Thermostat powers on but does not turn the boiler on; boiler remains in standby

Safety: Isolate mains power to the boiler before touching any wiring.

• Step 1 – Verify demand: Increase the setpoint temperature well above room temperature and check if the thermostat indicates a heating demand (flame icon or similar).
• Step 2 – Check wiring mode: Confirm whether your Immergas boiler is expecting a simple on/off contact or BUS communication, and wire the thermostat accordingly.
• Step 3 – Inspect terminals: At the boiler, ensure the thermostat wires are connected to the correct terminals (e.g., TA, COM/NO, or BUS). Tighten any loose screws.
• Step 4 – Test contact: For on/off systems, temporarily bridge the boiler thermostat terminals with a short wire. If the boiler starts, the issue is in the thermostat or its wiring.
• Step 5 – Check boiler settings: In the boiler menu, confirm that external thermostat control is enabled and not overridden by internal programming.
• Step 6 – Replace thermostat: If wiring and boiler settings are correct but no call for heat is sent, the thermostat output relay or BUS driver may be faulty.