Hot water cylinder thermostats

Where hot water is stored in a cylinder and heated by a separate boiler, a thermostat is required to avoid the cylinder being overheated.

Gravity feed copper cylinders

Typically found on older systems, although these can still be purchased for new systems. The water supply for these is via a cold water storage tank, usually in the loft or on a shelf above the cylinder.

The cylinder is formed from thin copper, and usually has a foam covering. Old ones may be bare metal.

The thermostat is purchased as a separate item, and placed against the metal surface of the cylinder. It is secured in place with an elastic strap or spring. Where the cylinder has foam insulation, this must be cut away where the thermostat is fitted, so the thermostat is in contact with the metal.

The thermostat is normally positioned about 30cm up from the bottom of the cylinder.

Mains pressure cylinders

Mains water is connected to the cylinder via a pressure reducing valve. There will also be other items such as an expansion vessel and safety valves.
This type of cylinder is oftem made from stainless steel.

The thermostat is normally supplied preinstalled on the cylinder, so all that is required is to connect wiring to the thermostat.


3 wire changeover cylinder thermostat Cylinder thermostats are usually changeover types with three terminals. The terminals are:

When the stored water is cool and heat is required, the Common and Calling terminals are connected together.
When the water reaches the required temperature, the Common and Satisfied terminals are connected.

Normally, 4 core heat resistant flex is used to connect the thermostat to the wiring centre. (Earth, Common, Calling, Satisfied). Cylinder thermostats do not have any neutral connection, just permanent live and two switched lives.

Some heating systems do not require all three terminals. For these, only the Common and Calling terminals are required, and 3 core flex can be used.


Where the cylinder is not insulated with foam, or the foam is thin, additional insulation is often added in the form of a jacket or other covering. This should NOT be placed over the thermostat or the wiring.

Temperature setting

Cylinder thermostats are normally set to about 60C. Above this the water will be dangerously hot, and below this, bacteria may grow in the cylinder and become a health hazard.