The hot water cylinder thermostat controls the temperature of the water in the cylinder. They are mounted against the outer wall and are usually set about 1/3 of the way up from the base. The normal setting for the hot water thermostat is around 60 degrees centigrade - that’s about as hot as your hand can stand. Hotter than this is dangerous, is a waste of energy, and can also contribute to increased lime scale deposits in hard water areas.
The hot water cylinder thermostat is fastened to the cylinder by a retaining strap round the cylinder itself. In order for the stat to work properly it must have good contact with the wall of the cylinder. The thermostat senses the temperature of the water inside the cylinder and switches the hot water system on or off according to the selected temperature. A stat which is loose will not work properly as the temperature sensed will be false.
Buying the Replacement Cylinder Thermostat
When buying a replacement it's best to get the same type and model if possible. The wiring for the new thermostat will be clearly detailed in the instruction leaflet but replacement with a matching model makes things easier. If you have any doubts at all about the wiring connections, get a professional electrician to advise or to do the job. It should be noted that new stats are often double insulated meaning that no earth wire is required.
Removing the Old Thermostat
Turn off the boiler and isolate the power for this circuit at the consumer unit, making sure that the power to the thermostat itself has been disconnected. Remove the thermostat from the cylinder by undoing the retaining straps. Remove the cover of the thermostat to expose the wiring connections and use a mains tester to check that the power is definitely off.
Labelling the Wires
Check the markings on each wiring terminal and label each wire accordingly with a small piece of tape. You will need these details later. If, on inspection, there are no markings on the wiring terminals, you will need to consult an electrician rather than doing it yourself. Disconnect all the wires and insulate the end of each as you go so that they cannot come into contact with the metal cylinder or any other conductor.
Fitting and Wiring the New Thermostat
Remove the cover of the new cylinder stat and feed the cable through any retaining clamp or gland. Carefully following the wiring diagram, and using your labels for identification, connect each wire to its appropriate terminal. Refit the cover and fix the stat to the cylinder with the securing straps. The new unit should sit tight to the surface to enable it to function correctly.
Reconnect the power supply at the consumer unit, switch the boiler back on, and test the operation of the new thermostat. With the boiler set to hot water check the thermostat works properly. Make sure that the programmer is set to hot water being on. Turning the temp up should fire up the boiler and turning it down, switch it off.