HVAC Equipment Sizing Calculator
Estimate heating and cooling loads for residential and light commercial spaces using the Manual J simplified method. Results help determine appropriate HVAC equipment capacity in BTU/hr and tons.
Space Dimensions
Climate & Envelope
Occupancy & Internal Gains
Formulas Used
Cooling Load Components (BTU/hr):
- Conduction: Q = U × A × ΔT (U = assembly U-value, A = area ft², ΔT = outdoor − indoor °F)
- Solar Gain: Qsolar = SHGC × Awindow × Ipeak × CFclimate (Ipeak = 200 BTU/hr·ft²)
- Infiltration (sensible): Qinf = 1.1 × CFM × ΔT where CFM = ACH × Volume / 60
- Infiltration (latent): Qlat = 0.68 × CFM × ΔW (ΔW = humidity ratio difference, gr/lb)
- Occupants: 250 BTU/hr sensible + 200 BTU/hr latent per person (ASHRAE 62.1)
- Lighting/Equipment: Q = W/ft² × Area × 3.412 BTU/W
- Total Cooling: Qtotal = Qsensible + Qlatent
Heating Load Components (BTU/hr):
- Conduction: Q = U × A × ΔT (ΔT = indoor − outdoor °F)
- Infiltration: Qinf = 1.1 × CFM × ΔT
- Total Heating: Sum of all conduction + infiltration (no internal gains credited)
Equipment Sizing: Design load = Calculated load × 1.15 (15% safety factor per ACCA Manual S). Cooling capacity rounded up to nearest 0.5-ton increment.
Assumptions & References
- Based on ACCA Manual J (Residential Load Calculation, 8th Ed.) simplified method
- Equipment sizing per ACCA Manual S (Residential Equipment Selection)
- Standard door size assumed at 20 sq ft per door
- Building footprint assumed square for perimeter/wall area estimation
- Peak solar intensity of 200 BTU/hr·ft² per ASHRAE Fundamentals Handbook
- Occupant sensible/latent gains per ASHRAE 62.1 (250/200 BTU/hr per person)
- Infiltration latent load uses humidity ratio differences by climate zone (ASHRAE climate data)
- U-values based on ASHRAE 90.1 prescriptive envelope requirements by insulation level
- No credit taken for internal gains in heating load calculation (conservative, per Manual J)
- 15% safety factor applied per Manual S for final equipment selection
- Results are estimates only — a full Manual J by a licensed engineer is required for permits