This power adapter will provide power to your thermostat from any standard electrical outlet. You could use this in case you don't already have power coming up from your furnace to your thermostat. This transformer converts 110VAC to 24VAC for use with your thermostat. Simply connect one end to the "Rh" terminal and the other end to the "C" terminal. You can connect either wire to either terminal (there is no polarity).