Boiler display works but does not respond to room thermostat demand

Safety: Isolate power before altering control wiring.

• Check thermostat: Ensure the thermostat is set above room temperature and that any batteries are fresh.
• Manual demand test: If using a simple on/off thermostat, temporarily link the boiler's room stat terminals to simulate demand. If the boiler starts, the thermostat or its wiring is faulty.
• Inspect cable: Look for damage to the thermostat cable, especially at entry points.
• Replace thermostat: Fit a compatible Viessmann-approved controller or thermostat and wire according to the installation manual.