- 1). Determine the input BTU of your furnace. Input BTU is the amount of energy the furnace consumes. It will be listed in the product's manual, and it is on record with the manufacturer. Some furnaces will also have a specification plate right on them. This plate will also list input BTU. If you are unable to find input BTU, you will have to call the manufacturer.
- 2). Determine the efficiency of your furnace. Efficiency is how well the furnace transforms the energy it consumes into heat energy. It is another specification that can be found either on the specification plate, or in the product's manual. The manufacturer will also be able to tell you the furnace's efficiency if you can't find it.
- 3). Multiply input BTU by efficiency to get output BTU.
For example, if your furnace had 12,000 input BTU and an efficiency of 80 percent, you would multiply 12,000 by .8 to get 9,600 output BTU.
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