Immersion Heater Not Working — How to Diagnose
Check the steps below first — if you're not confident, get it fixed safely today.
Post a job — we'll find you an engineer →Do not attempt to replace the immersion heater element yourself unless you are Part P certified (qualified to work on domestic electrics). This is 240V mains wiring.
An immersion heater that's stopped working is one of the most common hot water problems in UK homes, especially in older properties or those with hard water. Whether you've woken up to a cold shower or noticed the cylinder isn't heating overnight, the good news is that the fault is usually straightforward to identify — and often fixable without calling an engineer. Most failures come down to three culprits: a tripped safety thermostat (which can reset itself), a corroded heating element that's reached the end of its life, or a simple electrical fault like a blown circuit breaker. This guide walks you through the diagnostic steps you can safely perform yourself, what the costs look like if something needs replacing, and crucially, when you need to bring in a qualified electrician rather than risk a 240V repair.
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Most likely cause & what to check
Check the basics first: is the immersion heater timer set to 'on' or 'boost'? Has the circuit breaker for the immersion tripped in your fuse box? Reset any tripped breaker and try again.
Locate the inspection cap on the side of the hot water cylinder — this is a round plastic cap about 10cm in diameter. Remove it by pressing and twisting or unscrewing.
Inside you will see a small red or white reset button — this is the thermostat cutout. Press it firmly. This can trip if the water overheated or there was a power surge.
Turn the immersion on (set the timer to boost) and wait 30–40 minutes. Feel the hot water pipe leaving the cylinder — it should become warm if the element is working.
If pressing the reset has no effect and there is still no heating, the thermostat or element has failed. A thermostat replacement is around £15–30 for parts and can be done by an electrician. An element replacement is £50–150 for parts plus £50–80 labour.
If the immersion has worked fine for years but recently stopped, the element has almost certainly corroded and failed — this is normal wear. In hard water areas, scale buildup shortens element life.
Call a qualified electrician or plumber with electrical experience to test and replace the element. Do not attempt this yourself unless qualified.
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Frequently asked questions
How long does an immersion heater element typically last?
In soft water areas, a good element should last 8–15 years; in hard water regions, expect 4–8 years before scale buildup causes failure. If your immersion has worked reliably for many years and suddenly stopped, element corrosion is the most likely cause.
Can I replace the immersion heater element myself?
No — unless you're Part P certified to work on domestic electrics, you must not attempt this. It's a 240V mains connection and a serious shock risk. Always hire a qualified electrician or plumber with electrical accreditation.
Why does the thermostat cutout keep tripping?
Repeated trips suggest either a faulty thermostat, a failing element causing overheating, or an electrical fault in the wiring. A single trip after a power surge is normal, but if it happens often, call an electrician to diagnose the root cause.
Is it worth replacing an old immersion heater or should I upgrade to a combi boiler?
That depends on your broader heating setup and budget. An element replacement is cheap (£100–230 total), but if your boiler is also ageing or you're planning renovations, it might be worth exploring a combi or heat pump long-term — chat with a heating engineer for advice tailored to your home.
How do I know if it's the element or the thermostat that's failed?
If the reset button on the thermostat cutout makes no difference and the water still won't heat, both components may have failed or the element may be dead. A qualified electrician can test with a multimeter to pinpoint which one, then advise on repair or replacement.
Does hard water affect immersion heater lifespan?
Yes, significantly. Scale deposits build up on the element in hard water areas, reducing efficiency and eventually causing it to fail much sooner than in soft water regions. Regular maintenance and descaling products can help extend life, but replacement will still be needed sooner.