App or thermostat indicates that the boiler is not responding; heating does not start even when there is a call for heat.

Danger: Boiler and Relay wiring may carry mains voltage. Switch off power at the breaker and boiler service switch before touching any wires. If you are not trained, contact a heating professional.

• Check boiler status: Verify the boiler has power, pressure, and no lockout error on its own display. Reset the boiler if needed according to its manual.
• Inspect Relay wiring: With power OFF, open the Relay wiring cover and confirm the two thermostat control wires are firmly clamped in the correct terminals (COM/NO or dedicated thermostat input) as per Netatmo and boiler manuals.
• Check continuity: If you have a multimeter and skills, test continuity of the two control wires from Relay to boiler to ensure no break in the cable.
• Bypass test (advanced): With power OFF, temporarily bridge the boiler thermostat terminals with a short wire. Restore power briefly: if the boiler starts heating, the issue is likely in the Netatmo Relay or its wiring.

If wiring is correct and the boiler responds to a manual bridge but not to the Netatmo Relay, the Relay’s internal dry‑contact relay is likely faulty and the Relay should be replaced.