16 National Oilheat Research Alliance Heating System Efficiency vs. AFUE Factors that cause the heating system efficiency to vary from the AFUE: • Location of the boiler or furnace (Do the jacket and distribution losses help heat the living space or not?) • Chimney Draft (The chimney height, what the chimney is made of, and the air temperature all affect on and off cycle losses.) • The source of combustion and draft relief air • Burner design and operating pressure • Boiler or furnace sizing and firing rate relative to the heat load • The zoning of the distribution system • Pipe or duct design and installation • Source and consumption of domestic hot water • The mass of the boiler or furnace • Boiler or furnace control system It is also affected by air leaks and insulation of the building envelope. stack temperature minus the temperature of the combustion air) is calculated. An efficiency calculation is done to relate the stack temperature to the excess air and arrive at an efficiency percentage, or combustion efficiency. When using digital test equipment, the smoke test is done first. Combustion efficiency describes the appliance when adjusted at its maximum reliable efficiency. Combustion efficiency only describes on-cycle losses. It does not include jacket losses, off-cycle losses, distribution losses, or outdoor air infiltration losses. It is possible in some older units to adjust the firing rate to get the combustion efficiency above 80%. But that number will ignore jacket losses that could be 8%, off-cycle losses of at least 15% and infiltration losses that could be 10%. The true efficiency is probably 30-40 percent lower than the combustion efficiency indicated. Annual Fuel Utilization Efficiency (AFUE) This is the US Government Department of Energy’s attempt to provide a rating for boilers and furnaces. It is based upon standardized testing and calculation procedures developed by the DOE in the 1980s. Manufacturers must perform these tests on every model of residential boiler or furnace they sell. These AFUE results are printed on a yellow label that must be affixed to the unit. They are also listed in several heating equipment directories. Unfortunately, equipment manufactured before 1985 does not have an AFUE rating and cannot be compared to today’s equipment without further analysis. The objective of the AFUE labeling program is to supply consistent efficiency information to help consumers select and install high efficiency equipment. While there are some serious flaws in this procedure, it is the only consistent information available for comparing units. AFUE indicates the efficiency of the burnerboiler or burner-furnace combination for a standard set of operating conditions. The test procedure approximates on-cycle heat loss and cool-down characteristics and then calculates off-cycle and infiltration losses for “standard” conditions. There are many real world cases where off-cycle and infiltration losses are significantly different that those predicted by AFUE. AFUE is used to rate the overall efficiency of new furnaces and boilers over an “average” heating season for “typical” operating conditions. While the AFUE test cannot and does not try to take all factors that determine efficiency into account, it does give a way to compare the
NORA Advanced Efficiency
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