HVAC System Sizing Calculator — Tonnage Estimator
Estimate the required HVAC system size (in tons of cooling/heating) for a residential or light commercial space using a simplified Manual J–style load calculation.
Formulas Used
Total Cooling Load (BTU/hr):
Q_total = (Q_env + Q_solar + Q_occ + Q_int + Q_inf) × F_climate × F_insulation × F_stories × F_shade × F_internal
- Envelope Conduction:
Q_env = Volume × U_eff × ΔT— where Volume = sqft × ceiling height (ft³), U_eff = 0.133 BTU/(hr·ft³·°F), ΔT = 20°F design difference - Solar Gain:
Q_solar = A_window × SHGC × I_peak × F_orientation— SHGC = 0.40 (double-pane low-e), I_peak = 200 BTU/(hr·ft²) - Occupant Load:
Q_occ = N × 250 BTU/hr— ASHRAE sensible heat per person (light activity) - Internal Load:
Q_int = sqft × 3.0 BTU/(hr·ft²)— residential appliance/lighting baseline - Infiltration:
Q_inf = Volume × ACH × 1.1 × ΔT— ACH = 0.5, 1.1 = ρ·Cp for air in BTU/(ft³·°F) - Tonnage:
Tons = Q_total / 12,000— 1 ton of cooling = 12,000 BTU/hr - Equipment Sizing: Round up to nearest 0.5-ton increment per industry practice