Thermostats communicate with HVAC units via low voltage control wiring. Each function of a thermostat needs an appropriate wire that will run from the thermostat to the HVAC unit. Depending on what kind of HVAC unit you have, the number of wires could range from two all the way up to eight. Two wire systems are heating only, and by adding additional functions, you add another wire for each.
Turn off power to the HVAC unit. There will be a switch on the side of the unit that controls power to it. Turn it to the "off" position.
Remove the access panel on the front of the furnace and set it aside.
Look for the control board inside the furnace. It will have a terminal strip on it where the low voltage control wiring hooks up to. Terminals on the terminal strip will most likely be "R, Y, W, G and C." If your furnace has a second stage cooling or heating, you may have "Y2 and W2." The wires on the furnace control board will be connected up to the same labeled terminals on the thermostat.
Look at and make note of which wires go to which terminals. The terminals that control the heating are "R and W." The "R" terminal is the power terminal and usually has the red wire connected to it. The "W" terminal is the heating terminal and usually has the white wire connected to it. The terminals that control the cooling are "Y and C." If you have air conditioning, you will notice that there are two wires connected to these that run to your outside condensing unit. There will also be a wire going from the "Y" terminal to the thermostat, and it is usually colored yellow. If your thermostat uses power from the furnace, the black wire will run from the "C" terminal to the thermostat. The "G" terminal controls the furnace blower fan which can be operated separately at the thermostat from the heating and cooling system. The wire that is usually connected to this is the green wire.
Check to see if your system has the additional terminals "Y2 and W2." These terminals exist if your furnace or air conditioner has a "second stage" of heating or cooling. What these do is allow the furnace or air conditioner to first come on with a low setting to see if it can meet the heating or cooling requirements. If after a certain time, the temperature requirement isn't satisfied, the furnace will kick up to a higher setting to finish the heating or cooling needs. For the second stage heat, the "W2" terminal is used and this is usually connected with a brown wire from the furnace control board to the thermostat. For second stage cooling, the "Y2" terminal is used and is connected with the light blue wire from the furnace control board to the thermostat.
Put the unit's access panel back on after identifying all the thermostat wires.
Turn the power back on to the HVAC unit.