A thermostat doesn't have to be interactive to make a difference in your energy bill. By automatically "setting back" temperatures when the house is unoccupied, programmable thermostats can save on energy costs — as much as 1 percent for every degree of setback over an eight-hour period.

There's just one hitch: You have to take the time to program them. "Some of the first generation of programmables were painful to use," says This Old House plumbing and heating expert Richard Trethewey. "People got fed up. They'd just press the "hold" button and do it manually."

Today's digital thermostats are a different breed. Not only are they easier to set up, with bigger displays and customized options for every day of the week, they've acquired even more energy-saving and consumer-friendly smarts. For instance, there are models that can "learn" how long it takes to heat or cool a house and then how to most efficiently maintain a room's temperature within half a degree of the setpoint. Others automatically change setback times when Daylight Savings kicks in, and automatically switch from heating to cooling. And if there's ever a power outage or the batteries die, they remember and restore all the settings.

To reap the maximum benefit from any of these devices, which range in price from $40 to $300 and up, it has to work in concert with your heating and cooling system. A good first step is to call the local HVAC dealer who sells the furnace, boiler, or heat pump you now own to find out what your options are. If it makes sense to get a high-end programmable, then let the dealer install it or recommend someone who can. These units are not found on home-center shelves, in any case, and an expert installation will help ensure that your thermostat performs at its full potential, delivering all the comfort and energy savings it possibly can.

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