The heating capacity of a furnace is measured in thousands of BTU (British Thermal Units). Furnaces are rated by the amount of fuel energy consumed when running, called Input BTU. Different furnaces of the same Input BTU have different efficiencies, measured in percentages. For example, a furnace with an Input BTU can have an efficiency of 80%. That means the furnace will produce 80,000 BTU of heat. If that same furnace has an efficiency of 95%, the furnace will produce 95,000 BTU of heat. The actual production of heat, called the Output BTU, will be the main factor in deciding which heating unit is right for your home.
There is only one correctly sized furnace for your home. If your heating unit is too big, it will turn on and off too frequently, or "short cycle", which will cause a loss of efficiency and even damage your heating unit over time. If your heating unit is too small, it will run too often and on the coldest days of the year, may not be able to adequately heat your home.
Additional Sizing Methods
There are other ways to calculate the correctly sized furnace for your home.
Manual J Load Calculation
This is the proper and scientific method for calculating AC size taught to HVAC technicians and recommended for use by professionals in the trade. It consists of taking information about your home’s construction materials, insulation levels, number of windows, sizes of rooms, etc, and making a calculation based on those factors to determine the appropriate heating and cooling requirements needed.
Does your neighbor have the same size home as you? If he has a properly sized air conditioner, then the same size unit may also work for you.
Ask a Contractor
Most air conditioning professionals give free in-home estimates for installing new air conditioning, during which they will recommend a unit size. While we don't encourage using contractors for quotes if you don't intend to hire them, a contractor familiar with the homes in your neighborhood will likely be able to give you an idea over the phone of what size you might need.
If the unit you have now is the correct size for your home, replace it with the same size. How do you know what size you currently have? Look at the name plate on your furnace and locate the model number (not serial number). You are looking for 2 digits in the model number that match the numbers below to indicate Tons or BTU.