Most homes have switches that control such things as lighting, garbage disposals, pumps and automatic garage door openers. A device, which most of us don't think of as a switch, is the thermostat. It controls the home's heating system.

Unless you live in a home built in the last 10 years or have upgraded the original thermostat, chances are good that you have a small box mounted to the wall. The box has a small lever or thumb dial that is adjacent to several hash marks and numerals which represent increments of temperature. Sound familiar?

Essentially, the purpose of that little box, which acts as an automatic switch, is to turn a heating system be it furnace, boiler or air conditioner on and off at the appropriate times.

The problem: Old thermostats aren't "smart." They turn on the furnace when the space gets a bit cool and turn it off when the desired temperature is reached. Now, that's not all that bad. Certainly, it's been working for a lot of you for years and years. Unfortunately, you're probably the same folks who complain about high utility bills and how cold your home is when you get up in the morning.

Wouldn't it be great to have a "smart" thermostat that would make your home more comfortable, conserve energy and lower your utility bill? One that would turn on just before you get out of bed in the morning; that would turn itself off about the time everyone left the home to avoid wasting precious energy, yet smart enough to have the home toasty just in time for your return at the end of a long day; that would turn off just about the time you snuggle into your warm bed. Sound good? Well, such smart thermostats have been around for more than a decade and continue to improve.

They're called setback or programmable thermostats. In most cases, they offer all of the features described and can be used with your old clunker furnace or boiler.

Programmable thermostats can be found in most hardware stores or home improvement centers and are designed for do-it-yourself installation. There are two basic models from which to choose; the analogue or "clock" mechanical style and the digital electronic. They range in price from about $35 to more than $100 depending upon the features. Mechanical thermostats are generally less expensive and don't have nearly as many features as do the electronic models. These are "super smart." Some electronic thermostats can be programmed day to day or for weekdays versus weekends, or to raise and lower the temperature several times a day. What's more, like computers, as technology improves, the product becomes more affordable.

When shopping for a replacement thermostat it's always a good idea to take along the make and model of your furnace and thermostat. This will help with the selection of a new model. One of the first things that you'll need to know is whether you are replacing a low-voltage, millivoltage or line-voltage model. Since most modern units are low voltage, it's important to be sure.

Once you have chosen a model, be sure that it comes with installation instructions and in an unopened package. Often, you'll come across returned merchandise which no longer has instructions or is missing a part or two. Many manufacturers will boast of a toll-free hotline to assist with installation. This is typically stated in a prominent location on the packaging.

The first step in the replacement process is to turn off the power to the existing thermostat to avoid injury. Use an electrical tester to verify that the power is off.

Remove the old thermostat by removing the cover to expose the mounting plate. Loosen the screws that attach the mounting bracket and pull it away from the wall. Disconnect the wires from the old thermostat and label each wire with the old terminal designation. Keep the wires from falling back into the wall by wrapping them around a pencil.

Separate the new thermostat from its mounting plate (often the mounting plate is one with the thermostat). Pull the wires through the mounting plate making sure to keep them separate. Level and attach the new thermostat mounting plate to the wall using the screws provided. It's especially important that the thermostat be level since the "switch" in many thermostats is a mercury bulb as opposed to a magnetic or electronic switch.

Next, match and connect the wires as labeled to the correct terminals on the mounting bracket or the back of the thermostat. If only two terminals and two wires exist, it generally doesn't matter which wire is connected to which terminal.

Use a small amount of fiberglass or foam insulation to fill the hole in the wall where the wires exit the wall. This will curtail any negative impact that drafts could have on the thermostat. Snap the thermostat onto the mounting plate and install the cover.

Finish the job by programming the thermostat according to the instructions provided by the manufacturer and in keeping with your "program."

Turn the power back on and start counting your savings dividends in the newfound comfort of your home.

We have received letters from some of you who are interested in how to obtain more information about the Vacu-Stack anti-downdraft fireplace device that we wrote about in a recent column.

Vacu-Stack is manufactured by Improved Consumer Products, Inc., Box 1264, Towne Street, Attleboro Falls, Mass. 02763.