I am just a lowly homeowner with a heat pump so I don't comprehend gas furnace sizing that well. Below is a post from another site. Why would a high-efficiency replacement gas furnace have to be less btus than the original less efficient furnace? Doesn't a 95% efficient 100,000 btu furnace put out the same 100,000 btus as an 80% furnace, only using less gas? Please educate me. Thanx.
"An easy way to determine the size of a new furnace is to multiply the input of the old unit by 60% then divide the efficiency of the new furnace into the result. For instance, if your old atmospheric furnace was 100,000 Btu times 60% = 60,000 Btu output. If your new furnace will be 94% AFUE divide 60,000 Btu by 94% =64,000 Btu. That is the input you need to match your old furnace. In this instance, if you match size for size the system will be 36% over sized."