HVAC Load Calculator (Manual J Estimate)
Estimates residential heating and cooling loads using ACCA Manual J simplified methodology. Results help size HVAC equipment appropriately.
Building Dimensions
Climate & Location
Envelope Construction
Infiltration & Ventilation
Internal & Solar Gains (Cooling)
Formulas Used
Envelope Conduction (Heating & Cooling):
Q = U × A × ΔT
where U = overall heat transfer coefficient (BTU/hr·ft²·°F), A = surface area (ft²), ΔT = design temperature difference (°F)
Infiltration Load:
Q_inf = 0.018 × Volume (ft³) × ACH × ΔT (heating)
Q_inf = 1.1 × CFM × ΔT (cooling sensible, CFM = ACH × Volume / 60)
Ventilation (ASHRAE 62.2):
CFM_vent = 0.03 × Floor Area + 7.5 × Occupants
Q_vent = 1.1 × CFM × ΔT (sensible)
Solar Heat Gain:
Q_solar = SHGC × A_window × I_solar × Shading Factor
where I_solar ≈ 150 BTU/hr·ft² (peak, adjusted by orientation)
Latent Cooling Load:
Q_latent = Q_latent_occupants + Q_latent_infiltration + Q_latent_ventilation
Q_latent_inf = 0.68 × CFM × ΔW (ΔW = humidity ratio difference, approximated by climate zone latent fraction)
Equipment Sizing:
Recommended Size = Total Load × 1.15 (15% safety factor per Manual J), rounded up to nearest 0.5 ton
Assumptions & References
- Based on ACCA Manual J (8th Edition) simplified load calculation methodology.
- Building footprint assumed square for perimeter/wall area estimation; actual irregular shapes may vary.
- U-values represent whole-assembly performance (framing + insulation), not cavity-only R-values.
- Attic ceiling load includes a 1.15× penalty for elevated attic temperatures above outdoor design temp.
- Ground floor cooling load uses a 0.5× factor because ground temperature moderates heat gain.
- Solar irradiance baseline of 150 BTU/hr·ft² represents peak summer conditions; adjusted by orientation multiplier.
- SHGC estimated from window U-value: single pane ≈ 0.65, double standard ≈ 0.40, double low-e ≈ 0.28, triple ≈ 0.22.
- Internal gains: 250 BTU/hr sensible + 200 BTU/hr latent per occupant (ASHRAE activity level: seated, light work); lighting 1.5 BTU/hr·ft²; equipment 1.0 BTU/hr·ft².
- Latent load fraction by climate zone per ASHRAE 169-2020 climate data (Zone 1 = 55%, Zone 7 = 12%).
- Ventilation rates per ASHRAE 62.2-2022: 0.03 cfm/ft² + 7.5 cfm/person.
- A 15% safety/design factor is applied to final loads for equipment selection per Manual J guidance.
- This tool provides a simplified estimate. A full Manual J calculation by a licensed HVAC engineer is required for permit and equipment selection.
- References: ACCA Manual J (8th Ed.), ASHRAE Fundamentals Handbook, ASHRAE 62.2-2022, ASHRAE 169-2020.