A21
Beretta
External Control / Thermostat Input Fault
Boiler displays A21, heating demand not recognised, room thermostat appears to call for heat.
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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
Open circuit in room thermostat wiring, Faulty external control (e.g., Beretta remote controller), PCB input fault on thermostat terminals
How to Fix / Troubleshooting
Safety: Isolate power before working on control wiring.
- Check controls: Ensure the room thermostat or programmer is set to call for heat and that batteries (if any) are good.
- Inspect wiring: Check the low-voltage thermostat cable from the controller to the boiler for damage or loose connections.
- Bypass test (technician): Temporarily link the thermostat terminals at the boiler to simulate a call for heat. If the boiler starts, the external control or wiring is faulty.
- PCB check: If bypassing does not start the boiler, the PCB thermostat input may be defective.
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Repair Difficulty
Medium
3/5
Required Part
Room Thermostat / Control Interface
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