304
Ariston
CH Demand Signal Fault
Boiler shows 304, hot water works but heating does not respond to thermostat
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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, Broken CH demand wiring, Stuck diverter valve in DHW position, PCB not recognising CH request
How to Fix / Troubleshooting
Safety: Turn off power before accessing wiring or valves.
- Check thermostat settings: Ensure the room thermostat is calling for heat (temperature set above room temperature, schedule active).
- Bypass test: If safe and permitted, link the CH demand terminals at the boiler. If heating starts, the thermostat or its wiring is faulty.
- Diverter valve check: If radiators stay cold but hot water is fine, the diverter valve may be stuck in DHW mode. Feel the CH flow pipe from the boiler when CH is demanded; if it stays cold, the valve is suspect.
- Engineer repair: Diverter valve motor head or cartridge replacement should be done by a professional.
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Repair Difficulty
Hard
4/5
Required Part
Diverter Valve Assembly
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