Room thermostat calls for heat but the furnace does not start; the display shows standby with no error code.

Safety: Only change user-level settings you understand; refer to the Defro user manual.

• Check the operating mode on the Defro controller (heating + DHW vs. DHW only vs. standby).
• Review the time schedules to ensure heating is enabled at the current time.
• Verify that any external safety contacts (e.g., fire alarm, emergency stop) are closed and not tripped.
• Inspect room thermostat wiring and confirm correct terminals are used.

If the controller logic appears correct but the boiler still does not start, a technician should check internal inputs and outputs.