There are several different kinds of programmable thermostats, and depending on what your typical schedule is will determine which one you should get. Every programmable thermostat will follow the schedule you set and maintain up to four pre-programmed settings throughout the day within two degrees. Different models come with different features like, vacation modes, touch pad screens, sensor indicators, phone or voice programming and more.

To figure out which of the three different kinds of programmable thermostats is right for you, think about your weekly schedule, work hours, school times, and the times when you sleep or eat. You will program you schedule differently if you are very regular throughout the week, as opposed to someone who has a different schedule every day. Your three options include the 7-day, 5+2-day, or the 5-1-1-day.

7-day models: For people whose schedule varies day to day, a 7-day model will be the best for you because they give you the most flexibility, and let you set different programs for different days — usually with four possible temperature periods per day.

5+2-day models: For people who have the same schedule every weekday, and another for weekends.

5-1-1 models: For people who have the same schedule every weekday, and two different schedules for the weekends, say on Saturday and Sunday.

Smart Thermostats are wifi enabled, allowing you to control them remotely. Some models also learn your habits and adjust temperature settings accordingly. Read more here.

Before installing a programmable thermostat select the location for where it will be placed. Often times you will want to install it in the exact spot the old thermostat was. Sometimes though, the old thermostat may be placed in an undesirable location that too close to sources of heat, cold, drafts, or direct sunlight. Ideally your thermostat should be placed on an interior wall that does not experience dramatic fluctuations in temperature due to other influences.

Since this project entails rewiring anywhere from 2-10 wires, make sure to shut off your electricity during the replacement. If the job is more than just a simple replacement, like the thermostat needs to be moved or other additions are being made to your heating and cooling system, be sure to call a HVAC professional.

Each individual model is different and will have its own set of directions, be sure to read them carefully before beginning. In general the steps are as follows:

1. Turn off service switch at boiler.
2. Pull off cover from existing thermostat.
3. Unscrew thermostat from its wall-mounted sub-base.
4. Loosen wires connected to thermostat's sub-base.
5. Remove the old sub-base.
6. Hold sub-base of new thermostat to wall and connect red wire to "R" screw terminal, and white wire to "W" screw terminal.
7. Drill new screw-mounting holes for sub-base.
8. Push plastic wall anchors flush into holes and screw sub-base to wall.
9. Insert batteries into the thermostat and snap on the cover.
10. Turn the service switch back on.
11. Program the desired on/off time and temperature of the thermostat.

You can also watch a video from This Old House: How to Install a Programmable Thermostat

Just having a programmable thermostat isn't enough, you must set it properly as well to insure that you are getting the most savings you can. Your instruction manual will provide you with guidelines and suggestions for programming the settings correctly. The following is an example of how one day of your week might be set.
 

Programmable Thermostat Setpoint Times & Temperatures

Setting	Time	Setpoint Temperature (Heat)	Setpoint Temperature (Cool)
Wake	6:00 a.m.	≤ 68° F	≥ 78° F
Day	8:00 a.m.	Setback at least 8° F	Setup at least 7° F
Evening	6:00 p.m.	≤ 68° F	≥ 78° F
Sleep	10:00 p.m.	Setback at least 8° F	Setup at least 4° F

If you use your thermostat properly, you could save about $180 annually in energy costs. Read more here.

ENERGY STAR Rules of Thumb for Proper Use of Programmable Thermostats:

1. Keep the temperature set at its energy savings set-points for long periods of time (at least eight hours), for example, during the day, when no one is at home, and through the night, after bedtime.
2. All thermostats let you temporarily make an area warmer or cooler, without erasing the pre-set programming. This override is cancelled automatically at the next program period. You use more energy (and end up paying more on energy bills) if you consistently “hold” or over-ride the pre-programmed settings.
3. Units typically have 2 types of hold features: (a) hold/permanent/vacation; (b) temporary. Avoid using the hold/permanent/vacation feature to manage day to day temperature settings. “Hold” or “vacation” features are best when you're planning to be away for an extended period. Set this feature at a constant, efficient temperature (i.e. several degrees warmer temperature in summer, several degrees cooler during winter), when going away for the weekend or on vacation. You'll waste energy and money if you leave the “hold” feature at the comfort setting while you're away.
4. Cranking your unit up to 90 degrees or down to 40 degrees, for example, will not heat or cool your house any faster. Most thermostats, including ENERGY STAR qualified units, begin to hear or cool at a set time, to reach setpoint temperatures sometime thereafter. Units with adaptive (smart/intelligent) recovery features are an exception to this rule — Adaptive recovery units are constantly calculating the amount of time required to heat or cool the house, so that it reaches that temperature when the homeowner programmed it. By “examining” the performance of the past few days the thermostat can keep track of the seasons. In this way, your house is always at the comfort levels when occupied, but saving the most energy when unoccupied.
5. Many homes use just one thermostat to control the whole house. If your home has multiple heating or cooling zones, you'll need a programmed setback thermostat for each zone to maximize comfort, convenience and energy savings throughout the house.
6. If your programmable thermostat runs on batteries, don't forget to change the batteries each year. Some units will indicate when batteries must be changed.