It typically takes about a decade’s worth of energy savings to recoup the investment in a new HVAC system, Dream Group’s Lunsford says, though that time frame can vary greatly depending on fuel price fluctuations.

Less apparent in dollar terms—but still valuable—is the increasing comfort level in your home and lowering your household’s drain on non-renewable fossil fuels.

You’ll get a bump up in salability when you’re ready to move on, says Frank Lesh, president of Home Sweet Home Inspection Co. in Indian Head Park, Ill. That doesn’t mean adding a $5,000 furnace will add $5,000 to the sale price. Rather, potential buyers are less likely to push for repairs or negotiate a credit if the HVAC is in good shape.

But before you do, conduct an energy audit of your home. Lunsford, also manager of consumer education for the U.S. Green Building Council’s Chicago Chapter, says he rarely recommends replacing a furnace as the first step in making a home more energy efficient.

Start by sealing it against air leaks:

• Do-it-yourself caulking and weatherstripping help, as does adding insulation in the attic.

• Professional air sealing, which is more effective, can cost as much as $5,000 for a large house, he says.

The payoff: Energy costs should go down, and you might get by with a smaller HVAC system.