Dear Jim: I have talked with contractors about getting a new gas furnace. Most say a condensing furnace is best. How is a condensing gas furnace different, is it efficient and how is the comfort with one? – Stu E.

Dear Stu: A condensing gas furnace is the most efficient design available and installing one can lower your utility bills by 40 percent or more. The newest modulating-burner condensing furnaces will also greatly improve comfort. Most manufacturers offer lifetime heat exchanger warranties.

In addition to the burner design, improved comfort is a function of the type of blower motor and thermostat. Since the furnace blower and thermostat are also used for central air-conditioning, they impact summertime comfort, actually to a great extent than in the heating mode.

A condensing furnace extracts extra heat from the exhaust gases before they exit the furnace. This requires an extra heat exchanger inside the furnace which condenses the water vapor in the hot exhaust gases back into water.

When gas burns inside the furnace, it basically creates carbon dioxide and hot water vapor. By cooling the exhaust gases down enough to make this water vapor condense, the latent heat is captured for your house. This is the opposite effect of how the evaporation of perspiration cools your skin.

Since most of the heat stays indoors, the exhaust gases are cool enough to be vented outdoors through a small plastic pipe. If you choose a model with sealed combustion, combustion air is also drawn from outdoors. This is efficient, quiet and safe (less chance of carbon monoxide poisoning).

The primary options are a variable modulating burner, a two-stage burner, or a single-stage burner. A modulating burner instantaneously varies the heat output (in 5 percent increments) to the changing heating needs of your house. It can maintain room temperatures to within one-half degree of the thermostat setting and can average sensors in six different rooms.

A two-stage burner offers the next best level of savings and comfort. It varies its heat output from about 60 percent to 100 percent depending on the heating needs of your house. On milder days, it runs at low heat output and automatically switches to high fire when more heat is needed. Some new models allow you to control the furnace from a computer or telephone.

Installing a variable-speed blower in your new gas furnace will vastly improve the comfort and also indirectly improve efficiency. These special motors have built-in microprocessors that monitor and vary the speed of the blower depending upon the heating and air-conditioning needs. Some new thermostats offer the option of increasing or slowing the blower speed for comfort and noise control.

Write for (instantly download – www.dulley.com) Update Bulletin No. 933 – buyer’s guide of 16 condensing gas (and propane) furnaces listing efficiencies, heating stages, blower speeds, warranties, comfort features, and sizing/payback charts. Include $3 and a business-size SASE, and send to James Dulley, 6906 Royalgreen Drive, Cincinnati, OH 45244.