Forced Air Furnace Sizes

A forced-air furnace uses gas or electricity to provide heat that is forced through ducts by a fan to warm your home.


Forced-air furnaces vary in design, power and efficiency. Don’t buy a furnace that you suspect is a bargain or simply by the most powerful one you can afford because there is more to sizing a furnace than just looking at a number. Learn all you can about furnace sizes before selecting a new one for your home and you may even learn whether the one you already have is appropriate.

Furnaces and air conditioning systems are typically rated based on British Thermal Units (BTUs). This size standard is calculated based on the output potential of the system. One BTU is the equivalent amount of energy needed to raise the temperature of one pound of water by 1 degree Fahrenheit. You may also see furnaces sized as “tons.” One ton is equal to 12,000 BTU.

Output BTU

While the BTU rating of a furnace will tell you how much power it has, the actual usefulness of the furnace is best calculated by figuring its efficiency. This is often listed as the furnace’s output BTU. The input BTU is the total available energy a system has, while the output BTU is the actual amount of power it will release into your home. For example, a 100,000 BTU system that runs at 95 percent efficiency is actually giving you 95,000 BTU of heating power. Both input and output BTUs are typically listed on the unit.

Manual J Factors

One of the best methods used in calculating the proper size of furnace for your home is the Manual J method. This industry standard involves taking numerous factors about your home into account to determine its furnace size needs. Not only does it take square footage into account but it also considers the quality of insulation, construction materials, number of windows, sizes of rooms and the climate in which your home is located. This method is taught to HVAC technicians to help you in choosing the right system.

Why Size Matters

There is a good reason you should find out the ideal size for a furnace in your home. The main reasons you’ll want to size your furnace properly are energy bills and effective heating. A furnace that is too small may not be able to heat your home adequately and if it can manage to do it, the furnace will need to run almost continuously. This will lead to high electric bills. On the other hand, an oversized furnace will short cycle, meaning it heats up too quickly and shuts itself off too often. This will cause too much wear and tear and shorten the unit's lifespan.

About the Author

Lee Morgan is a fiction writer and journalist. His writing has appeared for more than 15 years in many news publications including the "Tennesseean," the "Tampa Tribune," "West Hawaii Today," the "Honolulu Star Bulletin" and the "Dickson Herald," where he was sports editor. He holds a Bachelor of Science in mass communications from Middle Tennessee State University.