In replacing or installing a thermostat in your home, be sure you know which thermostat wires are to be connected to which specific terminals. If your thermostat wiring follows conventional rules, the green wire supplies power to the fan, and should be connected to the Fan Relay. The wires are color coded, and are assigned to specific terminals. Some wire colors you see will include white, green, black, and sometimes light blue.
If, when you finish wiring your thermostat, you discover that your HVAC fan is not working, you should check that your thermostat is properly wired.