Selecting Furnace Size (BTU)
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 will have different output
BTU depending on the furnace efficiency. Select a furnace for your home according to the output BTU, as this is the actual
heating capacity of the furnace. You can easily calculate the output BTU by multiplying the input BTU by the efficiency percentage.
For example a 100,000 BTU furnace at 80% efficiency will produce 80,000 BTU of heat while a 100,000 BTU furnace at 95% efficiency
will produce 95,000 BTU of heat output.
There is only one correctly sized furnace or boiler for your home. A unit that is oversized will turn on and off too frequently,
called "short cycling". Short cycling causes the unit to lose efficiency, and moisture can accumulate in the heat exchanger
which might damage the unit over time. A unit that is undersized will run too often, and on the coldest days of the year may not
be able to adequately heat the home.
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Available Sizing Methods
Method 1: Manual J Load Calculation
This is the proper and scientific method for calculating furnace or boiler 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.
You can purchase a downloadable copy of Manual J software for $49
if you wish to use this method.
Method 2: Use Our Equipment Sizing Estimator
This is an online tool that will give you a rough estimate. It won't be exact, but when used in conjunction with other information
it can provide a fairly close approximation.
Method 3: Compare your home to similar homes in your area.
Does your neighbor have the same size home as you? If he has a properly sized heating system, then the same size system may also
work for you.
Method 4: Ask a contractor.
Most HVAC professionals give free in-home estimates for installing new heating systems, 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.
Method 5: If you are replacing an existing unit, 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 somwhere inside the unit (remove the service panel to find it). Remember,
furnaces are rated by input BTU but you will want to determine what the actual output BTU is (also commonly listed on the name plate)
when selecting the right size replacement unit (see above).