The number of thermostat wires varies depending on if you have a furnace, heat pump, air conditioner and how many stages each of these units has. The typical home heating system with two stages of heating and a single stage air conditioner will use six thermostat wires in order to correctly function.
Turn the power off to the furnace by turning off the corresponding breaker in the circuit breaker panel.
Mount the wall plate of the thermostat to the wall. Secure it to the wall using drywall anchors and screws. Pull the thermostat wiring through the hole in the wall plate and check it with a level to ensure it is level after installation.
Strip the outer insulation of the thermostat wiring back until approximately 2 inches of the six individual wires inside are exposed. Strip the individual wires back so 1/4 inch of copper wire is exposed.
Connect the red wire to the "R" terminal on the thermostat. Connect the white wire to the "W" terminal. Connect the green wire to the "G" terminal. Connect the yellow wire to the "Y" terminal. Connect the black wire to the "C" terminal. Connect the light brown wire to the "W2" terminal.
Open the access panel on the furnace and check to see that the wires are attached to the same terminals on the control board as they are on the thermostat. If not, hook them up to the same terminals.
Close the access panel and turn the power back on to the furnace.
Turn on the furnace and set the thermostat to your desired setting. The furnace should operate when the thermostat calls for heat.