Hot water cylinder not heating — vented and unvented
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Most likely cause & what to check
Determine your cylinder type first: a vented cylinder is an older copper tank fed by a cold water storage tank in the loft. An unvented cylinder is a pressurised tank directly connected to the mains (usually has an expansion vessel and safety valves on it).
Check the programmer: is hot water scheduled for the current time? For vented cylinders, the heating circuit must also be calling — the boiler heats the cylinder via a coil inside it.
For vented cylinders: check the cylinder thermostat dial (on the side of the cylinder). It should be set to 60°C. If it's been turned right down, the boiler won't fire for hot water.
Listen during a hot water call — you should hear the motorised valve click open, then the boiler fire. If the valve doesn't open, it has likely failed (this is a very common fault on older vented systems).
For unvented cylinders: check the immersion heater element and thermostat (if electrically heated). An unvented cylinder should be serviced annually by a qualified engineer — the safety valves and expansion vessel need checking.
Check that the cold water supply to the cylinder is not shut off — there's usually an isolating valve on the inlet pipe.
If the cylinder is warm at the top but cold at the bottom, the immersion heater element at the lower position has failed (if fitted). The upper immersion heats only the top portion of the cylinder — this is the "boost" position for quick hot water.
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