinder to the new 60°C target. Be patient — it typically takes around an hour for the stored water to reach the new set temperature from cold, so do not expect an instant change.

6. Check the hot water temperature at a tap

   After allowing sufficient time for the cylinder to heat up fully (one to two hours), run the hot tap in your bathroom or kitchen for a minute and carefully test the temperature. It should feel very hot but not scalding. If you have a digital kitchen or bath thermometer, a reading of around 50°C at the tap (after a TMV) is ideal. If the water is still scalding hot after waiting, the TMV may need adjustment — call a plumber.

7. Check your cylinder insulation

   While you are at the cylinder, inspect the insulation. Run your hand over the outer surface — if it feels noticeably warm to the touch, heat is escaping and you are paying to heat the room rather than the water. Measure the jacket thickness if possible; it should be at least 80mm. If the jacket is thin, damaged or missing, fitting a new British Standard (Kitemark) cylinder jacket is a simple DIY job that could save you £45–£194 per year depending on your current insulation level.

8. Set a regular hot water schedule on your programmer

   If your programmer or smart thermostat allows it, review your hot water heating times. Most households only need one or two heating periods per day — for example, one early morning period for showers and one early evening period for baths or washing up. Avoid setting continuous heating unless your cylinder loses heat very quickly, as each extra heating cycle costs money.

9. Monitor and recheck after a few days

   Check the temperature at the tap again a few days later to confirm the system is maintaining 60°C consistently. If the water seems cooler than expected, the thermostat may not be making good contact with the cylinder, or there could be an underlying fault with the motorised valve or programmer. If the water is hotter than expected despite the 60°C setting, the thermostat may be faulty — in either case, call a heating engineer.

Typical costs

Typical UK ranges as a guide only — prices vary by region (expect the top end, or 20–30% more, in London and the South East) and by how accessible your system is. Always get a written quote.

Frequently asked questions