A British thermal unit is a unit of measurement of heat energy. One Btu is defined as the amount of energy required to raise the temperature of 1 pound of water by 1 degree Fahrenheit at a pressure of 1 atmosphere. As a precise unit of measurement of heat energy, the definition of a Btu is problematic because the amount of energy required to raise the temperature of water by a degree depends on many factors, including the starting temperature of the water and the method of heating. However, Btu ratings are commonly used to describe the heat output of appliances such as furnaces, space heaters and fireplaces.
The heat output of 1 Btu is very small, roughly the same as the heat created by burning a single kitchen match, and all heating appliances produce thousands of Btu per hour. The Btu-per-hour rating is the number typically used to describe the heating capacity of a given furnace or heater.
When you attempt to calculate the number of Btu required to effectively heat 1 square foot in a specific home, several variables come into play. The quality of the home's insulation is important, as are the presence or absence of significant air leaks in the building, the living habits of the home's residents, the number of windows in the home, and the sun and wind exposure of the home on its site. All of these factors affect the amount of energy needed to adequately heat each square foot in the home.
Perhaps the most important factor that influences the amount of energy required to heat a home is the home's geographic location. In parts of the country where winters are cold, a furnace will need to produce more Btu to heat 1 square foot of living space on a typical winter day than it would in a comparable home in a warm climate. For example, a home in Minnesota may require between 50 and 60 Btu per square foot for adequate heating, while a home in Florida may only require between 30 and 35 Btu per square foot.
Within each climate zone, variables unique to each home will have an impact on the amount of energy required to heat the home. A well-insulated home in Minnesota may require 50 Btu per square foot, but a poorly insulated home in the same place may require 60 Btu per square foot.
After you've determined the heating requirements for your climate and estimated the energy efficiency of your home, you can multiply the per-square-foot Btu number by the square footage of your home to calculate the size of furnace you'll need. For example, a well-insulated, 1,600-square-foot home in Minnesota requires 80,000 Btu, or 50 Btu x 1,600 square feet.
A final factor to consider in selecting a furnace is the efficiency of the appliance. A 90-percent-efficient furnace rated at 90,000 Btu input, meaning that it consumes 90,000 Btu of fuel per hour, will output 81,000 Btu per hour. An 80-percent-efficient, 90,000-Btu furnace, however, will output only 72,000 Btu per hour. It is the output of the heater that you need to consider when determining what size furnace you need for your home's square footage. Using the example of the 1,600-square-foot Minnesota home, the 90-percent-efficient, 90,000-BTU furnace would be adequate, but the 80-percent-efficient unit would not.