Unvented cylinder not producing hot water
Check the steps below first — if you\'re not confident, get it fixed safely today.
Find a Gas Safe engineer near you →⚠️ Unvented (pressurised) hot water cylinders operate at mains pressure and must only be serviced by a G3-qualified engineer. Do not tamper with the pressure relief valve, expansion vessel, or immersion heater on an unvented cylinder.
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Most likely cause & what to check
An unvented cylinder (e.g. Megaflo, Telford, Heatrae Sadia) is a sealed system operating at mains water pressure. It has no cold water tank in the loft — it is fed directly from the mains.
Check the expansion relief valve (a brass valve, typically on the pipework near the top of the cylinder) — if it is continuously dripping, the expansion vessel has failed and pressure cannot be safely absorbed. This is a G3 engineer job.
Check whether your boiler is firing for hot water — look for fault codes. If the boiler is working but the cylinder is not heating, the coil inside the cylinder or a motorised valve may have failed.
Check the immersion heater backup: unvented cylinders usually have an immersion heater. Find the red switch and turn it on — if the water heats up after 45 minutes, the heating circuit is the problem, not the cylinder itself.
In all cases, servicing or repairing an unvented cylinder requires a G3-qualified plumber. Call and describe the symptoms — especially if the expansion valve is dripping, as this is a safety issue.
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