DHW Priority Fault
Grant
Diverter or Motorised Valve Fault (Y-plan/S-plan)
Boiler heats domestic hot water cylinder but does not supply central heating, even when CH is calling.
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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
Stuck 3-port mid-position valve, Failed 2-port CH zone valve, Faulty microswitch in valve head, Wiring centre fault
How to Fix / Troubleshooting
Safety: Isolate power before removing valve heads or accessing wiring centres.
User checks:
- Confirm that the room thermostat is turned up and programmer is set to CH ON.
- Listen near the motorised valve(s) for movement when CH is selected.
Engineer steps:
- Identify the 3-port or 2-port valves controlling CH and DHW circuits.
- Check the valve actuator for movement and test the end-switch output that signals the Grant boiler to fire.
- Replace the valve head or complete valve if seized or if the microswitch has failed.
- Verify correct wiring in the wiring centre to ensure CH demand is passed to the boiler.
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Repair Difficulty
Medium
3/5
Required Part
3-port motorised valve
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