- One way to identify and describe a gas furnace is by the direction of the air flowing through the heating unit, or the location of the warm-air outlet and the return-air inlet on the furnace. Gas furnaces can be described as upflow, downflow (counterflow), highboy, lowboy, and horizontal flow. Air can flow up through the furnace (upflow), down through the furnace (downflow), or across the furnace (horizontal). The arrangement of the furnace should not significantly affect its operation, or your inspection.
BTUGas furnaces can be classified by their different capacities. A furnace capacity can be described by its BTU output. The appropriate BTU is determined by the heating requirements of the structure, which is the amount of heat the unit needs to produce in order to replace heat loss and provide the occupants with a satisfactory comfort level.
AFUEFurnaces can be identified and described by heating efficiency. The energy efficiency of a natural gas furnace is measured by its annual fuel utilization efficiency (AFUE). The higher the AFUE rating, the more efficient the furnace. The U.S. government has established a minimum rating for furnaces of 78%. Mid-efficiency furnaces have AFUE ratings of between 78 to 82%. High-efficiency furnaces have AFUE ratings of 88 to 97%. Old standing-pilot gas furnaces have AFUE ratings of 60 to 65%. Gravity warm-air furnaces can have efficiencies that are lower than 6
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