HVAC
Furnace Size Calculator
Estimate the furnace input capacity a home needs, allowing for efficiency losses.
Result
This is a preliminary sizing estimate using industry rules of thumb. Actual equipment selection requires a room-by-room load calculation (ACCA Manual J) and duct design (Manual D) by a qualified professional, oversized equipment short-cycles, dehumidifies poorly and fails early. Furnaces are rated by input; the heat actually delivered is input × AFUE.
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About this calculator
What is the difference between input and output BTU?
Input is the fuel energy consumed; output is the heat delivered into the house. A 100,000 BTU input furnace at 95% AFUE delivers 95,000 BTU of usable heat.
Does a higher AFUE mean a smaller furnace?
It means less input for the same output. The heat the house needs does not change. The fuel required to produce it does.
The formula
input BTU = (area × climate factor) ÷ AFUE
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