Assess Your Home

• 1). Measure the number of square feet in your home that need to be heated by the furnace. Calculate square feet in each room, closet or hallway by measuring the length and width of a room along the wall and multiplying these dimensions to find the number of square feet in the room.
  
  
• 2). Examine your home's windows and doors to assess how much heat you expect to lose through them. Newer or double-paned windows typically lose less heat than older ones.
  
  
• 3). Compare the number of windows in your home to typical homes. If you have more windows than the average home, you may need a bigger furnace to compensate for the heat loss.
  
  
• 4). Consider how well insulated your home is. Rooms that were added on later without proper insulation will have more heat loss. Basements are also potential insulation problems if you plan to heat them. Homes with two or more stories help insulate themselves because the first story does not have a cold roof above it.
  
  
• 5). Determine what zone of the United States you live in, with Zone 5 covering the coldest areas in the north and Zone 1 being the warmest areas in the south. See the map in the References to find the zone for your home.
  
  

Calculate Furnace Size

• 1). Decide whether your home has high, medium or low furnace needs based on your assessments of the windows and insulation.
  
  
• 2). Multiply the number of square feet in your home by the number of British thermal units, or BTUs, that corresponds to your zone of the country. Choose a number on the high, medium or low end of the range depending on your assessment of your furnace needs. The recommended BTUs per square feet are 30 to 35 for Zone 1, 35 to 40 for Zone 2, 40 to 45 for Zone 3, 45 to 50 for Zone 4 and 50 to 60 for Zone 5.
  
  
• 3). Round up to the next size of furnace available, making sure that you are looking at the output BTUs of the furnace rather than the input. Manufacturers may not make furnaces in the exact size that you estimate your home needs, but it will not hurt to have a furnace that is up to 25 percent bigger than you estimate you need.