HVAC Equipment Sizing Calculator (Manual J Estimator)

Estimates residential heating and cooling loads using Manual J simplified methodology to help size HVAC equipment properly.

Conditioned Floor Area (sq ft) Average Ceiling Height (ft) Number of Stories

Design Temperature Difference (°F) Difference between indoor setpoint (70°F) and outdoor design temp Climate Zone

Wall Insulation Level Ceiling/Attic Insulation Level Window Type Window Area (% of floor area)

Number of Occupants Infiltration / Air Tightness Duct Location

Conductive Heat Loss/Gain: Q = U × A × ΔT

Infiltration Load: Q_inf = 0.018 × CFM × ΔT, where CFM = (Volume × ACH) / 60

Ventilation Load (ASHRAE 62.2): CFM_vent = 0.01 × A_floor + 7.5 × (N_occupants + 1)

Solar Gain: Q_solar = SHGC × A_window × I_solar × orientation factor

Latent Cooling Load: Q_latent = Q_sensible × latent fraction (varies by climate zone)

Duct Loss: Total Load = Subtotal × Duct Multiplier (1.00–1.20 depending on duct location)

Tons of Cooling: 1 ton = 12,000 BTU/hr

Based on ACCA Manual J 8th Edition simplified residential load calculation methodology.
Indoor design conditions: 70°F heating setpoint, 75°F / 50% RH cooling setpoint.
Floor U-value assumed 0.05 BTU/hr·ft²·°F (insulated floor over unconditioned space).
Peak solar irradiance assumed 200 BTU/hr·ft² with 0.5 average orientation factor (accounts for N/S/E/W exposure mix).
Internal gains: 250 BTU/hr sensible per occupant (ASHRAE 55), 3 W/ft² lighting at 50% diversity, 1.5 BTU/hr·ft² appliances.
Latent fraction by climate zone: Zone 1–2: 20–25%, Zone 3–4: 25–30%, Zone 5–7: 10–20%.
Duct loss multipliers: conditioned space 1.00, semi-conditioned 1.10, unconditioned attic/garage 1.20 (ACCA Manual D).
Building geometry assumes a roughly square footprint; irregular shapes may require adjustment.
This tool provides a preliminary estimate only. A full Manual J calculation by a licensed HVAC professional is required for equipment selection, permits, and code compliance.
References: ACCA Manual J (8th Ed.), ASHRAE Fundamentals Handbook, ASHRAE 62.2-2022, IECC 2021.

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