Most accurate method of determining furnace size is to have professional perform a heat loss calculation following ACCA or ASHRAE guidelines. Heating factor is a value that’s based on climate and expressed in BTUs, or British Thermal Units.

Across the U.S., the heating factor can vary from 5-45 BTUs. The heating factor will be highest in the Upper Midwest, where it is 15-45 BTUs per square foot. Manufacturers often rate their furnaces in terms of BTUH capacity, such as 100,000 BTHs, which indicates a heating output of 100,00 BTUs per hour. For example: If your home is 2,400 square feet and you figure 35 BTUs each square foot, you’ll need an 84,000BTUH-capacity furnace. That same home, using a heating factor of 15 BTU Hs, would require an output capacity of 36,000 BTUs.

If your home is well insulated, start by using the 15 BTU heating factor. If your home has a lot of windows, patio doors, or skylights, you may need a higher capacity furnace. The output (BTU) capacity of a furnace is not the same as its efficiency rating.