This is from the
Iowa State Energy Center (link) ... and basically dispels the myths about setbacks. Read on:
-- Mike
According to the U.S. Department of Energy, turning down the thermostat 8 degrees F for eight hours a day will save 8-10 percent on home heating costs. An easy way to take advantage of these savings is to lower the thermostat temperature while away from home or sleeping.
Any thermostat, even a digital electronic model, is essentially an on/off switch for the heating system. When it senses the room temperature has dropped, it signals the furnace to provide heat until the temperature rises to the selected temperature.
Some older thermostats may be inefficient, allowing temperature variations of up to 5 degrees F. Inefficient thermostats cause the furnace to cycle on/off frequently, wasting a great deal of energy. If it's time for a replacement, consider installing an automatic "setback" or programmable thermostat. While you might forget to turn down the heat before you leave for work in the morning, a programmable thermostat won't.
Programmable thermostats help save energy because they store and repeat multiple daily temperature settings, which you can manually override without affecting the rest of the daily or weekly program. Many models can even store six or more temperature settings a day. New programmable thermostats are also usually highly accurate—within .5 degrees F of the actual setting.
On a winter night, for example, when the house doesn't need to be heated to 72 degrees F, simply setting back the thermostat can save money. It is simple to program the thermostat to 65 degrees F from 11 p.m. to 5 a.m., and then bring it back to 72 degrees F so it is nice and warm to wake up to. In a well insulated home the temperature change may go unnoticed.
Myths about Setbacks
A common misconception associated with programmable thermostats is that a furnace works harder than normal to warm the home back to a comfortable temperature after the thermostat has been set back, resulting in little or no savings. Years of research and numerous studies have shown that the fuel required to reheat a home is roughly equal to the fuel saved as the home drops to the lower temperature. This will result in fuel savings between the times the temperature stabilizes at the lower level and the next time heat is needed. The longer the house remains at the lower temperature, the more energy saved.
Another misconception is that the higher the thermostat is raised, the more heat the furnace will put out or the faster the house will warm up. Furnaces put out the same amount of heat no matter how high the thermostat is set. The difference is the length of time it must stay on to reach the set temperature. In the winter, a significant amount of money can be saved by setting back the thermostat for as little as four hours per day. The savings can be attributed to a building's heat loss in the winter, which depends greatly on the difference between the inside and outside temperatures. By turning the thermostat back 15 degrees F for eight hours; it is possible to save 5-15 percent a year on the heating bill.