Do You Need a Cylinder Thermostat With a New Boiler?

Yes, if your new boiler heats a hot water cylinder, it should have a cylinder thermostat. The cylinder thermostat tells the system when the stored water has reached temperature, so a valve closes and the boiler stops firing for hot water. The guidance that goes with Building Regulations expects stored hot water heated by a boiler to have a cylinder thermostat, a timer and boiler interlock, and a boiler replacement is when missing controls are normally put right. With a combi there is no cylinder, so there is no cylinder thermostat.

What does a cylinder thermostat do?

A cylinder thermostat measures the temperature of the stored water and switches the hot water circuit on and off around a set point. On most systems it is a small box strapped to the side of the cylinder about a third of the way up, or a sensor in a pocket in the cylinder. Unvented cylinders come with one fitted by the manufacturer.

When the water is below the set point, the thermostat calls for heat. A motorised valve opens to let hot water from the boiler flow through the cylinder's coil, and the boiler fires. When the water reaches temperature, the thermostat switches off, the valve closes and, through the valve's switch, the boiler is told there is no demand. That last step is boiler interlock.

Why boiler interlock matters

Boiler interlock means the boiler only fires when something is actually asking for heat: the room thermostat for the heating, or the cylinder thermostat for the hot water. Without it, a boiler can keep firing when the cylinder is already hot and the rooms are warm, heating water it then has nowhere to send. That wastes gas and makes the boiler cycle on and off.

On a typical system with a cylinder, interlock is done through the wiring: the cylinder thermostat controls the hot water valve, the room thermostat controls the heating valve, and the switches on those valves are what turn the boiler on. It is not a separate gadget. It is the controls wired so that each one has a say.

Old systems that have no cylinder thermostat

Plenty of older London homes have a cylinder with no thermostat at all. The most common reason is a gravity hot water system: the boiler heats the cylinder by natural circulation, without a pump or a valve on the hot water side, so the water gets as hot as the boiler makes it for as long as the boiler is on. These systems are simple and have lasted decades, but they cannot control hot water temperature properly and do not have boiler interlock.

Many modern boilers are not designed to run a gravity circuit, so replacing the boiler is usually the point where the system is converted to fully pumped, with a motorised valve and a cylinder thermostat added. That conversion should be on your quote as a line item. The layout changes involved are part of any system boiler replacement where the existing controls are out of date.

Unvented cylinders have more than one thermostat

An unvented cylinder does not rely on one thermostat. It normally has:

• A control thermostat that sets the normal stored temperature, doing the same job as a strap-on cylinder thermostat.
• A high-limit thermostat that cuts the heat supply if the water gets far too hot, for example if the control thermostat or valve fails.
• A temperature and pressure relief valve as the final safety device, which discharges water if both of the above fail.

If the cylinder has an immersion heater, that has its own thermostat and cut-out too. These are safety controls, set by the manufacturer and checked by a G3 qualified engineer. They are not something to adjust or bypass. How an immersion fits in with a new cylinder is explained in do you need an immersion heater with a new cylinder.

What temperature should a cylinder thermostat be set to?

Around 60°C is the usual setting for stored hot water. Health and Safety Executive guidance on legionella says hot water should be stored at 60°C or above, because the bacteria can grow in water kept lukewarm for long periods. Setting it much lower to save gas raises that risk. Setting it much higher wastes energy, adds to the risk of scalding at the tap and speeds up limescale in hard-water areas, which covers most of London.

If the water at the taps is too hot for a household with young children or elderly relatives, the answer is usually a thermostatic mixing valve at the outlet, not a lower cylinder setting. Your installer should set the thermostat at commissioning and note it on the paperwork.

Smart controls and wireless cylinder sensors

Some smart heating systems can control hot water as well as heating, with a schedule in an app and, on some products, a wireless sensor on the cylinder. That can replace the strap-on thermostat as the device that calls for hot water, but it does not replace the high-limit safety cutout on an unvented cylinder, which stays wired in. If you are choosing controls with a new boiler, check that the product you like handles hot water on a cylinder and not just heating. Is a smart thermostat worth it with a new boiler covers the wider choice.

What to check on your quote

If you are keeping a cylinder, your quote for a new system or regular boiler should say whether the existing cylinder thermostat and motorised valves are being kept, replaced or added, and confirm the controls will give full boiler interlock. If it says nothing about controls, ask. If you are converting to a combi, the cylinder and its thermostat come out. Our survey looks at the cylinder, valves and wiring along with the boiler, and the fixed written quote lists the controls you will end up with. Book a free survey to see what your system needs.