Remove the front panel from your furnace.
Locate the label indicating the number of BTU's that your furnace is capable of inputting. This is referred to as the furnace's input rating and is most often located behind the service door of the furnace.
Determine the furnace's efficiency level. This should be written as a percentage on the label beside the input rating.
Multiply the furnace's input rating by its efficiency. For example, if your furnace had an input rating of 200,000 BTU's and an efficiency level of 90 percent. You would multiply 200,000 by 0.90 to get 180,000. This number would then be the number of BTU's at which your furnace is effectively operating.
- If your furnace does not specify an efficiency level it is most likely an older furnace. If this is the case, it is fair to assume that the furnace operates at around 80 percent efficiency.