HVAC Load and Duct Sizing Calculator
Calculate the heating/cooling load for a room and determine the required duct diameter or rectangular duct dimensions based on airflow requirements.
Room Load Inputs
Duct Sizing Inputs
Formulas Used
Conduction Load: Q = U × A × ΔT (BTU/hr)
Infiltration Load: Qinfil = 1.1 × CFMinfil × ΔT | CFMinfil = (ACH × Volume) / 60
Total Load: Qtotal = (Qwall + Qceil + Qwin + Qinfil + Qint) × SF (SF = 1.15 cooling, 1.10 heating)
Required CFM: CFM = Qtotal / (1.1 × ΔTsupply) (ΔTsupply = 20°F cooling, 30°F heating)
Round Duct Diameter: D = √(4A / π) where A = CFM / Velocity (ft²)
Rectangular Duct Width: W = A / H (given duct height H)
Equivalent Diameter (ASHRAE): Deq = 1.30 × (a·b)0.625 / (a+b)0.25
Friction Loss (Darcy-Weisbach approx.): ΔP/100ft = 0.109 × (V/1000)1.82 / D1.22 (in. w.g.)
Assumptions & References
- Standard air density: 0.075 lb/ft³ at sea level, 70°F (factor 1.1 in sensible heat equation).
- U-values are representative ranges per ASHRAE 90.1 and IECC; actual values depend on specific construction assemblies.
- Infiltration ACH: Poor = 1.5, Average = 0.8, Good = 0.5, Excellent = 0.25 (ASHRAE Fundamentals).
- Occupant sensible heat gain: 250 BTU/hr per person (sedentary, ASHRAE 55).
- Lighting heat gain: 3.4 BTU/hr per ft² (average residential/light commercial).
- Supply air temperature differential: 20°F below room for cooling; 30°F above room for heating.
- Recommended duct velocities: Main trunk 800–1200 FPM; branch ducts 600–900 FPM; residential 400–700 FPM.
- Equivalent diameter formula per ASHRAE Handbook of Fundamentals, Chapter 21.
- Friction loss approximation valid for galvanized sheet metal ducts with standard roughness (ε ≈ 0.0003 ft).
- Safety factors: 15% for cooling, 10% for heating (common industry practice).
- This calculator provides estimates for preliminary design; a full Manual J/D calculation is required for final system design.