If you're in the market for a gas furnace, AFUE, BTU and GAMA should be familiar initials to you.

If they're not, the Trane Home Comfort Institute, a consumer information service on heating and cooling, says you probably need to do a little more homework before you buy.

AFUE stands for Annual Fuel Utilization Efficiency, which is the efficiency rating of the furnace. The higher the AFUE, the more efficient the furnace. The more efficient the furnace, the lower the heating bill.

BTU stands for British Thermal Unit, a unit of measure of heat. The higher the AFUE, the more BTUs you'll get for your heating dollar. The capacity of a gas furnace is measured in BTUs per hour of operation.

GAMA stands for Gas Appliance Manufacturers Association, the national trade association whose members produce over 90% of the gas appliances made in the U.S. It verifies the AFUEs of the furnaces made by its members.

According to the Trane Home Comfort Institute, the AFUE is a rating method developed by the U.S. Department of Energy that indicates what percent of the energy consumed is converted to useable heat. It measures the average annual seasonal efficiency of the furnace including on and off cycling. The rating is determined by dividing the total energy used into the total heat produced. AFUEs for gas furnaces range from 57 to 98%.

The federal government requires that all new furnaces meet or exceed an efficiency level of 78% AFUE. It also requires that all new furnaces be tested according to its standards so that consumers can compare the efficiency ratings of various units.

GAMA publishes a directory of those ratings twice each year. Your local heating dealer should have such a directory and will be able to show you the rating for the system you're considering.

If your present furnace is 10 years old or more, it's probably in the 60-70% AFUE range. In effect, that means you're wasting 30 to 40 cents out of every dollar you spend for heat. By replacing an old, low efficiency unit with a new high efficiency model, more of the fuel you pay for is turned into heat.

For example, replacing an old 60% AFUE furnace with a new 92% AFUE unit can trim nearly 35% off your heating costs. If you're paying $800 per season, you'll save $278. If you're paying $1,000 per season, you'll save $348. Multiply that over the 15- to 20-year lifetime of the new furnace and you'll quickly realize how much money is involved.

The AFUE ratings of gas furnaces have increased over the past few years because of new furnace designs that allow the heating units to recover heat that would otherwise be wasted. Included among these new designs are induced-draft furnaces with AFUEs in the 80% range and condensing furnaces with AFUEs in the 90% range.