Dear Jim: When my old gas furnace was serviced this fall, it needed repairs and the technician indicated it was time to think about replacing it. What are my options for a new gas furnace and which ones save the most? – Ron H.

Dear Ron: Even if your old furnace was not beginning to need expensive repairs, installing an efficient one would cut your utility bills significantly. In addition to a 40 percent savings on your utility bills, a new efficient gas furnace provides better comfort and more quiet operation. Gas furnace efficiency is called AFUE, for annual fuel utilization efficiency.

Every house is unique, so there is not one “best” furnace for everyone. You must have a contractor do a thorough heat loss analysis of your home to determine the size (heating capacity) of furnace you need. Furnace capacity is rated by the amount of gas it consumes, not the effective heat it produces. You may be able to get by with a smaller, but more efficient model.

If you have a central air conditioner that also is old, you may consider replacing it at the same time with a heat pump. The heat pump can provide heat efficiently during milder weather with the gas furnace as backup. Choosing this approach will affect which furnace provides the best economic payback.

The gas furnaces that save the most money use a condensing design with two heat exchangers. The hot water vapor in the flue gases condenses in the second heat exchanger to capture more heat before it is lost out the flue. These all have efficiencies above 90 percent and do not require a chimney. They are vented outdoors through a small plastic pipe.

Less expensive models in the 80 percent range have only one heat exchanger and the flue generally goes into a chimney. In cold climates, your contractor will have to determine if your chimney needs a flue liner because the water vapor may condense inside the chimney. Having to upgrade a chimney can be almost as expensive as installing a better condensing furnace.

There are typically three categories of condensing furnaces. Standard ones have a single-output burner and a standard blower motor. The furnace is either on or off.

The next step up is a two-stage or modulating burner, which varies its heat output depending upon the instantaneous heating needs of your house. Some still use a standard blower motor.

The top-of-the-line models use an ECM variable-speed blower motor to improve comfort and efficiency. These also operate more quietly. There is a small $50 energy tax credit for upgrading to the variable-speed blower, but its installed cost is several hundred dollars more than a standard blower.

Write for (or instantly download at www.dulley.com) Update Bulletin No. 698 – buyer’s guide of the 19 most efficient modulating and two-stage gas-propane furnaces listing AFUEs, capacities, blower motor types, warranties, and sizing and savings charts. Please include $3 and a business-size SASE.

Send inquiries to James Dulley, Bangor Daily News, 6906 Royalgreen Dr., Cincinnati, Ohio 45244 or visit www.dulley.com.