The short version
- On a combi, the diverter valve is the most common culprit
- On a cylinder system, look at the motorised valve, cylinder stat and programmer
- A flow sensor fault can stop the boiler firing for hot water at all
- Scale in the plate heat exchanger causes weak flow rather than no flow
Combi boilers
The boiler decides whether to heat radiators or the tap water using a diverter valve. When the cartridge or its actuator sticks in the heating position, radiators work perfectly and taps stay cold.
- Diverter valve cartridge stuck or worn
- Diverter actuator or motor failed
- Flow sensor or turbine not detecting a tap being opened
- Scaled plate heat exchanger — usually weak, fluctuating heat rather than none
Cylinder systems
With a hot water cylinder, the same symptom points at the controls instead: a failed motorised valve head, a faulty cylinder thermostat, or a programmer that's lost its hot water schedule. It's worth checking the timer settings before booking anything.
What to tell us when you call
Whether you have a cylinder or a combi, whether the boiler fires when you open a hot tap, and any fault code shown. That usually decides which parts travel to your door.
Common questions
Can I fix a diverter valve myself?+
No — it's inside a sealed, gas-safe appliance and requires a registered engineer.
How long does the repair take?+
Most diverter jobs are completed in a single visit where the part is on the van.
Is it worth repairing on an old boiler?+
Often yes; it's a wear part rather than a terminal fault. We'll advise if the wider picture says otherwise.
Why is the hot water lukewarm instead?+
That points at scale or a partially stuck diverter rather than a full failure.
Cold taps, warm radiators?
Tell us the symptoms and we'll bring the likely parts with us.