Cylinder Thermostat Setting: The Right Temperature for Stored Water

For most homes, the right cylinder thermostat setting is 60°C. That is hot enough to stop legionella bacteria growing in stored water, without heating the tank more than you need. Lower settings save a little energy but increase the legionella risk, and higher settings increase scalding risk and heat loss. If anyone in the home is vulnerable to scalding, keep the cylinder at 60°C and limit the temperature at the taps with thermostatic mixing valves instead of turning the cylinder down.

This guide explains where the cylinder stat lives, how it works with your boiler and immersion heater, how to check the real water temperature, and how to choose the right setting for your household.

What does a cylinder thermostat do?

If you have a system or regular (heat-only) boiler, your hot water is stored in a cylinder, usually in an airing cupboard. The boiler heats the water indirectly through a coil inside the tank. The cylinder thermostat measures the water temperature and, when it reaches the set point, switches off the call for hot water. On most systems that closes the hot water zone valve or moves the three-port valve, and the boiler stops heating the cylinder.

Without a working cylinder stat, the boiler would keep heating the cylinder whenever the hot water timer is on, wasting gas and making the water dangerously hot. That is why a cylinder thermostat is one of the expected controls for stored hot water. If you are unsure how your valves are arranged, our guide to S plan vs Y plan heating systems explains the layouts.

Cylinder thermostat position: where to find it

Open your airing cupboard and look at the side of the cylinder.

• Strap-on stat: the most common type on vented copper cylinders. A small grey or white box with a dial, held on by a metal strap, usually around a quarter to a third of the way up the tank.
• Pocket stat: on many unvented and factory-insulated cylinders, the sensor sits in a pocket inside the tank, with the dial on a control panel on the cylinder.
• Immersion heater thermostat: separate from the cylinder stat. It sits under the immersion heater cap at the top or side of the tank and controls only the electric element.

The position matters. The stat measures the water where it is mounted. If it has slipped down the cylinder, or is mounted too high, the boiler may stop heating before enough of the tank is hot, or heat for longer than needed. A strap-on stat also needs good contact with bare copper, so it should sit under a cut-out in the foam insulation, not on top of it.

Hot water cylinder temperature: why 60 degrees?

The main reason is legionella. Legionella bacteria thrive in lukewarm stored water and are controlled by keeping hot water hot. The HSE’s guidance on controlling legionella in hot water systems says hot water should be stored at 60°C at least to kill legionella, and that water at outlets should reach at least 50°C within one minute of running. That guidance is written for workplaces and rented properties, but the same principle applies to any stored hot water system.

The balance looks like this:

Higher temperatures also speed up limescale build-up on the coil and immersion element in hard water areas, which reduces efficiency over time.

How to check and set your cylinder stat at 60 degrees

1. Find the stat and note the current setting.
2. Set the dial to 60°C. Some dials show numbers or a scale rather than degrees; check the label or manufacturer’s leaflet.
3. Let the cylinder fully heat. Run the hot water programme until the boiler stops heating the tank. How long that takes depends on the cylinder size, the coil and the boiler.
4. Measure at the nearest hot tap. Run the tap for a minute, then hold a cooking or digital thermometer in the flow. You should see at least 50°C, and close to 60°C at a tap near the cylinder.
5. Adjust if needed. If the water is much cooler than the dial setting, the stat may be loose, badly positioned or faulty.

Take care: water at 60°C can scald quickly. Keep hands out of the flow while measuring and supervise children nearby.

If you have a timer for hot water

The cylinder stat only works when the hot water programme is on. If your timer gives hot water for only a short period each day, the tank may never reach 60°C. Make sure the programme runs long enough for the tank to fully heat at least once a day. Some cylinders and controls also have a weekly pasteurisation cycle. Our guide to cylinder pasteurisation and legionella explains how that works.

Managing scalding without turning the cylinder down

Water at 60°C is hot enough to scald quickly, which is a concern for young children, older people and anyone with reduced sensation. The right answer is to store water hot and blend it down where it is used.

• Thermostatic mixing valves (TMVs) blend hot and cold water at the bath or basin to a safe outlet temperature.
• Thermostatic showers do the same for the shower.
• Building Regulations: Approved Document G covers hot water safety in England, including limiting the temperature of hot water delivered to baths in new and refurbished homes.

Fitting mixing valves is plumbing work, and in homes with vulnerable occupants it is worth having them checked regularly.

Unvented cylinders, combis and heat pumps

Unvented cylinders (mains pressure, often white with a pressure vessel and safety valves) have built-in control and high-limit thermostats as part of their safety system. You can adjust the control thermostat setting, but the safety devices must only be worked on by someone qualified for unvented systems. Our unvented cylinder service guide explains the annual checks these cylinders need.

Combi boilers have no cylinder or cylinder stat. Hot water temperature is set on the boiler’s control panel. See our guide to combi boiler hot water temperature settings.

Heat pump cylinders are often run at lower everyday temperatures for efficiency, with a regular legionella cycle using an immersion or boost to raise the tank temperature. Follow the installer’s settings.

Signs your cylinder thermostat is faulty

• Hot water is scalding even with the dial set to 60°C.
• Hot water never gets properly hot, even after a long heating period.
• The boiler keeps heating the cylinder long after the water is hot.
• The dial turns but you hear no click from the stat.

Cylinder thermostats are mains-wired and linked to the zone valves and boiler, so a replacement should be done by a heating engineer. If your boiler also seems to be cycling on and off, read our article on no hot water but the heating works for related faults.

Need help with your hot water cylinder?

If your cylinder thermostat is faulty, your hot water is too hot or never hot enough, or you want mixing valves fitted, our engineers can help. We work on vented and unvented cylinders and heating controls across Stoke-on-Trent, Newcastle-under-Lyme, Stafford, Crewe and the surrounding area. Find out about our central heating services or contact Gasmaster to book a visit.

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