Ariston
No Heat Demand Transfer (Relay/Bus Not Activating Boiler)
Thermostat calls for heat but boiler does not start; no error code shown on thermostat or boiler.
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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
Incorrect wiring between thermostat and boiler, Thermostat set to wrong control mode (bus vs on/off), Faulty relay contacts, Boiler external control input disabled in boiler settings
How to Fix / Troubleshooting
Safety: Isolate power to boiler and thermostat before wiring checks.
- Check control mode: In the thermostat installer menu, confirm whether it is configured for bus control or simple on/off relay, matching the boiler’s expected input.
- Verify wiring: Ensure the thermostat’s COM and NO relay terminals are wired to the correct boiler terminals (e.g. TA/COM). For bus mode, ensure bus wires are on OT+/OT− or BUS terminals.
- Boiler settings: On the boiler control panel, ensure external thermostat control is enabled and not overridden by internal programming.
- Test relay: With power on, command heating and listen for a click at the thermostat. If no click and no boiler response, the relay or bus output may be faulty and the thermostat may need replacement.
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Repair Difficulty
Medium
3/5
Required Part
Ariston Thermostat Relay Output
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