how many btus does a typical home furnace
Look at the manufacturer's plate on the current furnace, and read the output BTUs if they're listed. If not, multiply the input BTUs by the furnace's efficiency. For example, an older furnace rated for 100,000 input BTUs at 75-percent efficiency has 75,000 output BTUs. A newer, higher-efficiency furnace might have a lower rating for input BTUs, but still put out the 75,000 output BTUs you need.