Dear Jim: I made energy improvements to my house, but my 25-year-old gas furnace does not keep it as comfortable as before. Also, I think my gas bills should be lower. Which 2008 furnace models are most efficient? – Mike J.

Dear Mike: The highest efficiency gas (or propane) furnaces now have AFUE’s (heating efficiencies) of 96 percent and above. Most new designs targeted 95 percent to meet the just-expired federal energy tax credit requirement. Compared to your old furnace efficiency of about 65 percent at best, installing one can cut your utility bills by hundreds of dollars per year.

Your situation of reduced comfort is not uncommon after making energy improvements. What happened is that your house now requires less energy to stay warm, so your old furnace is oversized.

A furnace that is oversized (high heat output) for the heating needs of a house does not have to run very long each time your thermostat calls for heat. This results in excessive temperature swings and drafts indoors. Typically, when people become uncomfortable during winter, they set the furnace thermostat a little higher. This further increases their gas bills.

Before you select the gas furnace you want, have a contractor do a heating-load analysis of your house to determine how large a furnace it now needs. The amount of heat a house requires depends on the climate, construction method, amount of insulation, type of windows and doors, orientation to the sun and winter winds, etc. It requires a detailed computerized analysis.

Once you know how large (heating output) a furnace your house needs, the contractors can do payback analyses on new models of various efficiencies. In most cases, installing the highest-efficiency model is best in the long run. In order to get the highest-efficiency central air-conditioning for hot climates, the air handler and controls of the highest-efficiency furnace are required.

Since comfort and low gas bills are your primary concern, select one of the variable-heat output furnaces. In low-fire mode during milder weather, the furnace runs at about 60 percent of its maximum heat output. When the heating needs are greater, it automatically switches to the maximum.

Several companies now offer modulating-output furnaces with several stages of heat output to provide even more comfort. Several models have three stages and one offers 13 stages. In order to access all the stages, you must install their compatible thermostats. With any of these models, it is important to install a variable-speed blower. Most use a variable-speed General Electric ECM blower motor. This motor can also be used in a standard furnace for better comfort.

Write for (or instantly download at www.dulley.com) Update Bulletin No. 698 – the 2008 buyer’s guide of super-efficient modulating and two-stage gas furnaces listing AFUE’s, capacities, blower types, warranties, and sizing and savings charts. Please include $3 and a business-size SASE.

Dear Jim: We have a wood-burning fireplace insert in a wall finished with artificial stone. It is discolored from years of smoke and soot from the fires. What is the best method to clean the wall surface? – Amy H.

Dear Amy: Even the best wood-burning insert will allow some soot to escape indoors over many years. Many of the artificial stone walls use either shallow real stones or ones made of concrete. Check with the manufacturer. If you cannot determine the material type, test-clean a tiny spot with a typical acidic masonry cleaner. Saver Systems, www.saversystems.com, makes an environmentally safe masonry cleaner for fireplaces and masonry.

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