How to Reset an Immersion Heater, and When Not To

Where the immersion heater reset button is

Find the immersion first. On most cylinders it is a round or square plastic cap, roughly the size of a jam jar lid, on the top of the cylinder or on its side near the bottom. A flex runs from it to a wall switch nearby, often a double pole switch marked "water heater" with a small red neon. Some cylinders have two immersions, one high and one low, and each has its own cap. If you are not sure which part is which, our explainer on what is an immersion heater shows the parts.

Under the cap is the element head, the terminals the flex connects to, and a rod thermostat with a temperature dial. The reset button is on the body of that thermostat. It is usually red, sometimes white or black, often marked "reset" or with a small arrow, and on most modern thermostats it sits beside or just above the dial. There is no reset button on the wall switch and none on the outside of the cap.

If your cylinder is an unvented one, there may be a second reset on the cylinder itself, on the thermostat that controls heat from the boiler. That one is covered further down, because the advice for it is different.

First, make sure the cut-out is really the problem

A tripped cut-out gives a completely dead immersion: switch on, neon lit, no heat. Several much simpler things look exactly the same, so rule them out before taking any covers off.

• The wall switch is on and its neon is lit. If there is no neon, the problem is the supply, not the cut-out.
• The breaker or fuse for the immersion at the consumer unit has not tripped. It is usually labelled "immersion" or "water heater".
• Any timer or programmer on the immersion is set to be on now. Many are set for off peak hours only and will not heat during the day.
• On an Economy 7 or similar supply, the off peak circuit only runs at night, so a cold cylinder in the afternoon may be normal.
• Give it time. A full cylinder heated by the immersion alone takes a couple of hours from cold. Our guide, how long does hot water take to heat up, gives the figures.

How to reset an immersion heater safely, step by step

This is work on a mains voltage appliance next to water. If you do not own a proper voltage tester and know how to use it, stop at step two and call someone who does. Switching off at the wall is not enough on its own.

• Switch the immersion off at the wall switch.
• Switch it off at the consumer unit as well, at the breaker marked for the immersion or the water heater, or remove its fuse. If nothing is labelled and you are not sure which circuit it is, stop here.
• Unscrew or unclip the cap on the element. Most are held by one central screw or a quarter turn.
• Before touching anything inside, test the terminals with a voltage tester to prove they are dead. Old immersions often have a heat damaged flex, so look for cracked or brittle insulation while the cap is off. If you find any, do not restore power.
• Find the button on the thermostat and press it firmly. A tripped cut-out gives a definite click. If there is no click, it had not tripped and the problem is elsewhere.
• Refit the cap, restore power at the consumer unit, then switch on at the wall.
• Check the cylinder is warming within the hour. Feel the pipe at the top, carefully, rather than the tank itself.

Never press the reset with the cap off and the power on, and never tape, wedge or bypass the button. The cut-out is the thing that stops the element boiling the cylinder if the thermostat fails.

Why the reset button tripped in the first place

The cut-out is a one-shot safety device set well above the normal working temperature, at roughly 85 degrees on common immersion thermostats. It only trips when the water gets there. The usual reasons are:

• The immersion thermostat has failed with its contacts stuck closed, so the element kept heating past the set temperature.
• The boiler and the immersion were both heating the cylinder and the cylinder thermostat or motorised valve on the boiler side was not switching off.
• The element was switched on with the cylinder partly empty, for example after a drain down or a supply problem, so it ran hot in air rather than water.
• The thermostat rod is not pushed fully into its pocket, so it is sensing the wrong temperature.
• The cut-out itself has aged and become sensitive. It happens, but it is the least likely explanation and the one to arrive at last.

It tripped again: stop resetting it

A reset that holds for months was probably a one-off. A reset that trips again within days, or within hours, is telling you the water keeps overheating. Pressing it again puts the same fault straight back on the cylinder, and on an unvented cylinder the next thing to open is the temperature and pressure relief valve, which dumps scalding water into the tundish.

The usual repair is a new immersion thermostat, and sometimes the element as well if it has been damaged or is scaled up. Our page on an immersion heater not working explains how a thermostat and element are tested and told apart, and our immersion heater repair service covers the parts and the visit. If the water at your taps is scalding, or the cylinder is making banging or rumbling noises, switch the immersion and the boiler off and read our guide to a hot water cylinder overheating first.

Unvented cylinders have a second reset, on the boiler side

An unvented cylinder heated by a boiler has its own control thermostat and a separate manual reset cut-out for the boiler coil, usually in a housing on the side of the cylinder. If that cut-out trips, it shuts the motorised valve and the cylinder stops taking heat from the boiler, even though the radiators still work. From the outside it looks like no hot water but the heating works.

That cut-out is part of the cylinder's safety set, the controls a G3 qualified engineer has to check at installation and at every service. A trip means the stored water got too hot on the boiler side, which points to a failed cylinder thermostat, a motorised valve that stuck open, or a wiring fault. We would not advise resetting it without finding out why it went, and our unvented cylinder repair service deals with exactly this.

When to call us instead

Call us or send an enquiry if the cut-out trips again, if you cannot find the circuit to isolate it, if the flex or terminals look scorched, or if you would simply rather not open a live appliance. A photo of the cylinder label and the inside of the cap, taken with the power off, helps us bring the right thermostat. We have 20 engineers across London and our rate is on the pricing page, agreed before anyone attends. Call 020 4652 5111.