Home Economics: Deciding when it's time to replace the furnace

If your furnace is 15 years old or more, now might be the right time to exchange it for a more efficient model.

The obvious reason is to reduce your energy costs, keeping in mind that winter is on its way. Data from the U.S. Department of Energy show that if your current furnace has an AFUE (Annual Fuel Utilization Efficiency) rating of 50 percent, one with 90 percent will save you $44.24 a year on every $100 you now spend on fuel costs.

In addition, upgrading your furnace or boiler from 56 percent to 90 percent efficiency in an average cold-climate house will save 1.5 tons of carbon dioxide emissions each year if you heat with gas, 2.5 tons if you heat with oil.

But financial and environmental considerations are only part of the story. Effective May 1, Energy Department rules will require that all new gas furnaces installed in 30 Northern states, including Pennsylvania and New Jersey, have a minimum AFUE of 90 percent. Residents of states in the South and Southwest will still be able to go with furnaces with an AFUE of 80 percent.

The new efficiency rules affect gas furnaces only. Gas is the most common U.S. heating fuel, and most new central-heating systems use gas, Consumer Reports says.

An AFUE of 90 percent means that's the percentage of energy in the fuel that becomes heat, while the other 10 percent escapes up the chimney and elsewhere.

According to the Department of Energy, AFUE does not take into account heat losses from duct systems or piping, which can be as much as 35 percent of the energy output of a furnace when ducts are located in the attic.

Although older furnace and boiler systems had efficiencies in the range of 56 percent to 70 percent, modern conventional-heating systems can achieve efficiencies as high as 97 percent, converting nearly all the fuel to useful heat for your home.

The downside, of course, is the initial cost. High-efficiency gas furnaces, with 90 percent to 97 percent annual fuel utilization efficiency, can cost $4,000 to $12,500, including installation, according to QualitySmith.com, a website that vets contractors.

Installation can run several thousand dollars, depending on the complexity of the job. If your home requires new ductwork, expect an even higher price. And higher-rated gas furnaces require different venting to the outside, which could add extra construction and renovation costs to run PVC pipes through the side or the roof.

Achieving significant energy savings requires use of condensing technology, which yields a gain to 90 percent or higher AFUE, according to Alex Lekov and other researchers at Lawrence Berkeley National Laboratory in Berkeley, Calif.

But condensing furnaces need a different venting system because the buoyancy of the flue gases is not sufficient to draw them up a regular chimney, the researchers said. Plastic through-the-wall venting systems are typically used with such furnaces.

"This could mean additional product and renovation costs ranging from $1,000 to $1,500 for furnaces not near an exterior wall or under a roof," said Bill Cunningham, a senior product manager for Dallas-based Service Experts, which has franchisees in the Philadelphia area.

Cunningham said his firm was urging clients with aging furnaces to determine what the replacement costs might be both now and after the rules go into effect, adding that any furnace over 15 years old "should be considered aging."

Return on your investment might be an issue if you plan to sell your house soon - furnaces have little effect on resale value. Still, a very old furnace could be a turnoff for buyers.

Also affecting how quickly a new furnace will pay for itself, Consumer Reports says: the climate where you live, how well your home is insulated, and your local gas and electricity rates.

QualitySmith.com, which collects data from homeowners and participating contractors, has an extensive list of gas-furnace manufacturers and prices. (The latter don't appear on the makers' websites; QualitySmith says contractors purchase furnace units wholesale from manufacturers, then charge for the heater and installation.)

Also offered at QualitySmith are pros and cons of models from various manufacturers, as well as a way to obtain four estimates for installation and/or repair in your zip code. (Warning: You'll be required to enter some personal information.)

According to the experts, size is among the most important factors in choosing the right furnace for your home.

To decide size, a contractor first calculates the heating requirements, determining the area and the insulation level of each part of the house (floor, ceiling, exterior walls, and windows).

Also needed will be an estimate of air infiltration. You can consult a furnace dealer or an energy consultant, but computer programs and websites also are available to help. The important thing is to take the time to do this or to have it done for you.

Why? Because an oversize unit will waste energy through excessive cycling on and off, sending even more heat up the flue. And an undersized furnace may not provide enough heat for comfort during very cold weather.

Properly matching the size of the furnace to the heat load of the structure helps you save money in two ways, according to experts at Michigan State University:

The furnace unit operates more efficiently, saving costs on fuel. And you don't pay for a larger unit than you need.

Contact Alan J. Heavens at 215-854-2472, aheavens@phillynews.com or @alheavens at Twitter.