HVAC Equipment Sizing Calculator — Manual J Load Estimate
Estimates residential heating and cooling loads using the Manual J simplified methodology to properly size HVAC equipment. Enter your home's characteristics to calculate BTU/hr requirements.
Climate & Location
Building Envelope
Windows & Doors
Infiltration & Internal Gains
Formulas Used (Manual J Simplified)
Heating Load (each component):
Qheat = U × A × ΔTwinter
where U = 1/R (BTU/hr·ft²·°F), A = area (ft²), ΔT = Tindoor − Toutdoor design
Infiltration Heating:
Qinf,heat = 1.1 × CFM × ΔT
CFM = ACH × Volume(ft³) / 60
Cooling Sensible Load:
Qcool,sens = Σ(U × A × ΔTsummer) + Qsolar + Qinternal + Qinf,sens
Qsolar = SHGC × Awindow × 250 BTU/hr·ft² × Orientation Factor
Cooling Latent Load:
Qlat,inf = 0.68 × CFM × ΔW (gr/lb)
Qlat,occ = Occupants × 200 BTU/hr
Total Cooling: Qtotal = Qsensible + Qlatent
Tons: Tons = Qtotal / 12,000 BTU/hr per ton
Duct Loss Factor applied to all loads (1.0–1.3×)
Assumptions & References
- Based on ACCA Manual J 8th Edition simplified residential load calculation methodology
- Wall R-values include +R-4 for sheathing, air films, and drywall; ceiling includes +R-2 for framing/drywall
- Slab-on-grade floor uses effective U=0.10 BTU/hr·ft²·°F per ASHRAE 90.1 simplified method
- Peak solar intensity of 250 BTU/hr·ft² per ASHRAE Handbook of Fundamentals (summer peak)
- Occupant sensible gain: 250 BTU/hr; latent gain: 200 BTU/hr (ASHRAE 62.1 seated light activity)
- Outdoor humidity ratio approximated from relative humidity and dry-bulb temperature
- Indoor design conditions: 75°F / 50% RH summer; 70°F winter (ACCA Manual J defaults)
- Equipment sizing rounded up to nearest 5,000 BTU/hr heating; 6,000 BTU/hr (0.5 ton) cooling
- Manual J recommends equipment sized at 100–115% of calculated load (no more than 125%)
- A full Manual J by a certified ACCA contractor is required for permits and final design