Commercial HVAC Load Calculator
Estimates commercial building heating and cooling loads based on ASHRAE simplified load calculation methods. Calculates sensible heat gain/loss through walls, roof, windows, infiltration, and internal loads.
Building Envelope
Climate & Occupancy
Formulas Used
Conduction (Walls, Roof, Windows):
Qcond = U × A × ΔT [BTU/hr]
where U = U-value [BTU/hr·ft²·°F], A = area [ft²], ΔT = design temperature difference [°F]
Solar Heat Gain:
Qsolar = SHGC × Awindow × Ipeak × forient
where SHGC = Solar Heat Gain Coefficient, Ipeak = 200 BTU/hr·ft² (ASHRAE peak vertical surface),
forient = 0.5 (orientation diversity factor)
Infiltration & Ventilation (Sensible):
Q = 1.1 × CFM × ΔT [BTU/hr]
CFMinfil = ACH × Volume / 60 | CFMvent = Occupants × CFM/person
Internal Gains:
Occupants: 250 BTU/hr·person (sensible) + 200 BTU/hr·person (latent)
Lighting: W/ft² × Area × 3.412 BTU/W
Equipment: W/ft² × Area × 3.412 × 0.75 (sensible) / 0.25 (latent)
Unit Conversions:
1 Ton = 12,000 BTU/hr | 1 kW = 3,412 BTU/hr
Assumptions & References
- Building geometry assumed square footprint; wall area = perimeter × total height
- Roof load calculated on top-floor footprint only (no slab-on-grade floor loss)
- Peak solar intensity of 200 BTU/hr·ft² on vertical surfaces per ASHRAE Fundamentals
- Orientation diversity factor of 0.5 applied to solar gain (not all facades simultaneously sunlit)
- Occupant metabolic rates per ASHRAE 62.1: 250 BTU/hr sensible, 200 BTU/hr latent (office)
- Lighting assumed 100% converted to sensible heat gain
- Equipment load split: 75% sensible, 25% latent (ASHRAE Handbook of Fundamentals)
- Infiltration sensible factor: 1.1 BTU·min/(hr·ft³·°F) at standard air density
- Heating load conservatively excludes internal gains (peak sizing practice)
- Ventilation rates per ASHRAE 62.1 minimum requirements
- Results are estimates; detailed ACCA Manual N or energy modeling required for final design
- References: ASHRAE Handbook of Fundamentals (2021), ASHRAE 62.1-2022, ACCA Manual N