The heating capacity of a furnace, also referred to as size, is measured in thousands of BTUs (British Thermal Units). Furnaces are rated by the input BTU’s, that is, the amount of fuel energy that goes into it when running.
The heating capacity of a furnace, also referred to as size, is measured in thousands of BTUs (British Thermal Units). Furnaces are rated by the input BTU’s, that is, the amount of fuel energy that goes into it when running. Different furnaces of the same input BTUs will have different output BTUs if the furnace efficiency ratings are different. Select a furnace for your home according to the output BTUs, as this is the actual heating capability of the furnace. You can easily calculate the output BTUs of a furnace according to the efficiency rating of the unit and the input BTUs – simply multiply the efficiency percentage by the input BTUs. For example, a 100,000 BTU furnace at 80% efficiency will produce 80,000 BTUs of heat output, whereas as 100,000 BTU furnace at 92% efficiency will produce 92,000 BTUs of heat output.
There is only one correct size furnace for your home.
You don’t want a unit that is too big, as it will turn on and off too frequently, be inefficient, and accumulate moisture in the heat exchanger which can damage the unit over time. And you don’t want a unit that is too small as it won’t properly heat the home. You want just the right size. How do you determine which one is just right? There are several ways you might go about determining the correct furnace heating capacity (or size) unit:
1. Manual Load Calculation
This is the proper and scientific method as well as the most accurate. It is the method taught to HVAC technicians at school and recommended for use by professionals in the trade. It consists of taking information about your home’s constructions materials, insulation levels, sizes of rooms, etc, and then makes a calculation based on those to determine the appropriate heating and cooling requirements needed. The BTUs recommended by this tool should be close to output BTUs of the furnace you select. It should be noted, that while this is the most accurate approach for determining furnace size, according to a study by the US Department of Energy, less than half of heating and air conditioning contractors actually use Manual J Calculations to determine the proper size for heating or cooling equipment.
2. Compare your home to similar homes in your area.
Does you neighbor have the same size home as you? If he has a properly sized furnace, then the same size unit may also work for you.
3. Ask a contractor.
Most furnace professionals give free in-home estimates for installing new heating equipment during which they will recommend a unit size. While we don’t encourage using contractors for quotes if you don’t intend to hire any of 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.
4. If you are replacing an existing furnace, look at what size you have.
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 the furnace usually located somewhere inside the unit (remove service panel to find). Remember, furnaces are rated by input BTUs, but you will want to determine what the actual output BTU is (often also listed on name plate) when selecting the right size replacement unit.