Immersion Heater Not Working: Causes, the Reset Button and When to Call an Electrician

What Is an Immersion Heater and How Does It Work?

An immersion heater is essentially a large electric heating element — similar in principle to a kettle element — that sits inside your hot water cylinder and heats the water directly. It is controlled by a dedicated switch, usually a red rocker switch mounted in the airing cupboard or near the cylinder.

Unlike a gas boiler, an immersion heater uses only electricity. The element heats the water to a set temperature, and a thermostat cuts the power once that temperature is reached. There is also a thermal cut-out (safety thermostat) that trips if the water gets dangerously hot — for example, if the main thermostat fails.

Immersion heaters are found in a wide range of UK homes: as the sole hot water source in all-electric properties, as a backup in solar thermal or heat pump systems, and as an emergency fallback when a boiler has failed. Almost all domestic models in the UK are rated at 3kW and must be wired on a dedicated 20A radial circuit — a legal requirement, not a suggestion.

Why Is My Immersion Heater Not Working? The Most Common Causes

When your immersion heater has stopped working, the fault almost always falls into one of the categories below. We have listed them roughly in order of how often they occur.

• Tripped MCB or RCD in the consumer unit. This is the first thing to check. Find the consumer unit (fuse box) and look for a breaker labelled "Immersion" or "Water Heater." If it has tripped to the off position, reset it. If it trips straight back, do not keep resetting it — there is a fault that needs investigation.

• Tripped thermal cut-out (safety thermostat). Most modern immersion heaters have a separate thermal cut-out that trips if the water overheats — for example, when the main thermostat has stuck. The cut-out has a manual reset button. It is a safety feature, not a fault in itself, but repeated tripping does signal an underlying problem.

• Faulty heating element. The element is the single component that fails most often over time. When it burns out, you get no hot water at all, or the MCB trips as soon as power is restored. Limescale is the biggest accelerant — in hard water areas such as London and the South East, elements can fail well within five years.

• Faulty immersion heater thermostat. If the thermostat fails open, the element never comes on and the water stays cold. If it fails closed, the element never switches off, the water overheats, and the thermal cut-out trips. Either way, a replacement thermostat is usually the fix.

• Limescale build-up. Mineral deposits coat the element, insulating it from the water and causing it to overheat from the inside. Over years this cracks and burns out the element. It is the leading cause of early element failure in hard water areas.

• Faulty wall switch or wiring. The dedicated rocker switch can develop internal faults that cut power entirely. Similarly, loose connections at the element head can cause intermittent failures or complete loss of heating.

• Timer or programmer fault. If your cylinder has a timer, a failed programme or a clock that has reset after a power cut can make it appear the heater has stopped working. Check the timer is set correctly and that the clock shows the right time before assuming anything is broken.

• Poor cylinder insulation. An older cylinder with a degraded foam jacket loses heat rapidly, so the water may never reach a usable temperature even when the element is working fine. This tends to produce lukewarm rather than cold water.

How to Use the Immersion Heater Reset Button

The reset button is part of the thermal cut-out — the secondary safety thermostat that trips when the water has overheated. Finding it is straightforward, and pressing it is something any homeowner can do safely as long as they isolate the power first.

Where to look: On most UK cylinders the element head is fitted at the top or near the top of the tank. There will be a plastic or metal cover — sometimes labelled "Thermostat" or "Safety Cut-Out" — secured by a screw or a nut. The reset button sits beneath this cover; it is usually a small red or white circular button, or on older models a pin that pops up when the cut-out has tripped.

Step-by-step reset procedure:

1. Switch off the power at the consumer unit. Find the MCB labelled "Immersion" or "Water Heater" and switch it to the off position. Never touch the element head or any wiring with the power on.

2. Allow the cylinder to cool for at least 10 minutes. The thermal cut-out will not reset reliably if the water is still very hot.

3. Remove the thermostat cover. Unscrew or unclip it carefully, taking care not to tug on any wiring behind it.

4. Locate the reset button or pin. Press it firmly until you feel or hear a faint click. If the cut-out uses a pin, push it back in with a small flat-head screwdriver.

5. Replace the cover securely. Then restore power at the consumer unit and switch the immersion heater on at the wall.

6. Check whether it heats normally. After an hour or two the water should be noticeably warm. If the MCB trips again, or if the cut-out trips again within a day, do not keep repeating the reset — there is an underlying fault that needs a qualified electrician.

Important: pressing the reset button does not fix the root cause. It simply allows you to restore hot water temporarily while you investigate why the cut-out tripped in the first place.

Immersion Heater Thermostat: Temperature Settings and Legionella

The immersion heater thermostat is adjustable, usually in a range of 20–70°C. You will find the adjustment dial behind the same cover as the reset button — it may be a slotted dial or a rotary control.

The correct setting for a domestic hot water cylinder in the UK is 60°C. This is not arbitrary: Legionella bacteria, which causes Legionnaires' disease, thrives in warm stagnant water between 20°C and 45°C. At 60°C the bacteria are killed within 32 minutes; at 66°C within 2 minutes; at 70°C virtually instantly. Setting your thermostat below 60°C to save energy is a false economy and a genuine health risk.

