The thermostat calls for heat, but the furnace does not respond; the display shows standby.

Safety: Turn off power before altering thermostat wiring.

• Verify thermostat operation: Increase the setpoint well above room temperature and listen for a click or indicator on the thermostat.
• Check wiring at furnace: Ensure thermostat wires are firmly connected to the correct terminals.
• Bypass test (on/off systems): Temporarily bridge the thermostat terminals at the furnace. If it starts, the thermostat or cable is faulty.
• Control settings: On Ariston modulating controls, verify that the control is not in OFF or HOLIDAY mode.