Formula and input notes
Screening heating load (BTU/h) = conditioned area × heat-loss factor × adjustment factor
| Input | Practical note |
|---|---|
| Conditioned area | Use the floor area served by the proposed system, excluding unconditioned zones; area alone does not describe the building load. |
| Load factor | This user-supplied BTU/h-per-square-foot factor controls the screening load directly. It is not derived from the envelope or weather by this tool. |
| Ceiling/climate factor | Use this multiplier only for an adjustment you can justify; it combines effects that a full load calculation would model separately. |
Worked furnace size example
Furnace Size Estimator example: 721 ft² × 25 BTU/h·ft² × 1.1 = 19,828 BTU/h, equivalent to 5.81 kW of heat rate.
Compare load with delivered furnace output
Use the result to screen quotes; compare the verified design load with the furnace's output capacity, not input nameplate alone.
What final furnace selection still requires
Outdoor design temperature, envelope U-values, air leakage, ventilation, ducts, zoning, gains, and equipment output versus input ratings determine furnace sizing.
Questions specific to this calculation
Is the result furnace input or delivered heating output?
The calculator returns an estimated building heating load in BTU/h. Furnace input rating and delivered output differ by efficiency, so do not equate the screening load directly to the nameplate input capacity.
Why is this not a Manual J heating calculation?
It uses only area, a user-entered BTU/h-per-square-foot factor, and one multiplier. It does not separately model envelope heat transfer, infiltration, ventilation, design temperatures, ducts, or room-by-room loads.
Before selecting furnace capacity, compare this area-factor screen with a residential heating-load calculation performed under ANSI/ACCA Manual J; this page does not implement that procedure.