Furnace Size Calculator
Construction & Home ImprovementEstimate the required furnace input capacity, in BTU per hour, from floor area, climate zone, insulation, and efficiency.
Required Furnace Input Capacity
Approximate required rated furnace input capacity, in BTU per hour, to meet the heating demand at this efficiency.
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Odeh Ahwal0people find this calculator helpful
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Construction & Home Improvement calculators
Frequently Asked Questions
Why does a less efficient furnace need a higher rated capacity?
A furnace rated AFUE, annual fuel utilization efficiency, converts only that percentage of its fuel input into usable heat. A less efficient furnace wastes more energy, so it needs a higher rated input capacity to deliver the same amount of actual heat to the home.
What climate zones does this use?
A simplified 5-zone system from Zone 1, hot climates like southern Florida, through Zone 5, cold climates like the northern US, using commonly cited BTU-per-square-foot demand figures for each zone. This is a simplification of the more detailed 9-zone systems some references use.
Does this replace a Manual J load calculation?
No. This is a quick square-footage-based estimate. A full Manual J calculation, the HVAC industry standard for accurate sizing, accounts for window placement and area, air leakage, local design temperatures, and other details this simplified estimate does not.
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