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No Hot Water
Cold water runs fine but the hot taps stay cold, or run hot then go cold within seconds.
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What's happening?
No hot water is a symptom of one of three quite different systems, and the fix depends on which one you have. A stored system heats water in a cylinder, from the boiler coil or an immersion heater, and fails when the cylinder thermostat, the motorised valve or the immersion element stops working. A combi boiler heats on demand and fails when the diverter valve sticks or the plate heat exchanger scales up. An unvented cylinder adds a mains-pressure control set of its own. Cold water still running normally tells you the incoming supply is fine and the fault is on the heating or storage side of the hot water, not the mains.
What usually causes it?
- Immersion heater element burnt out, or its thermal cut-out tripped and needing a manual reset
- Cylinder thermostat failed, or set too low to bring the store up to temperature
- Motorised valve actuator seized so the boiler heats radiators but never the cylinder coil
- Combi diverter valve stuck on the heating side — heating works, hot taps stay cold
- Limescale in a combi plate heat exchanger, giving a brief burst of hot then lukewarm water
- Programmer or time clock lost its schedule after a power cut and is no longer calling for hot water
- Airlock in the hot distribution after the system was drained, so hot water reaches some taps and not others
Is it dangerous?
Losing hot water is not itself dangerous, but two related things are. Stored water sitting lukewarm for long periods is the condition Legionella bacteria grow in, which is why a cylinder is normally stored at around 60°C. And water hot enough to be safe from that is hot enough to scald in seconds, which is why bath and basin outlets are commonly limited by a blending valve. Never turn a cylinder thermostat down to a comfortable tap temperature to solve a scalding worry — the answer is a blending valve, not cooler storage.
Can I fix it myself?
Check the obvious controls first: the programmer is on and calling for hot water, the boiler has power and shows no fault code, and the immersion switch is on. If you have an immersion heater, its thermal cut-out has a small reset button under the cover — resetting it once is reasonable, but a cut-out that trips again is telling you the thermostat has failed. Do not remove a boiler casing, and do not touch an unvented cylinder's valves. Everything past the switch and the programmer needs a plumber, and a gas boiler needs a Gas Safe registered engineer.
When should you call a plumber?
Call a plumber when the controls are all set correctly and you still have no hot water, when hot water goes cold within seconds of running, or when only some taps run hot. Call sooner if you have an unvented cylinder and see water at the tundish, or if the cylinder itself is leaking. A gas boiler that is locked out or showing a fault code needs a Gas Safe registered engineer for the appliance itself.
What will the plumber actually do?
- Establish which system is fitted and prove where the heat stops — boiler, valve, cylinder or distribution
- Test the cylinder thermostat and immersion element, and check the immersion thermal cut-out has not failed
- Free or replace a seized motorised valve actuator and confirm the boiler is being called correctly
- Descale or replace a scaled plate heat exchanger on a combi, or fit a scale reducer on hard water
- Purge an airlock from the hot distribution and rebalance the flow to the affected outlets
- Reset stored water to a safe temperature and check the blending valve serving bath and basin
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