Gas Furnaces There are a variety of ways to describe different types residential gas furnaces. Gas furnaces can be classified by:
- the direction of the air flowing through the heating unit;
- the heating efficiency of the unit; and
- the type of ignition system installed on the unit.
Airflow in Gas Furnaces 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. BTU Gas furnaces can be classified by their different capacities. A furnace capacity can be described by BTU output. The BTU is determined by what is required by the heating unit for the structure, which is the amount of heat the unit needs to produce to replace heat loss and provide the occupants a good comfort level. AFUE Furnaces 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 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 from 78 to 82%. High-efficiency furnaces have AFUE ratings from 88 to 97%. Old, standing-pilot gas furnaces have AFUE ratings from 60 to 65%. Gravity warm-air furnaces might have efficiencies lower than 60%.
BTU and Efficiency BTU stands for British Thermal Unit. The BTU is a unit of energy. It is approximately the amount of energy needed to heat one pound of water 1 degree Fahrenheit. Once cubic foot of natural gas contains about 1,000 BTUs. A gas furnace that fires at a rate of 100,000 BTUs per hour will burn about 100 cubic feet of gas every hour. On a gas furnace, there should be a data plate. On that plate there might be written the input and output capacities. For example, the data plate may say, "Input 100,000 BTU per hour." And it may also say, "Output 80,000 BTU per hour." While this furnace is running, about 20% of the heat generated is lost out through the exhaust gases. The ratio of the output to the input BTU is 80,000 ÷ 100,000 = 80% efficiency. This is the "steady state efficiency" of the furnace. Steady state efficiency measures how efficiently a furnace converts fuel to heat, once the furnace has warmed up and is running steadily. However, furnaces cycle on and off as they maintain their desired temperature. Furnaces typically do not operate as efficiently as they start up and cool down.
As a result, steady state efficiency is not as reliable an indicator of the overall efficiency of your furnace. AFUE and Efficiency The AFUE is the most widely used measure of a furnace's heating efficiency. It measures the amount of heat delivered to your house compared to the amount of fuel that must be supplied to the furnace. Thus, a furnace that has an 80% AFUE rating converts 80% of the fuel that is supplied to heat. The other 20% is lost and wasted. Note that the AFUE refers only to the unit's fuel efficiency, not its electricity usage. The U.S. Department of Energy (DOE) determined that all furnaces sold in the U.S. must have a minimum AFUE of 78%, beginning January 1, 1992. Mobile home furnaces are required to have a minimum AFUE of 75%.
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