Depending on the heating system you have, you can have from two all the way up to eight wires that run from your furnace to your thermostat. Two wire thermostats are heating only thermostats. These thermostats have two wires, typically red and white, that run from the furnace to the thermostat. These are the simplest thermostats to wire, as there are only two wires to contend with. In order to install a two wire thermostat, all you need is a few tools and basic home improvement skills.
Turn the power off to the furnace by turning the correct breaker off in the circuit breaker panel.
Pull the control wiring through the hole on the thermostat wall plate and mount it to the wall using drywall anchors and screws. If you can, use a stud finder to get at least one of the screws into a stud which will allow the thermostat to be secured a little better.
Strip the outer insulation from the control wiring back approximately 2 inches to expose the red and white wires. Strip the ends of the red and white wires back approximately 1/4 inch with your wire stripper.
Connect the red wire to the "R" terminal in the thermostat. Connect the white wire to the "W" terminal in the thermostat.
Mount the thermostat to the wall plate by gently pushing it on until it snaps into place. Make sure the control wiring doesn't get pinched in the process. It should gently tuck in behind the thermostat.
Open the furnace access door and check to see that the red wire is connected to the "R" terminal and the white wire is connected to the "W" terminal. If not, connect them to these terminals.
Put the furnace access door back on and turn the power back on to the furnace. Set the thermostat to the desired setting and verify that the heat kicks on.