What Is a Cylinder Thermostat?

A cylinder thermostat is a device that attaches to the outside of a hot water tank. Usually fitted about one quarter to one third of the way up from the bottom of the tank cylinder, the thermostat can turn on a boiler if the water temperature is below the desired setting. The device typically shuts off the boiler when the water reaches the set temperature. Settings often remain around 140°F (about 60°C); otherwise the water can be too hot, raising the risk of burns as well as deposits in the tank or pipes.

Installing a cylinder thermostat is often accomplished by securing it to a retaining strap. This strap typically goes around the cylinder, and should be tight enough for the thermostat to be in direct contact with the wall. If contact with the cylinder wall is not flush, then temperature readings can be inaccurate.

The different types of cylinder thermostat can vary in shape, although most are box-shaped or elongated. Settings can be adjusted using a large dial, or a smaller dial with a slot for a screwdriver. Usually, the devices are identified by the manufacturer’s type and model, so when replacing or repairing a cylinder thermostat, one is often better off knowing the make of the old one to match the wiring. Purchasing a similar device typically makes reconnecting the wires, and reading the instructions, more convenient.

If a cylinder thermostat needs to be removed, then it is generally important to shut off the boiler first. The power for the device’s circuit should usually be isolated as well. After the straps are removed, then the cover of the thermostat can be taken off to test the wiring connections, with a mains tester, to be sure no power is getting through. It often helps to label wires that are unmarked so they can be re-connected to a new cylinder thermostat without any confusion. A new device can then be installed by attaching the appropriate wires and terminals; it can then be fit into the retaining strap against the surface of the cylinder.

When shopping around for a cylinder thermostat, one should typically look at the details that indicate what systems it is most suited for. Some thermostats can be installed on just about any hot water cylinder, while others are more specific. For most, it is usually important to keep the settings not too high, because the water can become hotter than necessary and energy can be wasted.