How do you determine what size furnace is needed in a house?

To determine the appropriate furnace size for a house, you need to conduct a load calculation, typically using the Manual J method, which considers factors like the home's square footage, insulation levels, window types, and local climate. The calculation will yield the heating requirements in British Thermal Units (BTUs) per hour. It's also important to account for the layout of the home and any specific heating needs in different areas. Consulting with an HVAC professional can ensure an accurate assessment and optimal furnace selection.