Hot water cylinder 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 →⚠️ Unvented cylinders (Megaflo-type) operate at mains pressure and high temperature. Do not tamper with the pressure relief valve or temperature and pressure relief valve — call a qualified installer.
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Most likely cause & what to check
Check the programmer or cylinder thermostat first — make sure hot water is scheduled and the cylinder thermostat dial (on the side of the cylinder) is set to 60°C.
For an immersion heater cylinder: check the immersion heater switch or timer, and check the consumer unit for a tripped breaker on the immersion heater circuit.
Check that the boiler is firing when hot water is called — you should hear the boiler ignite and a motorised valve click in the airing cupboard.
If the boiler fires but the water doesn't heat: the motorised (zone) valve or the cylinder thermostat may have failed. A heating engineer can test and replace these.
If the cylinder is lukewarm but not getting hot enough: the cylinder thermostat may be set too low or have failed. Set it to 60°C and see if that resolves the issue. If the cylinder can't reach 60°C, the thermostat needs replacing.
For unvented cylinders: call a qualified unvented cylinder installer — do not attempt DIY repairs on these units.
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Frequently asked questions
What temperature should a hot water cylinder be set to?
60°C is the recommended temperature — hot enough to kill Legionella bacteria but not so hot as to cause scalding. Never set below 55°C.
My cylinder is old and rusty — should I replace it?
If a cylinder is leaking or cannot maintain temperature, replacement is usually more cost-effective than repair. A new vented copper cylinder costs £200–400 plus installation.