Do you know the answer to this chilling wintertime question? Is it more energy-efficient to turn the thermostat down at night - or during the day when no one is at home - or to leave it at a constant temperature? Some say that turning the thermostat down or off intermittently can create more expense; that reheating the entire house from scratch in the morning and/or again in the evening is more expensive than leaving the furnace on all the time. These same folks will tell you that it is more energy-efficient to leave an air conditioner running 24 hours a day during the summer. The U.S. Department of Energy, however, has produced a leaflet that discounts these myths.

Consider this: Wouldn't a furnace or air conditioner wear out more quickly if operated 24 hours a day? And wouldn't that result in more equipment being replaced and more profit resulting for furnace and air-conditioning equipment manufacturers? You can save energy, lower your heating bill and cause your heating and cooling equipment to last longer by turning your thermostat down whenever possible.

Numerous studies have been made, and the conclusions vary, but it is generally agreed that substantial savings are probable when setback thermostats are used. Naturally, climate has a great deal to do with how much can be saved, but warmer climates win over colder ones. The reason: air conditioners are used more extensively in warmer climates and are more expensive to use because they are usually electrically operated. Although some furnaces are electric, most use fuel oil, coal, propane or natural gas. All are less expensive than electricity.

According to one manufacturer of thermostats, it is possible to save up to 24 percent of heating costs by setting back the temperature of a thermostat 10 degrees for two eight-hour periods per day. Setting back the thermostat 10 degrees for one eight-hour period per day, they say, can save 12 percent.

Some folks turn their thermostats down before going to bed and before leaving for work. One problem with this system is that in the morning when you get up and in the afternoon when you get home it takes a while to get comfortable. The modern answer to this dilemma is the programmable thermostat. It will turn the heater or air conditioner off when it isn't needed and then automatically turn it back on again in time to make things comfortable - before you get up or return home.

A thermostat is a device that automatically regulates the temperature of a system - a switch controlled by temperature. Thermostats are used in heating and cooling systems, and where temperature control is important - an oven, for example.

Most thermostats depend on the expansion of a substance responding to an increase in temperature. Lowering the temperature causes a contraction. Another widely used thermostat depends on two dissimilar strips of metal bonded together. Upon temperature change each strip expands or contracts at a different rate than the other. If one end of this pair of metal strips is fixed, a change in temperature will elongate one strip more than the other, causing the pair to curve. The result is a mechanical switch that is controlled by changes in temperature.

With a setback thermostat, the addition of a second temperature control allows the user to preset high and low temperature settings, and then change between those settings using a built-in timer. The most simple of these setback thermostats allows the user to preset one, two, three or even four on/off cycles with a pair of preset temperatures - one high and one low.

More sophisticated digital models have long-term programs with almost unlimited temperature and time controls. The first few generations of setback thermostats weren't so user-friendly.

When you install a thermostat for the first time, be aware that location is everything. Air traveling in and out of ducting, sunlight or exterior walls is taboo to a thermostat. For maximum comfort, use a setback thermostat.