A thermostat communicates with a furnace via control wiring that runs between the furnace and the thermostat. The control wiring is made up of individual 18-gauge wires. Each wire carries out a specific function: heating, air conditioning and fan control, for instance. Each wire must be connected to the same terminal on the furnace that it is on the thermostat. If wires are crossed, damage to the furnace or thermostat can result. This can lead to costly repairs.
Turn off power to the furnace. Either turn off the correct circuit breaker or turn off the switch on the side of the furnace.
Remove the access panel on the front of the furnace. It will either pull right off or you'll have to loosen a few screws to pull it off.
Run a thermostat wire from your furnace to where your thermostat will be located. Leave an extra foot at each location so you have enough wire to make the connections. Use wire staples to secure the thermostat wire to wood joists.
Strip the outer casing of the thermostat wire back approximately two inches at the furnace and by the thermostat. Strip each individual wire back 1/4-inch to expose enough bare copper wire to make the connection.
Connect the thermostat wires to the terminal strip in the furnace. Connect the wires as follows: red wire to "R" terminal, white wire to "W" terminal, yellow wire to "Y" terminal, green wire to "G" terminal, black wire to "C" terminal. If you have two-stage heating, connect the brown wire to the "W2" terminal and if you have two-stage cooling, connect the light blue wire to the "Y2" terminal. Put the furnace access panel back on when all the wires are connected.
Go to where the thermostat will be located and screw the wall plate of the thermostat to the wall. Level it before fastening it tight. Connect the wires to the same terminals in the thermostat as you did on the furnace. Push the thermostat on to the wall plate once all the wires are connected.
Turn the power back on and test out the thermostat. Try out the heating, cooling and fan-only selections to ensure the furnace operates on all of them. If it all works, you can set your thermostat as desired.
You may have to remove some drywall to get the thermostat wire in the wall. If there was an old thermostat at the location you will be installing the new one, you should be able to fish the wire through by taping it to the old wire and then pulling it through the wall. If you will be running your thermostat wire exposed part of the way before it enters the furnace, mount it in a way to minimise possible damage. Secure it high in a joist space to make it hard for things to accidentally hit it.