Talk about misplaced priorities: In the name of saving energy, many people think nothing of spending tens of thousands of dollars on replacement windows. But at the same time, they’ll willingly limp along with an obsolete furnace whose replacement would have a far greater payoff, dollar for dollar, on both their utility bills and their home’s comfort.

The bottom line is that, if improving your home’s energy efficiency is the main goal, replacing your windows is among the least cost-effective ways to do it. Here’s why: Although glass radiates heat at a substantially higher rate than walls or ceilings do, it represents only a small fraction of a home’s exterior surface area.

A typical 1,800-square-foot, one-story rancher, for example, will have a window area comprising something on the order of just 6 percent of the exterior envelope. In the same house, however, the ceilings represent a whopping one-third of the surface area.

Therefore, the most cost-effective way to improve energy efficiency in homes built before the 1980s is simply to increase attic insulation levels.

So is replacing windows the next logical step for improving energy efficiency? Not by a long shot. Consider that many pre-World War II houses still have their original "octopus" gravity furnaces. If you have a basement, chances are that your house once had — or may even still have — this type of heating system.

With their gas-squandering pilot lights and primitive heat exchanger designs, most gravity furnaces have dismal energy efficiencies of perhaps 60 percent (meaning the other 40 percent of your energy dollar is wasted up the flue). What’s more, their thinly insulated ductwork wastes yet more heat just getting it to the register grilles, quite possibly leaving you with a net efficiency of 50 percent or so.

Maybe your prewar house has already had its old gravity furnace replaced with a "modern" forced-air unit somewhere along the line. Or, maybe you’re not worried about your furnace at all because your house is only 30 years old. Alas, any forced-air unit predating the 1980s is likely to have an efficiency of perhaps 75 percent — and that one-fourth of your energy dollar being wasted is nothing to celebrate.

Replacing your furnace with a modern high-efficiency unit (typically around 95 percent efficient or better) not only will yield big savings on your energy bills, but will markedly improve your home’s comfort as well.

Most models have variable speed fans that are quieter and do a better job of maintaining a steady temperature. Other improvements, such as automatic flue dampers and electronic ignition, finally do away with longstanding sources of energy waste that have hobbled furnace efficiency for over a century.

Last but not least, the new electronic clock thermostat can be precisely tailored to your daily routine, conserving even more energy by turning off the heat at the times it’s not needed.

All in all, a new furnace and ductwork is likely to cost you less than new windows, and will probably have a much bigger impact on both your utility bills and your comfort. So why throw money out the window?

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