Dear Jim: Our gas rates have jumped 30 percent in one year and heating bills are killing our budget this winter. My 17-year-old gas furnace still works. Does it make sense to replace it with a super-efficient one? – Bob C.

Dear Bob: Heating bills across the country are the highest they have ever been with no relief in sight. Even if your older furnace still runs, from an economic standpoint, it would most likely be wise to replace it. With the proper furnace selection, your central air-conditioning bills can be lower too.

As compared to a 17-year-old furnace, a new furnace can save the typical family hundreds of dollars per year. Based on the efficiency of your old furnace, probably 60 percent at best, a new furnace can cut your utility bills by 40 percent. You can do the arithmetic to determine your own annual savings.

Not only will you have lower utility bills, but the comfort and quiet operation of a new system will surprise you. The contractor should install a computerized thermostat with it. This thermostat, coupled with the electronic controls in the new furnace, will maintain even room temperatures.

There are two basic designs of furnaces to choose from: condensing and noncondensing – designations that refer to the type of heat exchanger used. The condensing models are the most efficient and the best choice for most homeowners. The efficiencies of these condensing models range from about 90 to more than 95 percent.

These models are very efficient and so little heat is lost in the flue gases that a chimney is not needed. The gases are exhausted by a 2-inch-diameter plastic pipe through an outdoor wall. With no need for a new chimney liner, a condensing furnace often is cheaper to install than a less-efficient model.

Some models also offer sealed combustion for better efficiency. The combustion air is drawn in from outdoors through another plastic pipe instead of being drawn from inside your house. Being sealed, there are fewer indoor drafts, less noise and less chance of hazardous backdrafting.

For the ultimate in comfort and efficiency, although at a higher initial cost, is a two-stage heat output furnace with a variable-speed blower. This type of blower is needed if you want the best central air conditioning too. In all but the coldest weather, the gas burners operate at a low heat level. This allows the furnace to run more continuously with fewer uncomfortable on-off cycles. The blower also runs slower and quieter at this low level. During very cold weather, it automatically switches to high heat.

If your budget allows, also install a quality air cleaner. Since a two-stage unit runs more, the air cleaner is more effective for allergy sufferers.

Write for (instantly download – www.dulley.com) Update Bulletin No. 650 – buyer’s guide of the 16 most efficient gas (and propane) furnaces listing efficiencies, blower speeds, heating stages, warranties, comfort features, sizing and payback charts. Include $3 and a business-size SASE. Send to James Dulley, Bangor Daily News, 6906 Royalgreen Drive, Cincinnati, Ohio 45244.