Immersion heater not heating water
An immersion heater is a reliable workhorse in most UK homes, but when it stops heating your water, it can be genuinely frustrating. This problem is surprisingly common, especially in older properties or after a power cut, and the good news is that nine times out of ten, it's something you can diagnose yourself without calling anyone out. Most failures stem from simple issues — a tripped switch, a thermostat that's reset itself, or a timer that's been accidentally disabled. This guide walks you through the most likely culprits in order, starting with the easiest checks and progressing to when you'll genuinely need a qualified engineer. If you've worked through these steps and nothing's shifted the problem, the heating element itself may have reached the end of its life, and that's when a professional plumber should step in.
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Most likely cause & what to check
Check the immersion heater timer or switch — ensure it's turned on and set to the correct schedule.
Check your consumer unit for a tripped circuit breaker on the immersion heater circuit.
Check the thermostat on the immersion heater — it should be set to 60°C.
Press the reset button on the immersion heater thermostat (small red button under the top cap).
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Frequently asked questions
How long should it take for an immersion heater to heat a full tank of water?
A typical 3kW immersion heater will heat a standard 150-litre cylinder to 60°C in around 1.5 to 2 hours from cold, though this varies depending on the element size and your water hardness. If you're waiting significantly longer than this, there may be a fault with the element or thermostat.
Why does my immersion heater keep tripping the electrics?
Repeated trips usually indicate a short circuit in the heating element itself, which is a fault that needs professional attention for safety reasons. Never ignore this issue — a faulty element can pose an electrical risk, so contact a qualified electrician or Gas Safe engineer straightaway.
Can I replace an immersion heater myself?
Whilst removing and replacing the element itself is relatively straightforward, you'll need to isolate the power safely and be confident working with electrics. Most homeowners find it safer to call a qualified electrician or plumber, especially if you're unfamiliar with your consumer unit.
What's the difference between a single and dual immersion heater?
A dual immersion has two elements — typically one larger (3kW) and one smaller (2kW) — allowing you to heat water more economically. You can use just the smaller element during off-peak hours if you're on Economy 7, saving energy costs.