Safety: Turn off power before altering thermostat wiring.

Verify thermostat operation: Increase 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.

Thermostat calls for heat but furnace does not respond; display shows standby.

Possible Causes