When your radiators heat up but the taps run cold, the boiler is not necessarily completely broken. The fault may affect only the hot water controls, valves, sensors or cylinder, so identifying your heating system is the best place to start.
Start by identifying your system
If you have a combi boiler, it heats water directly as you open a tap and usually has no separate hot water cylinder. If you have a system boiler, you will normally find a hot water cylinder in an airing cupboard, along with a programmer and one or more motorised valves.
Some cylinder systems also have an immersion heater, which uses electricity to heat stored water. Follow the relevant path below, but stop immediately if you smell gas, see water near electrical components or the boiler repeatedly locks out.
Check whether the fault affects every hot tap or only one outlet.
Look at the boiler display for an error code before resetting it once.
Check the pressure gauge against the range recommended in the manufacturer’s instructions.
Confirm that the gas, electricity and water supplies have not been interrupted.
If you have a combi boiler
1. The hot water programmer or controls are set incorrectly
Combi boilers usually provide hot water on demand, but some controls can still affect the hot water setting. Check that the boiler has not been set to an unusual mode, such as heating only, and make sure any digital control is showing the correct time and programme.
Run one hot tap fully and listen for the boiler starting. If it does not respond, check for an error code and try one reset only, following the manufacturer’s instructions. Repeated resets can conceal a fault and should not be used as a long-term solution.
2. A diverter valve is stuck or failing
The diverter valve directs heat between the radiators and the hot water circuit. If it sticks in the heating position, the radiators can work normally while the boiler fails to heat water for the taps.
This is a common combi boiler fault, particularly on older appliances. A heating engineer may need to clean, repair or replace the valve, and it is not a component homeowners should dismantle themselves.
3. A hot water sensor or thermostat has failed
Combi boilers use sensors to detect water flow and regulate temperature. A faulty temperature sensor, flow sensor or associated wiring can prevent the boiler from recognising a demand for hot water, even though central heating continues to operate.
Error codes may point towards this problem, but a proper diagnosis usually requires electrical testing by a qualified engineer. Do not remove the boiler casing to investigate.
4. The plate heat exchanger is scaled or blocked
The plate heat exchanger transfers heat from the boiler to the mains water. In hard-water areas around South West London, limescale can build up inside it, restricting flow and reducing heat transfer.
You may notice lukewarm water, fluctuating temperatures or a hot tap that takes longer than usual to warm up. Descaling or replacing the heat exchanger can restore performance, depending on its condition.
5. The boiler pressure is too low
Low system pressure can stop some boilers from operating correctly. Check the gauge only when the system is cool and compare the reading with the manufacturer’s guidance, as many systems operate around 1 to 1.5 bar when cold.
If pressure keeps falling after it has been topped up, there may be a leak, faulty expansion vessel or pressure relief problem. Arrange a repair rather than repeatedly repressurising the system.
If you have a cylinder system
6. The programmer is not calling for hot water
Check that hot water is switched on at the programmer and that the time settings are correct. If there is a temporary boost function, try it once to see whether the cylinder begins heating.
A power cut, timer fault or incorrect setting can leave the heating working while the hot water remains off. Check the central heating controls before assuming the boiler itself has failed.
7. A motorised valve is stuck
Motorised valves control whether heat travels to the radiators, the cylinder or both. If the valve serving the cylinder is stuck or its actuator has failed, the boiler may fire for heating but never send hot water to the cylinder.
Typical signs include a cylinder that stays cold despite the programmer calling for hot water, or a valve that makes unusual buzzing or clicking sounds. Valve repairs should be carried out by a heating engineer.
8. The cylinder thermostat or an airlock is stopping heating
A failed cylinder thermostat may not signal that the stored water needs heating. The thermostat could also be set too low, although avoid adjusting it significantly without understanding the system, as stored hot water should generally be maintained at a safe temperature to reduce hygiene risks.
An airlock in the hot water pipework can also restrict flow, especially after plumbing work or a tank has been drained. Do not loosen fittings to release it, as this can cause leaks or scalding water.
9. The immersion heater or thermostat has failed
If your cylinder has an immersion heater, check that its dedicated switch has not been turned off and that the fuse or consumer unit has not tripped. A failed immersion thermostat or heating element can leave the cylinder cold even when the boiler and radiators work.
Where water is near electrical switches, wiring or the immersion heater, turn off the relevant supply if it is safe to do so and arrange urgent professional help. For cylinder systems, a specialist unvented cylinder service can identify faults and check safety controls.
Information to gather before calling
Before booking a visit, note the boiler make and model, any displayed error code, when the problem started and whether it affects all hot taps. It is also useful to mention any recent pressure drops, plumbing work, unusual noises or changes to the controls.
These details help an engineer prepare for the appointment and distinguish between a combi boiler fault, a control issue and a cylinder problem. Never attempt gas work, open the boiler casing or work on electrical components yourself.
