HVAC Load Calculation (Manual J Estimator)
Estimates residential heating and cooling loads based on ACCA Manual J methodology. Enter your home's characteristics to calculate the required HVAC system capacity in BTU/hr and tons.
Formulas Used
Conduction Load: Q = U × A × ΔT
- U = thermal transmittance (BTU/hr·ft²·°F) = 1/R-value
- A = surface area (ft²)
- ΔT = design temperature difference (°F)
Solar Heat Gain: Qsolar = SHGC × Isolar × Awindow × orientation factor
Infiltration (Sensible): Qinf = 1.1 × CFM × ΔT | CFM = (ACH × Volume) / 60
Infiltration (Latent): Qlat = 0.68 × CFM × ΔW (ΔW in grains/lb)
People Load: 250 BTU/hr sensible + 200 BTU/hr latent per occupant (ASHRAE 62.2)
Total Cooling: Qtotal = (Qwall + Qceil + Qwin + Qfloor + Qsolar + Qinf + Qpeople + Qappliance) × duct factor
Tons of Cooling: Tons = Total BTU/hr ÷ 12,000
Assumptions & References
- Based on ACCA Manual J (8th Edition) residential load calculation methodology.
- Assumes a roughly square floor plan for wall area estimation; irregular shapes may differ ±10–20%.
- Duct loss factor of 15% applied for ducts in unconditioned space (ACCA Manual D default).
- 40% of window area assumed to face south/west for peak solar gain calculation.
- Appliance/lighting internal gain estimated at 3 BTU/hr per sq ft (ASHRAE Handbook of Fundamentals).
- Latent load grain differences by climate zone sourced from ASHRAE 2021 Fundamentals, Chapter 14.
- Floor/slab conduction uses a reduced ΔT (50%) to account for ground-contact thermal buffering.
- This tool provides a preliminary estimate only. A certified Manual J calculation by a licensed HVAC engineer is required for equipment selection and permit compliance.
- References: ACCA Manual J (2016), ASHRAE 62.2-2022, ASHRAE Handbook of Fundamentals (2021), IECC 2021 Climate Zone Map.