Dear Jim: With gas prices so high now, I think I should replace my old furnace even though it still works. I want the most efficient design to save money and reduce pollution. What models should I consider? – Pete J.

Dear Pete: With today’s high energy prices, it probably does make good economic sense to consider replacing your old gas furnace. Depending upon how inefficient it is, installing the highest efficiency design will cut your gas bills by as much as 40 percent and lower your electric bills.

The most efficient new gas (all can be easily converted to propane) furnaces have AFUEs (efficiency rating) as high as 96 percent. This means only 4 percent of the heat from the gas is lost out the flue. Your old furnace can lose more than 40 percent of the heat content of the gas up the chimney. When a furnace is operating properly, burning of gas or propane produces little air pollution. The main concern is the carbon dioxide, which may cause global warming and climate change. Since a new high-efficiency model burns 40 percent less gas, it will produce 40 percent less carbon dioxide.

The most efficient gas furnace design uses a special gas valve that can control the amount of heat produced. Some are two-stage designs with a low- and a high-output burn rate. Others use a modulating gas valve that can vary the heat output in several percent increments.

The advantage of these types of furnaces is they vary the amount of heat produced depending on the instantaneous heating requirement of your house. During mild weather when your house loses less heat through the walls and ceiling, the furnace runs at the low-output level. When it gets very cold outdoors and more heat is needed, it automatically switches to high-output.

By matching the furnace heat output to the changing heating needs of your house, the furnace runs longer with few on-off cycles. Just as your car operates more efficiently using less gasoline on the highway than in stop-and-go traffic, so does your gas furnace when it runs in longer cycles.

To realize the most utility-bill savings and comfort, select a new furnace with a variable-speed ECM (electrically commutated motor) blower motor. When the furnace is running in low-output mode, the blower motor runs more slowly so the air blowing out of the registers still feels hot.

These motors are quiet and have a soft start so you may not even hear them start. It would be wise to install a matching thermostat from the furnace manufacturer to take full advantage of all its efficiency and comfort features.