Ariston CH Demand Signal Fault

Boiler displays error code 304. Hot water is working, but the heating does not respond to the thermostat.

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.

Possible Causes

Faulty room thermostat, Broken CH demand wiring, Diverter valve stuck in DHW position, PCB not recognizing CH request

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 remain 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 performed by a professional.

Hard 4/5

Diverter Valve Assembly

Check Price on Amazon

We may earn a commission from links on this page.