How it’s calculated
- UA = wall area ÷ R + window area × U + ceiling area ÷ R
- Conduction = UA × (T indoor − T outdoor)
- Infiltration = 0.018 × volume × ACH × ΔT
- Furnace input = load ÷ AFUE
Worked example
1,500 sq ft home: 1,200 sq ft R-13 walls, 200 sq ft U-0.30 windows, R-38 ceiling, 8 ft ceilings, average leakage, 70 °F in, 5 °F out.
Inputs
- Net exterior wall area (excluding windows)
- 1200 sq ft
- Wall R-value (whole wall)
- 13
- Window and door glass area
- 200 sq ft
- Window U-factor
- 0.3 Btu/h·ft²·°F
- Ceiling area under attic
- 1500 sq ft
- Ceiling R-value
- 38
- Conditioned floor area
- 1500 sq ft
- Average ceiling height
- 8 ft
- Air leakage (natural air changes per hour)
- Average (0.5 ACH)
- Indoor design temperature
- 70 °F
- Outdoor winter design temperature
- 5 °F
- Furnace AFUE
- 95 %
Results
- Design heating load
- 19,486 BTU/h
- Minimum furnace input rating
- 20,511 BTU/h
- Conduction loss
- 12,466 BTU/h
- Air leakage loss
- 7,020 BTU/h
Assumptions and limits
- Floors, basements, slab edges and duct losses are not included.
- 0.018 Btu/ft³·°F is the heat capacity of standard air.
- Internal and solar gains are ignored (conservative for heating).
Verify the result against the applicable local code, adopted code edition, manufacturer instructions and project requirements. See our methodology.
Questions
What size furnace do I need for 1,500 square feet?
It depends far more on climate, insulation and air leakage than floor area. This example house needs about 19,500 BTU/h at 5 °F — much less than old "50 BTU per square foot" rules suggest.
Why not just use BTU per square foot?
Per-square-foot rules ignore insulation, windows and climate and usually oversize equipment. Oversized furnaces short-cycle and heat unevenly.