How to Reset a Hot Water Cylinder (and When Not To)

What is the reset button on a hot water cylinder?

The small button, usually red, on or beside a cylinder thermostat is a high-limit cut-out. It is a safety switch separate from the normal thermostat. The normal thermostat keeps the water at the temperature you set. The cut-out sits behind it as a second line of defence, and it opens the circuit if the water gets far hotter than it should. Unlike the normal thermostat it does not close again by itself. It stays open until someone presses the button.

That design is deliberate. An overheating cylinder is a hazard, and a device that quietly reset itself would keep cycling a fault. The Part G guidance for unvented cylinders expects safety devices in addition to the control thermostat, with an acceptable approach being a cut-out that does not reset itself plus a temperature relief valve. So a tripped cut-out is not an annoyance to be pressed away. It is the cylinder telling you the normal thermostat did not do its job.

There is no reset for the cylinder as a whole. A programmer reset, a boiler reset and a tripped breaker are all separate, and none of them is a cylinder reset. If someone tells you to reset the cylinder, ask which device they mean.

How do I press the reset safely?

Only press it where you can do it without removing any cover. Some thermostats have the button exposed, with no tool needed. Many do not, and where it is behind a cover the manual for your cylinder says where. If reaching the button means unscrewing a cover, removing insulation or touching terminals, stop. That is work inside an electrical enclosure and it is not a householder job.

Where the button is reachable, the sequence is short.

• Switch the cylinder circuit off at the consumer unit, and off at the fused spur or double-pole isolator that feeds the heater.
• Wait for the cylinder to cool slightly. A tripped cut-out means the water was overheating, so it may be very hot.
• Press the button once, firmly. You may feel a click. Do not remove any cover to reach it.
• Restore the supply at the isolator and leave the heater to run.
• Check again after a couple of hours that the cylinder is warming and that the button has not popped out again.

Why does my cylinder keep tripping the reset?

A cut-out that trips once may have done so because of a brief fault, such as the normal thermostat sticking for a moment. One that trips again is telling you the cause is still there. Pressing it a third and fourth time hides the cause and risks overheating, which is why repeated resets are the wrong approach.

The usual causes are the normal thermostat failing closed so the element never switches off, an element or its wiring that has been wrongly connected so the thermostat is bypassed, scale burying the thermostat pocket so it reads a temperature that is too low, or a faulty cut-out that trips below its rating. Each is something an engineer tests with the power isolated.

On a boiler-fed cylinder the same logic applies to a separate cylinder thermostat or motorised valve. If the stored water keeps overheating, the valve may be stuck open, or the thermostat is no longer closing the circuit. The cure is replacing the failed part. The reset button was only ever a way to find out that something had gone wrong.

What should I do if resetting does not work?

If the button will not stay in, or it pops out straight away, either the cut-out has failed or the water is still too hot. Leave the power off. A cut-out that has failed can be replaced alone, usually as a combined thermostat and cut-out unit, which is part of an immersion heater replacement visit.

If pressing the button works but the cylinder still gives no hot water, the reset was never the problem. The tripped cut-out may have been a symptom of something else. Go back to the test order in our guide to an immersion heater that is not working, where the cut-out is one stage of four.

If the cylinder is unvented and the discharge pipe at the tundish is running hot, treat it as a safety issue. A discharge together with a tripped cut-out suggests the stored water overheated and the relief valve opened. Do not touch the relief valve or tundish pipework.

Repair or replace after a trip?

A tripped cut-out almost always ends in a repair, not a new cylinder. The thermostat or the combined thermostat and cut-out is replaced, the element is checked for scale damage, and the cylinder stays. The cylinder only enters the conversation if overheating has damaged it, for example a vented cylinder that has been distorted by repeated overheating, or scale so heavy that the element boss is compromised.

The sensible order is to find why it overheated, repair that, and only then consider the cylinder. A hot water cylinder diagnosis puts each control through its paces and names the one that failed.