Heat Load Calculator
Estimates the heating and cooling load for a room or building zone based on area, insulation, climate, and occupancy factors.
Formulas Used
1. Envelope Conduction Load (ASHRAE Fundamentals):
Q_cond = U × A × ΔT
Where U = overall heat transfer coefficient (BTU/hr·ft²·°F), A = surface area (ft²), ΔT = indoor–outdoor temperature difference (°F).
2. Infiltration Sensible Load:
Q_infil = 1.08 × CFM × ΔT (simplified as 0.018 × ACH × Volume × ΔT)
Where CFM = (ACH × Volume) / 60, and 1.08 = 0.018 × 60 (air density × specific heat constant for standard air).
3. Solar Heat Gain (Cooling only):
Q_solar = SHGC × A_window × I_solar
Where SHGC = Solar Heat Gain Coefficient, I_solar = peak solar irradiance by climate zone (BTU/hr·ft²).
4. Internal Gains (Cooling) / Credit (Heating):
Q_internal = Q_people + Q_lighting + Q_equipment
People: 250 BTU/hr sensible per occupant; Lighting: 3 W/ft² × 3.41; Equipment: 2 W/ft² × 3.41.
5. Total Load:
Q_total = Q_cond + Q_infil + Q_solar + Q_internal
Converted: 1 ton = 12,000 BTU/hr; 1 kW = 3,412 BTU/hr.
Assumptions & References
- Room geometry is approximated as a square floor plan; wall area = perimeter × ceiling height − window area.
- U-values for walls and roofs are representative of typical construction assemblies per ASHRAE Handbook of Fundamentals (2021).
- Infiltration ACH values are based on ASHRAE 62.2 and ACCA Manual J typical leakage rates by construction quality.
- Solar irradiance factors are simplified peak values by climate zone; a full Manual J calculation uses orientation-specific CLTD/CLF tables.
- Internal gains use ASHRAE 90.1 default lighting power density (3 W/ft²) and a modest equipment load (2 W/ft²); adjust for actual use.
- Latent load for hot-humid climates applies a 30% multiplier to infiltration load; a full psychrometric analysis is recommended for precise latent calculations.
- Equipment sizing includes a 15% safety factor per ACCA Manual S recommendations.
- This calculator provides a simplified estimate. For final equipment selection, a full ACCA Manual J load calculation performed by a licensed HVAC engineer is recommended.
- References: ASHRAE Handbook of Fundamentals (2021); ACCA Manual J (8th Ed.); ACCA Manual S; ASHRAE Standard 90.1-2022.