Immersion Heater Not Working? Causes and Fixes (UK)

The switch light is on, the timer says it should have been heating for hours, and the hot tap runs cold. Or you get a lukewarm bath that's gone cold halfway through. Or the breaker clunks off every time the immersion comes on.

In many London flats the cylinder sits in an airing cupboard with a single immersion element and nothing else to fall back on. When it fails, the hot water goes with it. Below: the order we'd check things in, what the reset button does, how an element is tested, why it might trip the RCD, and when to stop.

Why an Immersion Heater Stops Working

An immersion heater stops working for one of three reasons: no power is reaching it, the thermostat's over-temperature cut-out has tripped, or the element itself has failed.

The table shows what each symptom usually points to.

If your hot water comes from the boiler and the immersion is only a back-up, a cold tap with the boiler running is a different problem. See our guide to no hot water but the heating works.

Check the Switch, Timer and Consumer Unit

Start with the power: confirm the immersion switch is on, the timer is calling for heat, and the breaker or fuse in the consumer unit hasn't tripped.

Many "broken" immersions turn out to be switched off, or on a timer that was bumped. Work through these in order:

1. The immersion switch. It's usually a double-pole switch with a neon light, either in the airing cupboard or on the wall nearby. The light should come on when the heater is calling for heat. No light means no supply, or the thermostat is satisfied.
2. The timer. Check it has the right time, is set to "on" or an on-period that covers now, and hasn't been knocked to "off". If there's an override, try it.
3. The consumer unit. Look for a tripped breaker, RCBO or RCD labelled for the immersion or hot water. A breaker that has dropped once can be switched back on once. Don't reset it again and again.

The switch must be double-pole. Heatrae Sadia's installation manual says the heater must be wired through a double-pole isolating switch with a 3mm contact gap. If yours is a plain single-pole switch, or there's no isolating switch at all, tell an electrician.

If the light is on but nothing heats, the supply is reaching the heater. That points to the cut-out, the thermostat or the element, and it's what the next three sections cover.

The Immersion Heater Reset Button

The reset button is a small pin under the thermostat cap. Pushed in lightly until it stops, it resets the over-temperature cut-out. Heatrae Sadia says that should be done by a competent person, after the cylinder has cooled and the reason for the trip has been found.

The thermostat sits on top of the element where it screws into the cylinder, under a plastic cap or cover. Inside is a temperature dial and the reset pin. A Heatrae Sadia thermostat is adjustable from 10C to 60C and factory set to 60C. The cut-out is a separate safety device inside the same unit. It operates if the water temperature gets excessive, to stop the cylinder boiling.

This is the sequence an engineer follows, and the reason it matters:

1. Switch off at the double-pole switch and at the consumer unit. Prove it dead.
2. Wait for the cylinder to cool. Heatrae Sadia's manual says the cause must be investigated and rectified before the cut-out is reset.
3. Take the cap off. A tripped pin sits level with, or slightly proud of, the surface.
4. Push it in lightly until it stops.
5. Refit the cap, switch the power back on and see if the heater runs.

If it trips again, something is wrong. Don't press it repeatedly. Call an engineer to find out why it operated.

Building Regulations back this up. Approved Document G (2024 amendments, para 3.13) says vented hot water storage systems should incorporate a non-self-resetting energy cut-out for all direct heat sources. The manual reset is deliberate. It makes sure a fault gets noticed.

How to Test an Immersion Heater Element

An element is tested by measuring its resistance with a multimeter once the power is isolated and proved dead and the element disconnected. A 3kW element should read roughly 17 to 19 ohms, and an open circuit means it has burned out.

Most elements are around 3kW. Which? describes a typical immersion as "typically 3kW", and Heatrae Sadia rates its element at 2.8kW at 230V and 3.0kW at 240V. The resistance follows from the maths: 230 volts squared divided by 3,000 watts is about 17.6 ohms. Treat that as an approximate figure. Real elements vary a little, and a reading in that area is a healthy element.

What the readings suggest:

• About 17 to 19 ohms: the element's heating coil is intact. The fault is more likely the thermostat or the wiring.
• No reading, or a wildly different one: an open circuit. The element has failed and needs replacing.
• Normal reading, but the breaker trips: the element may still be leaking to earth, which a resistance test across the terminals won't show. An engineer checks earth separately.

