Hot Water Tank Thermostat

A hot water tank thermostat, also known as a cylinder thermostat, controls the temperature of the water stored in your hot water cylinder.

It works separately from the room thermostat that controls your central heating. When the stored water reaches the set temperature, the cylinder thermostat signals that further heating is no longer required.

Getting the setting right helps maintain a reliable supply of hot water while keeping stored water at an appropriate temperature.

What does a hot water tank thermostat do?

A hot water tank thermostat monitors the temperature of the water stored inside your cylinder.

When the water falls below the selected temperature, the heating controls can call for more heat. Once the required temperature is reached, the thermostat signals that heating is no longer needed.

This helps prevent the cylinder from continuously heating water and keeps the stored water close to the required temperature.

A cylinder thermostat shouldn't be confused with your room thermostat. Your room thermostat controls central heating, while the cylinder thermostat controls stored hot water.

What temperature should a hot water tank thermostat be set to?

For most homes with a hot water cylinder, around 60°C is the key setting to remember.

Where temperature is being used to control Legionella, hot water should be stored at a minimum of 60°C. Setting the cylinder significantly lower can create conditions that are more favourable for bacterial growth.

Setting it unnecessarily high isn't recommended either. Higher water temperatures increase the risk of scalding and can increase heat loss from the cylinder.

The temperature stored inside the cylinder also shouldn't be confused with the temperature that's appropriate at a bath, shower or tap. Where scalding is a significant risk, thermostatic mixing valves can be used to reduce the temperature closer to the point of use.

Why isn't my hot water tank thermostat working properly?

If your stored water is unexpectedly cold, excessively hot or inconsistent, the cylinder thermostat could be one possible cause.

Signs that may warrant investigation include:

• Water consistently failing to reach the expected temperature.

• Stored water becoming much hotter than the thermostat setting.

• Hot water temperature becoming unusually inconsistent.

• The cylinder continuing to heat when it should have reached temperature.

However, these symptoms don't automatically confirm a faulty thermostat. Problems elsewhere in the hot-water system or its controls can produce similar symptoms.

If the problem persists, a qualified heating professional can test the thermostat and associated controls to establish the actual cause.

Can a faulty cylinder thermostat cause no hot water?

It can, but a faulty thermostat is only one possible cause.

If the thermostat or its associated controls aren't operating correctly, the system may fail to call for heat when the stored water temperature falls.

Before assuming the thermostat needs replacing, check that your hot-water programmer is set correctly and that the problem affects the stored hot-water supply generally.

If your cylinder still isn't heating, the thermostat and wider hot-water controls should be professionally checked.

Can a faulty thermostat make the water too hot?

Yes. A fault that prevents the system from controlling the stored-water temperature correctly can potentially result in excessively hot water.

Water becoming significantly hotter than expected should be taken seriously rather than compensated for by repeatedly changing the thermostat setting.

This is particularly important with unvented hot water cylinders, which incorporate safety controls and should be worked on by someone with the appropriate competence for that type of system.

If your hot water suddenly becomes much hotter than normal, arrange for the system to be checked.

How do you adjust a hot water tank thermostat?

The exact controls depend on your cylinder and heating system, so check the manufacturer's instructions before making changes.

If the cylinder thermostat has an accessible temperature control, set it to around 60°C. Avoid reducing the stored-water temperature significantly below this in an attempt to save energy.

After making an adjustment, allow the cylinder time to heat before assessing whether the change has worked.

For more information on different heating controls, see iHeat's guide to adjusting water temperature on your boiler.

Does turning down the cylinder thermostat save energy?

Reducing unnecessarily high temperatures can reduce heat loss, but a hot water cylinder shouldn't simply be turned down as far as possible to save energy.

Where temperature is being used to control Legionella, stored hot water should be maintained at 60°C or above.

Other measures, such as ensuring the cylinder is adequately insulated and using your hot-water programmer appropriately, can help reduce unnecessary energy use without compromising the stored-water temperature.

When should a hot water tank thermostat be replaced?

A cylinder thermostat may need replacing if testing confirms that it is no longer controlling the stored-water temperature correctly.

Possible signs include:

• Water consistently becoming too hot.

• Water failing to reach the set temperature.

• Unpredictable hot-water temperatures.

• The thermostat failing to control when the cylinder is heated.

These symptoms can have other causes, so they don't prove that the thermostat itself has failed.

A professional can test the thermostat and associated controls before deciding whether replacement is necessary.

Can you replace a hot water tank thermostat yourself?

Replacing a cylinder thermostat can involve electrical connections and interaction with the hot-water controls, so it isn't a job to attempt unless you're appropriately competent to do so.

The correct thermostat must also be compatible with the cylinder and heating controls.

For an unvented hot water cylinder, work affecting its safety controls should be carried out by someone with the appropriate competence and qualifications.

If you suspect the thermostat has failed, arrange for it to be properly tested rather than attempting to bypass, rewire or modify the controls yourself.

Hot water tank thermostat FAQs

What temperature should a hot water tank thermostat be set to?

A hot water cylinder thermostat should generally be set to around 60°C. This keeps stored water at an appropriate temperature where temperature is being used to control Legionella.

Is 65°C too high for a hot water cylinder?

A setting of 65°C isn't necessarily abnormal, but around 60°C is generally sufficient for stored hot water where temperature is being used to control Legionella.

Higher temperatures can increase heat loss and the risk of scalding, so there is usually little benefit in setting the cylinder unnecessarily high.

Can I set my hot water tank thermostat below 60°C?

Stored hot water should generally be maintained at 60°C or above where temperature is being used to control Legionella.

If water at the tap, bath or shower feels excessively hot, don't automatically solve this by significantly lowering the cylinder temperature. Temperature can be controlled closer to the point of use where appropriate.

For more detail, see iHeat's guide to what temperature hot water should be at the tap.

How do I know if my hot water tank thermostat is faulty?

Possible signs include water becoming too hot, failing to reach the set temperature or becoming unexpectedly inconsistent.

However, these symptoms can also be caused by other parts of the hot-water system. The thermostat and associated controls should be tested before it's replaced.

Is a hot water tank thermostat the same as a room thermostat?

No. A cylinder thermostat controls the temperature of stored hot water, while a room thermostat monitors the temperature inside your home and controls the demand for central heating.

Changing your room thermostat won't directly change the temperature of the water stored in your cylinder.

Why is my hot water too hot even though the thermostat is set correctly?

If your cylinder thermostat is set correctly but the water is becoming excessively hot, the thermostat or another part of the hot-water controls may not be operating correctly.

Rather than repeatedly changing the setting, arrange for the system to be checked, particularly if the water is becoming dangerously hot.

Keeping your hot water cylinder at the right temperature

For most homes with stored hot water, around 60°C is the key cylinder thermostat setting.

The thermostat should control the stored-water temperature reliably without allowing the cylinder to become unnecessarily hot. If your water suddenly becomes too hot, too cold or inconsistent, the cause should be identified rather than simply changing the temperature setting.

Keep the cylinder thermostat separate from your room-heating controls, follow the instructions for your particular system and have suspected thermostat or control faults professionally checked.