This article reprinted courtesy of Smarthome.
As temperatures begin to drop, there’s no better time for upgrading your outdated, mechanical thermostat with a programmable thermostat, which gives you the ability to set your heating system to turn on only during the hours you’re home to enjoy it, for significant cost savings. Programmable thermostats can be used as direct replacements for most existing thermostats. Best of all, the upgrade process is much easier than you would imagine. Just follow the five steps described below.
| Step 1: Prepare to Remove Old Thermostat |
Start by removing the cover plate of your existing thermostat so that the wires connecting to the terminals are visible. You may only find two wires, or you may find as many as eight. The number of wires used depends on the type of HVAC system you have.
The covers of most mechanical thermostats just pop off with a screwdriver, although some are held in place with a screw or two. Note: Don’t actually disconnect the wires until you complete Step 2.
| Step 2: List the Connections |
You’ll see that each wire has a different color. Write down which color wires are connected to which terminals, and keep the list handy. This step is important for you to connect the new thermostat correctly. Your list should look something like this:
| Wire Color | Terminal |
| Red | R |
| White | W |
| Green | G |
| Yellow | Y |
| Orange | O |
Don’t be concerned if there are extra wires not connected to the original thermostat. It’s a pretty common occurrence.
| Step 3: Remove the Old Backboard & Install the New One |
Turn off the power to the furnace and disconnect the wires from the terminals. 
Remove the backboard for the old thermostat (chances are that you’ll need a screwdriver to do so). Once you’ve removed the old one, replace it with the backboard for the programmable thermostat.
| Step 4: Reconnect Wires |
Retrieve the list you wrote down in Step 2 and reconnect each of the wires that were connected to the old thermostat to the same terminal labels of the new thermostat.
Don’t worry if there are terminals on the new thermostat that are not connected to a wire, or if there are extra wires that were not connected to the old thermostat. It’s a common practice for installers to use a cable with more conductors than are needed when the system is installed. Carefully push the extra wires back into the wall, leaving only the wires connecting to the thermostat in sight.
| Step 5: Place Thermostat on Backboard |
Snap the new thermostat onto the backboard that you just installed on the wall. Turn on the power to the furnace and check the thermostat for proper operation. You’re now ready to enjoy your new, programmable thermostat and the benefits that come with it. Follow the instructions that came with your programmable thermostat to set the temperature for days, nights, weekends, and vacations.
Note: When selecting a programmable thermostat, it’s important to know if your system is a conventional system that runs off a fossil fuel or a “heat pump.” Heat pumps are basically air conditioners that can run backwards — they take warm air out of the house during hot weather, and “reverse” and extract warm air from outdoors to heat your home during cool weather. The outside condenser unit blows out cold air in this mode. If you have a heat pump, be sure to choose a model designed to work with your system.