Calculation Results
About This Calculator
Calculate required home heating output BTUs and input furnace nameplate rating based on climate heating zones (Zone 1 to 5), home square footage, insulation, and AFUE efficiency.
Frequently Asked Questions
What does AFUE stand for on a gas furnace?
AFUE stands for Annual Fuel Utilization Efficiency. A 96% AFUE furnace converts 96 cents of every dollar of natural gas into heat, exhausting only 4% out the vent. Older 80% furnaces lose 20% of their heat up the chimney flue.
Why should you avoid oversizing a gas furnace?
An oversized furnace heats the house too rapidly, short-cycles, creates hot and cold temperature swings, causes heat exchanger stress, and fails to dehumidify the home properly.
What is the difference between furnace Input BTU and Output BTU?
Input BTU is the amount of gas energy the furnace burns per hour. Output BTU is the actual heating energy delivered into the ductwork after combustion efficiency losses (Output = Input x AFUE).
How many BTUs per square foot are needed in cold northern states?
Cold northern states (Zone 4 and Zone 5 like Illinois, Minnesota, New York) typically require 45 to 50 BTUs per square foot for adequate heating during sub-zero cold snaps.