Immersion Heater Not Working: Causes and What to Check
Your immersion heater has stopped heating the water. How to check the switch, timer and circuit safely, what the thermostat cut-out does, and when the element or thermostat needs replacing.
When an immersion heater stops working, check the simple things first: the wall switch is on, the timer or booster is set correctly, and the circuit breaker or fuse has not tripped. If power is reaching it and the water is still cold, the usual causes are a failed element, a faulty thermostat or a tripped high-limit safety cut-out. A breaker that trips as soon as the immersion comes on usually points to a failed element. Anything under the cover is mains electrical work and should be left to a professional.
Key Takeaways
- Start with the switch, the timer and the consumer unit. Many 'dead' immersions are just switched off.
- A breaker or RCD that trips when the immersion switches on usually means the element has failed.
- Water that is only lukewarm can be a faulty thermostat or a scaled element in London's hard water.
- Some thermostats have a high-limit cut-out that trips on overheating. It is behind the cover, so do not open it live.
- An immersion on an unvented cylinder needs the correct cylinder-specific parts and safety controls.
If your immersion heater has stopped working, start with the checks that do not involve opening anything: make sure the wall switch is on, the timer or boost is set to heat, and the circuit has not tripped at the consumer unit. If power is reaching the immersion and the water is still cold, the likely culprits are a burnt-out element, a faulty thermostat, or a thermostat safety cut-out that has tripped because the water overheated. A breaker that trips the moment the immersion comes on almost always means the element has failed. Everything under the immersion cover is live mains wiring, so that part is a job for a professional.
Simple checks first
A surprising number of "broken" immersion heaters are simply switched off, often after someone has been saving electricity, changed a timer or had other work done in the house. Go through these before booking anyone:
- The wall switch. Usually a large double-pole switch near the cylinder, sometimes marked "water heater". Check the neon indicator if it has one.
- The timer or boost button. If you have an immersion timer, check the clock is right after any power cut and the on periods are set. Try the boost function for an hour.
- Dual switches. Older cylinders sometimes have a "sink" and "bath" switch for two elements. One may be off or failed while the other still works.
- The consumer unit. Look for a tripped breaker or RCD labelled immersion or water heater. If it has tripped, reset it once.
If the breaker trips again straight away, or trips as soon as the immersion switches on, leave it off. That is a fault to be tested, not a switch to keep flicking.
Why an immersion heater stops heating
| Symptom | Likely cause |
|---|---|
| Water stone cold, switch on, no trip | Failed element, failed thermostat or tripped high-limit cut-out |
| Breaker or RCD trips when immersion comes on | Element leaking current to earth, or water in the connections |
| Water lukewarm, takes ages to heat | Thermostat set low or faulty, one of two elements failed, heavy scale |
| Water scalding hot, then stops altogether | Thermostat stuck on, then the safety cut-out trips |
| Hot water runs out quickly | Short element only heating the top of the cylinder, or a cylinder sizing issue |
A failed element
The element is the long metal loop inside the cylinder that does the heating. London's hard water coats it in limescale over the years, which makes it run hotter and eventually fail. Some fail open, so they simply stop heating. Others break down and leak current to earth, which is what trips the RCD. Either way the fix is a replacement element, which means draining at least part of the cylinder.
A faulty thermostat
The thermostat is the rod that slots into the element head and switches the power on and off at the set temperature. If it fails open, the element never gets power. If it sticks closed, the water overheats until the safety cut-out stops it. A thermostat is a cheaper part than an element and can usually be changed without draining the cylinder.
Scale and a slow cylinder
If the water is warm but never properly hot, scale on the element and a thermostat set too low are common causes. It is also worth checking the problem is not really the cylinder or its controls. Our guide to hot water running out quickly covers the causes that are not about the immersion at all.
The reset button and thermostat cut-out
Many immersion thermostats have a built-in high-limit safety cut-out. If the water gets far too hot, usually because the main thermostat has stuck, the cut-out trips and switches the element off. On some types there is a small reset button on the thermostat. That is what people are looking for when they search for an immersion heater reset button.
The catch is where it sits: under the immersion cover, next to the mains terminals. Taking that cover off with the circuit live is dangerous. If the cover has to come off, the circuit must be switched off and isolated first, and anyone who is not confident working safely around mains wiring should leave it to a professional.
More importantly, a cut-out that has tripped is telling you something went wrong. If it trips again after being reset, the thermostat is very likely faulty and needs replacing. Repeatedly resetting it just lets the water overheat again.
Immersion heaters in unvented cylinders
If your hot water is stored in an unvented cylinder, such as a Megaflo or similar pressurised cylinder, the immersion heater is part of a system with its own safety controls. Unvented cylinders use immersion heaters and thermostats specified by the manufacturer, and work on the cylinder should be done by someone competent with unvented systems. If your unvented cylinder has no hot water at all, see our guide to an unvented cylinder with no hot water. If water is dripping from the tundish or discharge pipe, read our tundish dripping guide before resetting anything.
If the immersion is only a backup and your boiler normally heats the cylinder, check whether the boiler side is working too. A cylinder that is cold on both the boiler and the immersion may point to a shared problem such as a cylinder thermostat or controls fault.
Repair or replace?
For most working cylinders, replacing a failed element or thermostat is the sensible fix. The cylinder itself can last many years, and these parts are designed to be swapped. It makes less sense to spend on a new element if the cylinder is leaking, badly corroded, or undersized for the household, in which case replacing the cylinder may be better value. Our guide to immersion heater replacement costs sets out what affects the price, and our hot water cylinder service covers the bigger jobs.
When you book, say whether the breaker trips, whether the water is cold or lukewarm, what type of cylinder you have and whether you know how old it is. Photos of the cylinder label and the immersion head, taken without removing the cover, are very helpful. We confirm the price before any work starts.
Cold water and the immersion won't help?
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Frequently Asked Questions
Why has my immersion heater stopped working?
The most common reasons are that it has been switched off at the wall or timer, the circuit has tripped at the consumer unit, the element has burnt out, the thermostat has failed, or the thermostat's high-limit cut-out has tripped after overheating. Work through the switch, timer and breaker first, because those are quick and safe to check.
Where is the reset button on an immersion heater?
On immersion thermostats that have one, the high-limit reset is on the thermostat itself, under the cover on top of or on the side of the cylinder. That means the cover has to come off, exposing mains wiring. It should only be accessed with the circuit switched off and isolated, and a cut-out that trips more than once needs investigating rather than repeatedly resetting.
Why does my immersion heater trip the electrics?
A breaker or RCD that trips as soon as the immersion switches on usually means the element has failed and is leaking current to earth. Water getting into the wiring connections can do the same. Leave the circuit off and have the element tested and replaced.
How do I know if my immersion element is broken?
Typical signs are cold water even though power is reaching the immersion, a circuit that trips when it switches on, or hot water that takes far longer to heat than it used to. The element can be tested with a meter by a competent person with the power isolated.
Is it worth replacing an immersion heater element?
Usually, yes, if the cylinder itself is sound. An element or thermostat is a much smaller job than a new cylinder. If the cylinder is old, leaking or badly corroded, replacing the whole cylinder can make more sense.