The size of a household furnace is determined by its energy input rating, which is expressed in BTU per hour. An average furnace is 80,000 or 100,000 BTU/hr, but other commonly available furnaces can range from 40,000 to over 200,000 BTU/hr.
Selecting the correct size of furnace ensures maximum efficiency in heating a home. A furnace that is too small is unable to adequately heat the house on cold days and has a shortened lifespan because it turns on and off too frequently. On the other hand, an oversized furnace is more expensive and has a higher than necessary operating cost. In addition, too large furnaces do not run long enough to properly distribute the air, creating pockets of hot or cold air throughout the house.
There are a variety of techniques to calculate the correct size of furnace needed. One well-regarded method is the Manual J Load calculation, an equation that takes into account a number of different factors, such as room size, construction materials, the amount of insulation, number of windows and even how much sunlight the house receives. Due to the complexity of the calculation, the Manual J Load is best determined by a professional HVAC installer.