You'll find a variety of thermostat wires connected to the terminals in your thermostat. Two thermostat wires that are critical to the operation of the thermostat are the RH wire and the RC wire. The RC wire (C for Cooling), if connected properly, controls your HVAC cooling. The RH wire (H for Heating) provides power to your HVAC heating. If you are planning to replace your home thermostat, it will be essential that you know which wires to connect to the right terminals in your thermostat. Otherwise, you may find that your new thermostat is not working as it should.