We'd leave this to an engineer. The terminals are at mains voltage, the cylinder is full of hot water, and the thermostat has to be matched to the element. Heatrae Sadia's manual says any replacement thermostat must be of the same type.

What a failed element looks like

Hard water is the usual reason. Thames Water says most of the water in South-East England is hard, and scale builds up on a hot surface like an element. Scale insulates the element and makes it work harder to push heat into the water, so it runs hotter and heats more slowly. An old element can end up coated along most of its length, and eventually it can burn out. Our guide to hard water and hot water cylinders in London covers prevention.

Scale can also seize the element in the boss. If it won't unscrew with the right immersion spanner, stop. Forcing it risks damaging the cylinder boss, and that turns a small repair into a cylinder replacement.

Immersion Heater Keeps Tripping the RCD

An immersion heater that keeps tripping the RCD almost always has a fault to earth, usually in a worn element or in water that has got into the thermostat housing. Stop resetting it and have it tested.

An RCD trips when current leaks to earth. On an immersion, that tends to be one of these:

• A cracked or scaled element where the heating coil is touching the sheath, leaking current into the water.
• A damp thermostat or terminal housing from a weeping seal or condensation on the boss.

It can trip at once when the immersion switches on, or after a few minutes when the element is hot. Either way, the fix is the same. Switch the immersion off at the double-pole switch, leave it off, and call someone to test it.

Our guide to cylinder thermostat faults explains how to tell a thermostat fault from an element fault.

Is it a Part P job?

Approved Document P (2013, England) says replacements, repairs and maintenance aren't notifiable, but the work must still comply with BS 7671 and be tested. A new circuit or a consumer unit replacement is notifiable. So swapping an element isn't something you report to building control, but it isn't a free-for-all either. If the immersion needs a new circuit, that changes.

How Long Does an Immersion Take to Heat Water?

An immersion heater takes around three hours to heat a 180-litre cylinder from cold to 60C, according to Which?. Smaller cylinders are quicker, and scale or poor insulation slows it.

You can check the scale of this with a rough calculation. Heating 140 litres of water by 50C takes about 140 x 4.18 x 50 = 29.3 megajoules. A 3kW element delivers 10.8 megajoules an hour, so that's about 2.7 hours with no heat lost. Real cylinders lose some, so it takes longer. The same sum for 180 litres gives nearer 3.5 hours, so treat the Which? figure as a rule of thumb.

The time depends on:

• Cylinder size. More water means more energy.
• Starting temperature. A cylinder that's still warm needs far less.
• Scale on the element. A furred element transfers heat badly, so heat-up times creep up over the years.
• Insulation. A cylinder with a good jacket or factory foam holds the heat. A bare one loses it as fast as you put it in.

If yours takes much longer than it used to, scale on the element is the first suspect. It's also worth checking the thermostat. Heatrae Sadia sets them to 60C. HSE's legionella guidance says hot water should be stored at least at 60C and reach 50C at the outlets within a minute. That guidance is aimed at duty-holders such as employers and landlords, not householders, but a thermostat turned right down to save energy will leave the water lukewarm.

When to Call an Engineer

Call an engineer if the element tests as open circuit, if the cut-out or an RCD keeps tripping, if the element is seized or leaking, or if the cylinder is unvented.

Specifically, stop DIY and call if:

• The reset has tripped again after you pressed it once.
• The RCD or breaker trips whenever the immersion comes on.
• The switch light is on but the water stays cold, and the power checks are fine.
• Water is weeping around the immersion boss, or the element won't unscrew.
• You hear bubbling, see steam or have hot discharge from the tundish. Switch off and keep clear.
• The cylinder is unvented. Approved Document G says unvented systems are notifiable unless the installer is on a competent person scheme (3.41 and 3.42). The user notice (3.24) says: "Do not remove or adjust any component part of this unvented water heater; contact the installer". Fixed immersion heaters on these cylinders should comply with BS EN 60335-2-73 (3.43). An unvented cylinder is G3 work: call a G3 engineer.

Or WhatsApp a photo of the thermostat and switch to 07456 975436. For prices, see our guide to immersion heater replacement cost in London. If your cylinder is unvented and you want to know what the rules mean, read G3 unvented cylinders explained.

Frequently Asked Questions

Emergency Repairs London provides same-day and 24/7 emergency plumbing and hot water repairs across all 32 London boroughs. Lines are open 24/7 on 0207 046 1363.