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Hot Water But No Heating
Taps run hot as normal but the radiators stay stone cold — the heating side alone has failed.
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What's happening?
When hot water works and heating does not, the boiler is firing perfectly well and something is stopping heat reaching the radiators. On a combi, the usual suspect is the diverter valve — the component that decides whether the boiler's heat goes to the radiators or to the hot water plate, and which fails far more often stuck on hot water than the other way round. On a system or open-vented setup with a cylinder, the equivalent is a motorised zone valve stuck closed on the heating circuit, or a programmer set to hot water only. The other family of causes is control: a room thermostat with flat batteries, a wireless receiver that has lost its pairing, or every thermostatic radiator valve closed because the weather turned mild.
What usually causes it?
- A combi diverter valve stuck in the hot water position, or its actuator failed
- A motorised zone valve on the heating circuit seized or with a failed synchronous motor
- The programmer set to hot water only, or its settings lost after a power cut
- A room thermostat with flat batteries, or a wireless pairing that has dropped
- Every thermostatic radiator valve closed, or the pins under the heads seized shut
- A seized circulating pump that still allows the boiler to make hot water on demand
- An air lock in the heating flow, so the pump cannot establish circulation
Is it dangerous?
Low. Nothing here is hazardous, though a house without heating in a hard frost needs attention before pipes freeze. Take care with radiator valve heads and pins — the water inside is hot, and a stuck pin freed carelessly can let a valve leak.
Can I fix it myself?
Plenty of useful checks. Set the programmer to heating on, constant, and confirm it is showing the right time and day. Turn the room thermostat well above room temperature and change its batteries. Turn every thermostatic radiator valve fully open. Take the head off one thermostatic valve and press the small metal pin underneath: if it does not spring back, it is seized, and freeing it with gentle pressure or a squirt of releasing fluid often restores that radiator. Check the pressure gauge is in the band. If all of that is right and the radiators are still cold, the fault is in the valve or the pump.
When should you call a heating engineer?
Call a heating engineer when the controls are all set correctly and the radiators still stay cold, when only some radiators work, or when you can hear the pump running but nothing gets warm. A diverter valve or a zone valve actuator is a routine replacement and not usually a long job, but it is not a homeowner repair. Call the same day in freezing weather.
What will the heating engineer actually do?
- Confirm the boiler is receiving a heating demand at its terminals, isolating control from mechanical faults
- Test the diverter valve or motorised zone valve for movement and correct switching, and replace the actuator or cartridge
- Check the pump is running and actually moving water rather than spinning dry
- Free or replace seized thermostatic radiator valves and check the bypass arrangement
- Re-pair or replace a wireless thermostat and receiver and check the wiring centre
- Recommission, balance the radiators and prove heat to every room
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