Boiler shows A15, no response to room thermostat, heating stays off

Safety: Isolate power before accessing control terminals.

• Check controller: Ensure the Herz room thermostat or controller has power, correct time, and is calling for heat.
• Inspect wiring: At the boiler, verify that the room thermostat wires are firmly connected and not damaged.
• Bridge test: Temporarily bridge the room thermostat terminals at the boiler (if allowed by Herz manual). If the boiler starts, the external control or wiring is faulty.
• Reconfigure parameters: Check that the boiler is set to accept external control (e.g., on/off or BUS mode) according to the Herz installation manual.