Boiler displays A15, no response to thermostat, heating demand not recognised

Safety: Isolate power before working on control wiring.

Steps:

• Check controller display: Ensure the De Dietrich room thermostat or controller is powered and not showing its own error.
• Inspect wiring: At the boiler control terminals, verify that the thermostat wires are securely connected and not damaged.
• Bypass test (qualified persons only): Briefly bridge the room thermostat terminals at the boiler to simulate a heat demand. If the boiler starts, the external control or wiring is faulty.
• Check settings: In the boiler menu, confirm that the correct control mode (on/off, OpenTherm, BUS) is selected for your controller.

Professional help: For BUS or modulating controls, a technician may need to diagnose communication issues and replace the controller or PCB.