1 watt is approximately 3.412 BTU per hour. Most homeowners have difficulties figuring out what size of furnace do they need. E o = energy output (W, Btu/h) E i = fuel energy input (W, Btu/h) Annual Fuel Utilization Efficiency - AFUE However, if a furnace is rated at 100,000 BTUs and is 92% efficient, then the heat output will be 92,000 BTUs (100,000 × .92). In the burner, heat is released in the combustion process, and the energy equation must be used to determine the temperature change. But a furnace with an output of 91,000 BTU/h has an input of 105,000 BTU/h -- so that's an efficiency of 86.6%. A significant efficiency impact on burners is the ratio mix of the air and fuel. Fortunately, calculating furnace size (be it natural gas, propane, or oil) is not rocket science.. To help future furnace owners out, we have developed a simple furnace sizing calculator. For example, if a furnace has a listed input rating of 90,000 Btu's and an efficiency rating of 80%, it will produce 90,000 Btu input X .80 efficiency â¦ high efficiency gas or oil condensing furnace: 85 - 95 % ; Furnace Performance Indices. Then, to calculate the output on a given gas furnace, multiply it's efficiency rating by it's listed input rating to determine the actual Btu output of heat. What is a BTU? Thermal Efficiency. Calculation of the Air Requirement for Complete Combustion of Fuel Gases Example 1 Calculate the theoretical air-gas ratio at MSC of a fuel gas manufactured by the ICI 500 process containing (by volume) 14.8% CO 2 2.7% CO 48.9% H 2 33.6% CH 4 If this 1,900-square-foot home is in Washington, D.C., however, and the furnace youâre looking at has an 80 percent efficiency, the input rating should be 100,000 BTUs. 1 BTU = 1,055 joules, 252 calories, 0.293 watt-hour or the energy released by burning one match. Parameters for Properly Designed and Operated Flares Report for Flare Review Panel April 2012 Prepared by U.S. EPA Office of Air Quality Planning and Standards (OAQPS) It represents the ratio of annual heat output compared to the consumed fossil fuel energy. Combustor/burner or afterburner: , Turbine: Nozzle: , . Inlet/Diffuser: , (adiabatic, isentropic) Compressor or fan: , . How many BTU furnace would fit your home perfectly? To get started with a simple example (no turbomachinery), we will reexamine the ideal ramjet, picking up where we left off in Section 3.7.3. Essentially, there are two types of air/fuel burners: forced draft and natural draft. Î¼ t = thermal efficiency. Adjusting for burner efficiency: 37.21 divided by 0.95 = 39.17 cubic feet of natural gas consumed in one hour. It may be worth considering a more efficient furnace as less energy will be used to achieve the same heat output. Understanding furnace efficiency. 11. The British Thermal Unit, or BTU, is an energy unit. 3 Ideal Ramjet . 11. Î¼ t = 100 E o / E i (1) where . For a burner that is 95-percent efficient, divide by 0.95. The performance of a furnace is usually assessed by the following indices. Use a Smart Thermostat. If the model number of your furnace ends in 115, there's in a good chance that it has an input of 115,000 BTU/h. In the example: 40,000 BTU divided by 1,075 = 37.21. The efficiency of a furnace is measured by the Annual Fuel Utilization Efficiency (AFUE). If the output is 96,000 BTU/h, those numbers give you an efficiency of 83.5%.. The energy equation is given by: (1 + f) * ht4 = ht3 + f * nb * Q where ht is the specific total enthalpy, f is the fuel to air mass flow ratio, Q is the heat release, and nb is an efficiency factor. Furnaces typically have around an 80 percent efficiency rating, although more expensive, high-energy models have at least a 93 percent efficiency rating. 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