Fitting or replacing a cylinder thermostat
⚠️ Isolate the electrical supply before working on any cylinder thermostat wiring.
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Most likely cause & what to check
The cylinder thermostat is a strap-on sensor that clips around the outside of the hot water cylinder, typically one third of the way up from the bottom.
The correct setting is 60°C — hot enough to prevent Legionella growth but not so hot as to cause scalding or waste energy.
If water is consistently too hot or cold and adjusting the dial doesn't help, the thermostat sensor has failed and needs replacing.
Isolate the power supply. The thermostat has two wires — note which terminal each wire connects to before removing.
Undo the strap and pull the old thermostat away. Fit the new one at the same height with the sensor face in direct contact with the cylinder body.
Reconnect the wiring and restore power. Monitor the hot water temperature after a full heat cycle to confirm the thermostat is controlling correctly.
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