Input Parameters
About This Calculator
- Calculates wall, window, and infiltration heat losses
- Applies AFUE efficiency to determine required furnace input
- Output BTU matches heat load; Input BTU = Output ÷ AFUE
- Climate zone BTU/sq ft reference table for quick comparison
How this furnace size calculator works
This tool first calculates your room's heat loss using the same method as our heating load calculator — wall conduction, window conduction, and infiltration losses with orientation correction. It then converts the heat load to BTU/h and divides by AFUE efficiency to determine the required furnace input rating.
Understanding furnace sizing
- Furnace output BTU = heat load in BTU/h (what the building needs)
- Furnace input BTU = output ÷ AFUE (what the furnace consumes)
- A 60,000 BTU/h output at 80% AFUE requires 75,000 BTU/h fuel input
- At 95% AFUE, the same output needs only 63,158 BTU/h input
FAQ
What size furnace for a 2000 sq ft house?
Typically 60,000–90,000 BTU/h depending on climate zone, insulation, and AFUE. A well-insulated house in a moderate climate may only need 50,000 BTU/h.
Should I oversize my furnace?
No. Oversizing causes short-cycling, reduced efficiency, and uneven comfort. Slightly undersizing (within 10%) is better than oversizing.
What AFUE should I choose?
80% is standard and affordable. 90–95% condensing furnaces save 15–20% on fuel costs. 98% is premium with longest payback. Consider your climate and fuel costs.