At 60°C the water at the tap will be hot enough to scald, particularly for young children and elderly people. The solution is not to lower the cylinder temperature but to fit a thermostatic mixer valve (TMV) at the point of use — a shower or basin — which blends in cold water to deliver a safe temperature (typically 38–43°C) without compromising the cylinder temperature.

Setting the thermostat above 65°C wastes electricity and accelerates limescale formation. Stick to 60–65°C as your target range.

Modern immersion heater thermostats include an integrated thermal cut-out for overheat protection. If your thermostat is very old and lacks a separate reset button, it may not meet current safety standards and should be replaced by a qualified electrician.

What a Qualified Electrician Will Do — and When to Call One

The safe boundary for homeowner action is: check the consumer unit, press the reset button (with power isolated), check the timer, and confirm the wall switch is on. Everything beyond that requires a qualified electrician — and in some cases a plumber as well.

Call an electrician if:

• The MCB trips immediately or repeatedly after resetting.
• The reset button trips again within a day or two.
• You can smell burning near the cylinder or the wall switch.
• The wall switch feels warm or shows any scorch marks.
• There is any sign of water leaking near the element head (this can cause dangerous electrical faults).
• The element needs replacing — this involves draining the cylinder and disconnecting mains wiring.
• The thermostat needs replacing — again, mains wiring is involved.

What the electrician will do: They will carry out a visual inspection and use a multimeter to test the element for continuity (an open circuit confirms the element has blown). They will check the thermostat operation, inspect the wiring connections at the element head, and check the MCB and RCD in the consumer unit. If the element needs replacing, they will isolate the circuit, drain down the cylinder sufficiently, unscrew the old element with a dedicated immersion heater spanner, fit the new element with a fresh sealing washer, refill, check for leaks, and restore power. The whole job typically takes two to three hours.

If your cylinder is normally heated by a gas boiler: in many UK homes the immersion heater is only the backup, and the real problem is that the boiler has stopped heating the cylinder. Any fault on a gas boiler, its controls or its gas supply must be looked at by a Gas Safe registered engineer — never open the boiler casing yourself. If your system uses an oil boiler, use an OFTEC registered technician; for a heat pump, an MCS certified installer. Work on an unvented (pressurised) cylinder, including its safety valves, should only be carried out by an installer holding the relevant unvented hot water (G3) qualification.

How Long Does an Immersion Heater Last?

A well-maintained immersion heater can last 10 to 20 years overall, but the heating element itself is the most vulnerable component and typically needs replacing every 5 to 10 years — sooner in hard water areas where limescale accelerates wear.

If the cylinder itself develops corrosion, pinhole leaks, or has reached the end of its serviceable life, a full cylinder replacement becomes necessary. This is a larger job involving both an electrician and usually a plumber.

Regular maintenance extends element life: draining and flushing the cylinder every few years, checking the sacrificial anode if your cylinder has one, and fitting a water softener or scale inhibitor in hard water areas can all make a meaningful difference.

Running Costs: How Much Does an Immersion Heater Cost to Run?

Knowing what a working immersion heater costs to run helps you spot when something has gone wrong — a faulty thermostat that leaves the element on constantly will show up as an unusually high electricity bill.

At a typical electricity unit rate of roughly 24–27p per kWh (check your own bill — the Ofgem price cap changes every quarter), a 3kW immersion heater costs roughly 72–78p per hour to run. For a typical family of four heating water for around two hours a day, that is approximately £1.40–£1.57 a day, or £40–£50 per month.

If you are on an Economy 7 or Economy 10 tariff, running the immersion on the cheaper off-peak overnight rate can roughly halve your hot water electricity costs — a good reason to ensure your timer is set correctly and your thermostat is holding the right temperature so you are not reheating unnecessarily.

Step by step

1. Check the consumer unit first

   Go to your fuse box and look for an MCB labelled "Immersion" or "Water Heater." If it has tripped, reset it once. If it holds, your heater may now work — give it an hour and check whether the water is warming up.

2. Check the wall switch and timer

   Make sure the dedicated red rocker switch near the cylinder is switched on. If there is a timer, confirm the clock is showing the correct time and the programme is set to a heating period.

3. Isolate the power and locate the thermostat cover

   Turn the immersion MCB off at the consumer unit. Wait 10 minutes. Find the plastic or metal cover on the element head of the cylinder and unscrew or unclip it carefully.

4. Press the thermal cut-out reset button

   Look for a small red or white button (or a raised pin) and press it firmly. Replace the cover, restore power at the consumer unit, and switch on the heater at the wall.

5. Wait and check

   Allow one to two hours and check whether the water is heating. If the MCB trips again or the water stays cold, the element or thermostat has likely failed — stop resetting and call a qualified electrician.

Typical costs

Typical UK ranges as a guide only — prices vary by region (expect the top end, or 20–30% more, in London and the South East) and by how accessible your system is. Always get a written quote.

Frequently asked questions