Immersion heater not heating water
Check the steps below first — if you're not confident, get it fixed safely today.
Post a job — we'll find you an engineer →⚠️ Immersion heater wiring must comply with Part P Building Regulations. Replacing the heater element involves working with 230V wiring — if you are not confident with basic electrical connections, use a qualified electrician or plumber.
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
An immersion heater is essentially a large kettle element fitted into the top or side of a hot water cylinder. It is controlled by a dedicated switch — usually a red rocker switch in the airing cupboard.
Check the dedicated consumer unit circuit — there should be an MCB (miniature circuit breaker) labelled "immersion" or "water heater". If it has tripped, reset it and see if it holds. A repeatedly tripping MCB suggests a fault with the element or wiring.
Check the immersion heater thermostat — this is a small round white cylinder screwed into the back of the immersion heater head. The thermostat can fail and cause the heater to cut out. A replacement immersion thermostat costs £5–£10 and simply pulls out and clips back in on most models.
If the thermostat appears fine, the element itself may have failed. An element can be tested with a multimeter — unplug the heater from its wiring (after isolating the circuit at the consumer unit) and check resistance between the element terminals. Infinite resistance means a broken element.
Replacing an immersion element requires draining the cylinder down (close the cold feed valve, attach a hose to the drain cock, open a hot tap to break the vacuum), unscrewing the large hexagonal heater boss (requires a 55mm immersion spanner), and fitting the new element with a new fibre washer. Cost is £15–£30 for the element.
If the cylinder is over 20 years old or the element has failed multiple times, consider whether a full cylinder or heat pump water heater replacement would be more economical in the long run.
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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.