Fitting or replacing a cylinder thermostat
⚠️ Isolate the electrical supply before working on any cylinder thermostat wiring.
A faulty cylinder thermostat is one of the most common reasons why your hot water never seems to hit the right temperature. You'll notice it when you're constantly adjusting the boiler settings, scalding yourself at the tap, or finding your water's gone lukewarm halfway through a shower. This usually happens because the strap-on sensor gradually loses its ability to read the cylinder temperature accurately — a natural wear-out over 10–15 years. The good news is that replacing a cylinder thermostat is a straightforward job for most homeowners and costs very little. This guide walks you through diagnosing the problem, safely removing the old thermostat, and fitting a new one correctly. If you're uncomfortable working with electrical connections or the problem persists after replacement, it's worth calling a qualified heating engineer to rule out a boiler fault.
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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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Frequently asked questions
What temperature should my hot water cylinder actually be?
The recommended setting is 60°C. This is hot enough to prevent Legionella bacteria from growing (a genuine health concern), yet cool enough to avoid scalding and unnecessary energy waste. If your dial shows different markings, 60°C is usually marked clearly or sits roughly in the middle of the range.
How do I know if my cylinder thermostat has actually failed?
If you've adjusted the dial but water temperature remains stubbornly too hot or too cold, the sensor has likely failed. A properly working thermostat responds to dial changes within a heat cycle or two. If there's no response at all, the sensor is no longer reading the cylinder temperature correctly.
Can I replace a cylinder thermostat without turning off the electricity?
No — you must isolate the electrical supply before touching any wiring. Even though the voltage is typically low, it's a hard safety rule. Switch it off at the consumer unit or isolator, and if you're unsure which circuit it's on, turn off the main supply until you've located and labelled the correct breaker.
Why does my new thermostat feel loose or wobbly on the cylinder?
The strap should be tight enough that the sensor sits flush against the cylinder surface, but not so tight you'll crush the thermostat body. A slight amount of play is normal, but if it's genuinely loose, the sensor won't read the temperature properly and you'll have the same problem again. Tighten the strap until the sensor makes firm, even contact with the metal.