Megaflo Unvented 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 →Do not attempt to adjust the temperature or pressure relief valve, expansion vessel, or any sealed components on an unvented cylinder. Unvented systems operate under pressure and must only be serviced by a qualified plumber with Unvented Hot Water Systems certification (G3).
The Megaflo is the UK's most popular unvented hot water cylinder brand. When it stops producing hot water, the cause is usually a failed immersion heater element, a tripped thermal cut-out, a failed thermostat, or (less commonly) an issue with the boiler heating coil. Some checks are DIY-safe; others require a G3 qualified plumber.
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Most likely cause & what to check
Check whether the problem is with all hot water or just one outlet. If only one tap is cold, the problem is more likely in that tap's supply than the cylinder.
Check the consumer unit — the immersion heater has its own circuit with a dedicated switch or timer. Check the switch is on and the circuit breaker has not tripped.
Check the immersion heater thermal cut-out (also called the thermal safety cut-out or TCO). This is a red reset button on the immersion heater head, accessed by removing the white plastic cap on the side of the cylinder. Press the red button firmly to reset it.
If the TCO keeps tripping, the thermostat has failed and is allowing the element to overheat. The thermostat and element are combined in most immersion heaters — replacement costs £30–80 for the part and requires the cylinder to be drained and the element unscrewed (a plumber job).
Check whether the boiler is heating the cylinder correctly. If you have a twin-coil cylinder or a boiler with a hot water circuit, ensure the hot water is selected on the programmer and the boiler is firing when hot water is called for.
If the cylinder is warm but water is only lukewarm at the taps, you may have a thermostatic mixing valve (TMV) set too low. The TMV is fitted on the hot water outlet pipe — a G3 plumber can adjust it. Do not adjust it yourself as it is a safety device preventing scalding.
If the cylinder is completely cold and none of the above resolves it, call a G3 qualified plumber — the issue may be a failed motorised valve on the heating coil circuit, a faulty cylinder thermostat, or an internal coil problem.
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