HVAC Load Calculator
Estimate the heating and cooling load (BTU/hr) for a room or building zone based on area, insulation, climate, occupancy, and internal gains.
Formulas Used
Envelope Conduction (sensible):
Qcond = U × A × ΔT [BTU/hr]
Applied separately to net walls, ceiling, and windows. ΔT = |Toutdoor design − Tindoor|
Infiltration / Ventilation (sensible):
CFM = (ACH × Volume) / 60
Qinf = 0.018 × CFM × ΔT [BTU/hr]
(0.018 = ρ·cp for air ≈ 0.075 lb/ft³ × 0.24 BTU/lb·°F × 60 min/hr)
Internal Gains (cooling only):
Qpeople = N × 250 BTU/hr (sensible, seated/light activity — ASHRAE 2017 HOF)
Qlighting = Watts × 3.412 BTU/W·hr
Qequipment = Watts × 3.412 × 0.70 (70% heat fraction to space)
Total Cooling Load: Qcool = Qcond,cool + Qinf,cool + Qinternal
Total Heating Load: Qheat = Qcond,heat + Qinf,heat (internal gains omitted — conservative)
Unit Conversions:
Tons of cooling = BTU/hr ÷ 12,000
kW of heating = BTU/hr ÷ 3,412.14
Equipment Sizing (ACCA Manual S):
Cooling equipment ≤ 1.15× calculated load | Heating equipment ≤ 1.25× calculated load
Assumptions & References
- Floor plan assumed square for perimeter wall estimation; adjust window area manually for accuracy.
- Floor heat loss/gain is neglected (slab-on-grade or conditioned below).
- Latent (humidity) loads are not included; add ~20–30% to cooling load for humid climates.
- Solar heat gain through windows is not separately calculated; it is partially captured via the summer ΔT.
- Occupant sensible heat: 250 BTU/hr per person (ASHRAE Handbook of Fundamentals 2017, Table 1, Chapter 18).
- Air density/specific heat constant: 0.018 BTU/ft³·°F (standard sea-level conditions).
- ACCA Manual J (8th Ed.) is the industry standard for residential load calculations; this tool uses a simplified method suitable for preliminary sizing only.
- ASHRAE Standard 90.1 and IECC provide minimum U-value requirements by climate zone.
- For final equipment selection, consult a licensed HVAC engineer and perform a full Manual J/S/D analysis.