Begin with the home's design heating load
Furnace capacity should be compared with heat loss at the local winter design conditions. Floor area alone does not describe insulation, glazing, air leakage or outdoor temperature. Enter a property-specific design heating load from an appropriate assessment. ACCA Manual J provides a residential load-calculation method. This worksheet compares an existing load with a candidate appliance; it does not perform Manual J or select equipment under Manual S.
Distinguish fuel input from useful output
Estimated fuel input = required useful output ÷ (rated output/input efficiency ÷ 100)A furnace's input rating describes fuel energy entering the appliance. Its output rating describes useful heat delivered at the specified rating conditions. Those numbers are different. Enter the candidate's output in the output field, not its advertised input capacity. Use rated output divided by rated input for this conversion. NIST describes annual fuel utilization efficiency; that seasonal rating is not necessarily the appliance's rated output-to-input ratio.
A fifty thousand BTU per hour example
Suppose a completed heat-loss assessment gives a design load of 50,000 BTU/h. At an entered rated ratio of 90 percent, the simple input estimate is approximately 55,555.56 BTU/h. A candidate with 60,000 BTU/h rated output has output equal to 120 percent of the entered design load. Its displayed shortfall is zero. The calculation does not label that appliance approved: installation conditions, equipment-selection criteria and the manufacturer's performance data still matter.
Use the comparison without replacing equipment selection
The percentage row makes oversizing or undersizing visible. When candidate output is below the entered load, the shortfall row reports the numerical difference. Zero shortfall means only that the entered rating is at least the entered load. Altitude, airflow, external static pressure, fuel type and duct distribution can affect usable performance and require checks outside this form. Avoid adding an arbitrary safety percentage to a load calculation and then adding another equipment margin without justification. Metric mode accepts kilowatts; US mode accepts BTU per hour, which is a power rate rather than total energy. The converted-load row helps compare specifications. Keep the assessment and appliance data together for the installer's review.
Frequently asked questions
Can square footage alone size my furnace?
No. It omits important heat-loss inputs. Obtain a design heating load for the actual property before comparing appliance capacities.
Should I enter input or output capacity?
Use the candidate furnace's rated useful output. Input is fuel energy consumed and can be larger than the heat delivered.
Can I substitute AFUE for the rated ratio?
AFUE is seasonal. Prefer the rated output divided by rated input at the relevant conditions; do not assume the two efficiencies are identical.
Does a zero shortfall approve the candidate?
No. It only compares the entered numbers. Equipment selection, local conditions and installation performance need separate assessment.
What happens with zero efficiency?
It is rejected because division by zero cannot produce a finite input requirement. The efficiency must be greater than zero and no more than 100 percent.
Last reviewed: 2026-10-05 · How we calculate