How to Reset an Immersion Heater

What the immersion heater reset button actually does

A modern immersion thermostat has two parts inside the same housing. The first is the adjustable thermostat, which switches the element on and off to hold the temperature you set, usually around 60°C. The second is a safety cut-out, set higher, typically somewhere around 80 to 90°C depending on the maker. If the water ever reaches that point, the cut-out breaks the circuit and stays off until someone presses it back in. That is what the reset button is.

It exists because a thermostat can fail with its contacts welded shut. If that happens the element keeps heating with nothing to stop it, and in a vented cylinder the water can boil. The manual cut-out is a second line of defence, and it is deliberately not self-resetting: a person has to notice and act.

So a tripped reset button is a symptom, not the fault. Something let the water get far too hot, or the cut-out itself has become oversensitive with age. Resetting it restores hot water, but it does not tell you which of those happened. Keep that in mind before pressing it more than once.

How to tell the cut-out has tripped

A tripped cut-out looks exactly like a dead element from the outside, so rule out the simpler causes first. Work in this order, because each step takes a minute and costs nothing.

• Check the switch on the wall near the cylinder, usually a 20 amp double-pole switch labelled water heater. If it has a neon light, the light should be on when the switch is on.
• Check the timer or programmer if you have one. After a power cut many plug-in and wall timers lose their settings, and on Economy 7 the element may only get power overnight.
• Check the consumer unit. A breaker labelled immersion or water heater that has dropped, or an RCD that trips when the immersion is switched on, points to an earth fault in the element, not a tripped thermostat. Leave that one off.
• If power is reaching the switch, the timer is on and nothing has tripped at the consumer unit, but the cylinder stays cold after a couple of hours, the thermostat cut-out is the most likely cause.

If the switch or the cable to the cylinder feels hot, smells of burning or shows brown scorch marks, switch off at the consumer unit and do not reset anything. That is a wiring fault, not a thermostat trip.

How to reset an immersion heater, step by step

Manufacturers describe the reset as a simple press of the button once the supply has been isolated. It sits under the cover alongside live terminals, so the isolation matters more than the press itself. If you are not confident isolating a circuit and checking it is dead, stop here and get an engineer or electrician to do it.

• Switch off the wall switch for the immersion, then switch off the breaker for it at the consumer unit as well. Some older installations have a single-pole switch that breaks only the live side, and labels are not always right.
• Find the immersion on the cylinder. A top-entry immersion has a round cover on the dome at the top of the cylinder. A side-entry one sits low down on the side, and Economy 7 cylinders often have two, one near the top and one near the bottom.
• Unscrew or unclip the cover. Some are held by a single central screw, some twist off, and some have a small screw at the side.
• Look on the thermostat head, the plastic block with the temperature dial, for a small button. It is often red and may be marked reset or have a tiny separate cap over it. Some makes fit the cut-out as a separate unit on the element head instead, with its own button.
• Press it firmly. If it had tripped you will feel or hear a distinct click. If nothing clicks, the cut-out had not tripped and the fault is elsewhere.
• Check the dial while the cover is off. It should sit at about 60°C. A dial turned up to maximum is a common reason for nuisance trips.
• Refit the cover fully before restoring power, then switch on at the consumer unit and the wall switch.

It keeps tripping: what that means

One trip after years of service can happen for a harmless reason, such as a very hot spell or a thermostat set right at its limit. A trip that returns within a day or a week has a cause that will not fix itself. The common ones, roughly in order of how often an engineer finds them:

• A failing thermostat. The rod thermostat loses its accuracy with age and scale, lets the water overshoot, and the cut-out does its job. The fix is a new thermostat, usually a cheap part.
• A thermostat not pushed fully into its pocket. The rod has to sit all the way down inside the tube in the element head. If it has been pulled back, it reads the temperature in the wrong place.
• The boiler heating the cylinder too. On a system with both a boiler coil and an immersion, a faulty cylinder stat or motorised valve can let the boiler overheat the water and trip the immersion cut-out even when the immersion is off.
• Scale around the thermostat pocket and element. Scale insulates the thermostat rod from the water, so it reacts late and the water overshoots. On hard London water this builds up quickly.
• A worn cut-out that trips below its rated temperature. Again, the answer is a new thermostat.

Do not bridge, tape down or bypass a cut-out that keeps tripping. It is the only thing stopping the water boiling if the main thermostat sticks on.

Resetting an immersion on an unvented cylinder

Unvented cylinders, the sealed type fed straight from the mains with no cold tank in the loft, use an immersion specified by the cylinder maker, so the thermostat may look different from a standard one; the cylinder manual shows where its reset is. The label on an unvented cylinder tells users not to remove or adjust any component and to call the installer, so on these systems the reset is a job for a G3-qualified engineer.

An unvented cylinder has its own safety chain: the immersion cut-out, a separate cut-out for the boiler coil, and a temperature and pressure relief valve that discharges to the tundish if all else fails. Building Regulations Part G section G3 expects these systems to be installed by a competent person, with a non-self-resetting cut-out on every heat source, and makers require a qualified installer for any later work on them, so a replacement must be the type the cylinder maker specifies. If the immersion trips repeatedly on an unvented cylinder, or you see water running from the tundish, switch the immersion off and have the cylinder checked rather than resetting again. Other cylinder faults are covered in our hot water guides.

When to call someone instead

Call an engineer if the reset button will not click in, if it clicks but the water is still cold after two or three hours, if it trips a second time, or if the RCD goes as soon as the immersion is switched on. Each of those means a component has failed, and our guide on immersion heater stopped working covers the full fault list. If you want to confirm the element before booking, how to test an immersion heater walks through the meter readings.

On a visit the engineer isolates and proves the supply dead, tests the thermostat and cut-out for continuity, measures the element across its terminals and to earth, and replaces whichever part has failed. Our immersion heater repair service starts with the £60 call-out at any hour, and the repair price is agreed with you on site before any work starts. For typical part and labour costs across the market, see immersion heater replacement cost.