Glow-worm
Heating Demand / Diverter or Control Fault
Boiler fires for hot water but not for central heating
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Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
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Possible Causes
Faulty room thermostat or programmer, Stuck diverter valve (on combi models), Faulty heating demand wiring, Motorised valve failure (on system/regular boilers)
How to Fix / Troubleshooting
Safety: Isolate power before accessing wiring centres or valves.
- Check controls: Ensure the programmer is set to heating ON and the room thermostat is turned up above room temperature.
- Check boiler display: Many Glow-worm models show an icon or code when there is a heating demand. If no demand is shown, the issue may be external controls.
- Test with manual override: On external motorised valves, move the manual lever to the open position and see if radiators heat up.
- Engineer tasks:
- Test room thermostat and programmer outputs.
- Check and replace a faulty diverter valve assembly on combi boilers.
- Repair or replace failed motorised valves on system boilers.
Control wiring faults should be handled by a competent heating engineer or electrician.
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Repair Difficulty
Medium
3/5
Required Part
Diverter Valve Assembly
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