Thermostat calls for heat (heat icon on), but radiators or underfloor heating remain cold.

Safety first: Boiler and wiring centre contain mains voltage. Isolate power before opening any covers or touching wiring.

Steps to diagnose and fix:

• 1. Confirm boiler status: Check the boiler display for fault codes or lockout indicators. Reset the boiler if necessary following the manufacturer's instructions.
• 2. Check wiring centre indicators: Many Danfoss wiring centres have LEDs showing zone and boiler demand. Verify that the boiler demand LED lights when the thermostat calls for heat.
• 3. Inspect actuator operation: Touch the thermal actuators on the manifold (carefully). They should warm slightly and open after a few minutes of demand. Cold actuators may be disconnected or failed.
• 4. Verify wiring continuity: With power OFF, check continuity of the boiler demand circuit from the wiring centre to the boiler input. Repair any broken wires or loose terminals.
• 5. Replace faulty components: If the wiring centre's boiler relay does not close when signaled, replace the wiring centre or relay module. Replace any non-functioning thermal actuators.

When to call a professional: Hydronic system diagnosis is best handled by a heating engineer, especially if multiple components may be at fault.