Furnace BTU Sizing

Notes

• Input and output are different numbers, and the box shows the bigger one. A 100,000 BTU furnace at 90% AFUE puts 90,000 BTU/hr into the house. Compare furnaces on output, or you are comparing how much gas they burn rather than how much heat you get.
• Upgrading efficiency without resizing is the classic error. Replace an 80% furnace with a 96% one of the same input and the house receives 16,000 BTU/hr more than it did — roughly 20% extra. The new furnace short cycles, the house swings warm and cool, and the owner concludes high efficiency is uncomfortable. It was the sizing.
• 5% over is the normal rounding step — furnaces come in 20,000 BTU increments, so landing exactly on the calculated load is impossible.
• These BTU-per-square-foot figures assume average insulation. A tight house with good windows can need a third less; a 1950s house with an uninsulated attic and single glazing can need a third more. The figure above is where the conversation starts.
• A Manual J is what turns this into a specification. It accounts for window area and orientation, air leakage, insulation, ceiling height and duct losses. On a furnace it matters more than on air conditioning, because heating loads vary more between houses than cooling loads do.
• Do not size by what was there before. The old furnace was very likely oversized too — contractors have rounded up for decades — and copying its input perpetuates the error into another twenty-year appliance.