Many factors determine the number of British thermal units needed to heat a house, but in a moderate climate with a high-efficiency furnace, a 2,000 square foot home requires a 50,000 to 60,000-BTU furnace output. The general rule of thumb is that furnaces need to provide 25 to 30 BTU output per square foot in moderate climates and 40 BTU output per square foot in colder regions.
The furnace output is less than its input. This difference is the unit's efficiency, and manufacturers express it as a percentage. In the decade from 2000 to 2010, many furnaces became much more efficient. Older furnaces often had efficiency ratings of 80 percent, with 20 percent of the energy they produced lost through the exhaust system. The improvements make it common for new furnaces to have efficiencies of 95 percent or more. The greater efficiency allows homeowners making furnace replacements to choose a smaller unit.
Factors like insulation and sun exposure also affect the required furnace size. The materials from which the home is constructed and the layout of the house provide either positive or negative benefits. Landscaping may provide windbreaks in the form of trees and shrubs that reduce the heat required in a home. However, if the landscaping includes evergreens that shade the home and reduce solar heating, it requires additional heat.