Manual J Heating Load (Simplified)
Simplified heating load estimate.
Example
You enter
- Floor area (ft²) 1500
- Above-grade wall area (ft²) 1200
- Window area (ft²) 200
- Ceiling area (ft²) 1500
- Insulation level average
- Window type double
- Outdoor design temp (°F) 10
- Indoor design temp (°F) 70
You get
- Conductive 21300 BTU/hr
- Infiltration 6480 BTU/hr
- Total heating load 27780 BTU/hr
- Tons 2.315
Checked against published worked examples from: ACCA Manual J residential heating-load identity.
Details, formula, and sources
Formula or tableHeating load = U×A×ΔT for walls, ceiling and windows + U×A×0.5ΔT for the floor + infiltration sensible (1.08 × CFM_inf × ΔT). Simplified estimator only; ACCA Manual J 8th ed. is the code-compliant method.
Source and editionACCA Manual J, 8th edition. NOAA NCEI heating design temperatures by location.
Free accessACCA Manual J licensed. NOAA design temps free at ncei.noaa.gov.
What governsEstimate. AHJ and licensed mechanical contractor govern. ACCA Manual J / D / S supersede rules of thumb.
API field namesfloor_area_ft2, wall_area_ft2, window_area_ft2, ceiling_area_ft2, insulation_level, window_type, outdoor_design_F, indoor_design_F, conductive_BTU_hr, infiltration_BTU_hr, total_BTU_hr, tons
- Indoor design temperature 70 °F unless user suppliesACCA Manual J 8th ed. typical
- Outdoor design temperature user-entered (look up the NOAA NCEI 99% design value for the site)NOAA NCEI
- Air density factor 1.08 BTU/(hr × CFM × °F)ASHRAE Fundamentals by name (sea level, 70 °F)