Importance of Size

• Many homes have an inappropriately sized furnace because they have not assessed their needs properly. It is a mistake to assume that the size is not important. A furnace that is too big for a house will turn on and off too frequently. This is known as short cycling. This wastes energy and increases utility bills. In addition, a unit that is too large will cause moisture to accumulate in the heat exchanger, which could shorten the life of the furnace. If you choose a furnace that is too small for your home, it will have to work too hard to maintain the desired temperature. This results in long running times and lots of wasted energy. When the weather gets extremely cold, it may not be able to achieve the desired temperature.
  
  

Manual J Load Calculation

• The scientific way to calculate the appropriate size furnace for your home is called the Manual J Load Calculation. It is a standard taught to heating, ventilation and air conditioning technicians in which several factors about your home are plugged into a complex formula to arrive at a perfect sized system. Factors include construction material, number of levels and windows, size of rooms and other basic information about the structure. The only problem with the method is that it is not nearly as accurate for old structures as it is for new construction. Nonetheless, this is how most HVAC professionals decide which size you need.
  
  

Comparisons

• If you don’t want to rely on the standard formula for your home, you can use a less scientific quick estimate method of your own. One way to do this is to get nosy around your neighbors. Search your area for a few houses that are similar to yours in size and construction. Go to the door and tell the occupants you are looking around to find out about furnace sizes. Inquire what size unit they have and about their utility bills. A survey of a few similar houses will likely provide you with a close approximation of what you need. You can compare your notes with the advice of an HVAC pro.
  
  

What You Had Before

• If you are replacing an old furnace with a new one, consider how the old one functions. Think about whether it runs too often and struggles to keep up with the demands. Consider whether it cycles on and off a lot. Perhaps your existing unit is just worn out, but it gave you many years of excellent service. Use these memories of the existing system to determine whether you need one that is bigger, smaller or exactly the same as what you had before.