Heating systems are sized according to British Thermal Units (BTUs) per hour, which is defined as the amount of heat needed to raise a pound of water by 1 degree Fahrenheit. A pound of air-dried wood produces about 7,000 BTUs, a gallon of fuel oil generates about 140,000 BTUs, and a kilowatt-hour of electricity makes about 3,400 BTUs. To calculate the output BTU of a furnace, multiply the input BTUs by the efficiency rating, typically expressed as a percentage. For example, a furnace with 100,000 input BTUs that operates at 90 percent efficiency produces 90,000 output BTUs.
The most accurate way of sizing a furnace, according to Consumer Reports, is to call a contractor who uses Manual J of the Air Conditioning Contractors of America. These calculations take into account the climate; the design and location of the house; the number of inhabitants; the number of doors, windows and other openings; the insulation; and even the orientation of the walls toward the sun. Such contractors can also use Manual J to determine if your heating ducts are correctly sized for the furnace you need.
One way to estimate your current furnace needs is to look for similar houses in your neighborhood and find out what furnaces are used there. If you can, ask the homeowners questions about their installations. Another method is to multiply the square feet of living space by 40 to 45 BTUs. For example, 1,000 square feet of space needs a furnace that produces 40,000 to 45,000 BTUs per hour. However, this calculation typically leads to oversized installations.
Michaels Engineering recommends a more accurate method for calculating the BTUs. First multiply the July therm usage, available from the gas bill, by 12. For example, 25 therms for July times 12 equals 300. Then, subtract that total from the annual therm usage, also from the gas bill. For example, an annual 1,500 therms minus 300 equals 1,200 therms. You then multiply this number by 50 and then by the efficiency rating of the current furnace. For example, if the furnace is 82 percent efficient, multiplying 1,200 by 50 by 0.82 produces 49,200, which is the number of BTUs per hour needed from the furnace.