Reply: The single most important thing most people can do to make their homes more energy efficient is to replace antiquated heating-cooling systems with modern equipment. Furnaces, boilers, air conditioners and water heaters made more than 15-20 years ago were designed with little thought about conserving energy. Fuel costs have risen dramatically since then. Some older homes still have boilers that were originally designed to burn coal. Replacing these old clunkers can achieve great savings.

Replacement costs sometimes can be recaptured in two to three years, though five to seven years is more common. A good heating-cooling contractor ought to be able to run you through the calculations.

The cost-payback equation includes many variables - the design of your house, climate, system design, fuel costs, plus the installer's skills. Sometimes the payback in energy savings will take seven or eight years or longer. The average U.S. homeowner changes addresses every seven years. Make sure to consider if you will be living there long enough to recoup your investment.

Another caution: Just like high performance autos, some of the ultra-efficient heating-cooling equipment on the market is very delicate and prone to breakdowns. Money saved in fued in fuel costs could well be lost in repeated service calls. Deal with an experienced heating and cooling professional who knows what brands to stay away from. In fact, dealing with superior tradespeople is very important to the cost vs. payback equation. Poor installation can dramatically reduce the performance of high-efficiency equipment.

For instance, a common mistake made by inexperienced people is failing to install a new indoor evaporator coil when replacing a heat pump or central air conditioner. Whole-house air conditioners and heat pumps are split systems whereby the indoor and outdoor condensing units are designed to work together. It is important to match the indoor and outdoor units in accordance with the system manufacturer's specifications. Studies have shown that in almost half of replacement installations, the evaporator coil does not match the condensing unit.

Consequently, the homeowner can lose as much as 15% in operating efficiency. A mismatch also can lead to premature system failure.

Placement of the outdoor unit also impacts efficiency. The compressor should be installed in a cool, shaded place, usually on the north side of the house, never east or west. Sunlight will heat up the compressor, reducing its efficiency. The installer also should make sure the compressor has adequate airflow around it and will not be choked by vegetation.

Where you do opt for central heating heating/cooling, consider paying more for a "zoned" system that will enable you to control temperatures independently in various parts of the house. Rooms that go unoccupied for long periods can be set lower than the main living areas. A properly designed zoning system can save as much as 30% in heating/cooling costs. They also make for greater comfort, ironing out those cold and hot spots typical of most homes.

Automatic setback thermostats typically show some of the shortest payback periods of any energy conservation devices. You program these to modulate home temperature throughout the day. For instance, you might choose a relatively chilly 60-65 degrees while you sleep and during work hours, then set it to rise to around 70 about the time the first person arrives home. Setback thermostats usually pay for themselves within a year or two.

Finally, don't sneer too much at the simple tips you read about. Some of the greatest returns come from doing things that entail no expense.

For instance, turning down the thermostat from 70 degrees to 65-66 can save hundreds of dollars a year in fuel costs. A rule of thumb that works in many places is a 2% reduction in fuel costs for every degree turned down.

It's also a good idea to dial down the temperature on your water heater to 120 degrees. The government mandates new water heaters to be preset at this temperature to help prevent scalding accidents. Olnts. Older water heaters typically were set to 140 degrees. Turning down the heat not only is a safety precaution, it saves considerable energy and fuel costs. People typically mix hot and cold to achieve a water temperature that doesn't cause discomfort. Heating the water way high only to dilute it with a lot of cold water is a waste of both energy and water.

Finally, water heater insulating wraps are a good low-cost investment with a rapid payback for water heaters stored in cold basements and utility rooms. Also, be sure to clean or replace a/c filters very often, even monthly during prime cooling seasons. Dirt and dust can cut down substantially on the equipment's efficiency.