Phoenix Cooling Load Calculator (Manual J Estimate)
Estimates the residential cooling load for Phoenix, AZ using a simplified Manual J methodology. Enter your home's characteristics to calculate the required AC tonnage and BTU/hr.
Home Dimensions
Envelope Construction
Windows & Doors
Infiltration & Ventilation
Internal Gains
Formulas Used
1. Conduction Load: Q = U × A × ΔT
- U = overall heat transfer coefficient (BTU/hr·ft²·°F)
- A = surface area (ft²)
- ΔT = 110°F − 75°F = 35°F (Phoenix design outdoor − indoor setpoint)
2. Solar Gain: Qsolar = SHGC × Awindow × Ipeak × CForient × CFshade
- Ipeak = 250 BTU/hr·ft² (Phoenix peak west-facing, ASHRAE)
- SHGC = Solar Heat Gain Coefficient of glazing
3. Infiltration (Sensible): Qinf = 1.1 × CFM × ΔT | CFM = (ACH × Volume) / 60
4. Infiltration (Latent): Qlat = 0.68 × CFM × ΔW | ΔW = 28 gr/lb (Phoenix)
5. Internal Gains: Occupants (250 BTU/hr sensible + 200 BTU/hr latent each) + Lighting (W × 3.412) + Appliances
6. Total Load: Qtotal = Qconduction + Qsolar + Qinfiltration + Qinternal + Qlatent
7. Tonnage: Tons = Qtotal / 12,000 BTU/hr per ton
Assumptions & References
- Phoenix, AZ ASHRAE 0.4% cooling design dry-bulb: 110°F (ASHRAE Fundamentals Handbook, Chapter 14)
- Indoor design temperature: 75°F (ACCA Manual J, 8th Edition standard)
- Design temperature difference (ΔT): 35°F
- Phoenix design grains difference: 28 gr/lb (ACCA Manual J Table 1, Phoenix, AZ)
- Peak solar irradiance (west-facing vertical): 250 BTU/hr·ft² (ASHRAE HOF Table)
- Sensible heat factor for air: 1.1 BTU·min/(hr·CFM·°F)
- Latent heat factor for air: 0.68 BTU·min/(hr·CFM·gr/lb)
- Occupant sensible gain: 250 BTU/hr; latent: 200 BTU/hr (ASHRAE 62.2, sedentary activity)
- Lighting conversion: 1 W = 3.412 BTU/hr
- Wall area estimated from square footprint assumption per story
- Ceiling area = ground floor area ÷ number of stories (top floor only)
- Manual J recommends equipment sized within +15% / −0% of calculated load (ACCA Manual S)
- This tool does not account for duct losses, mechanical ventilation (ERV/HRV), or radiant barriers
- References: ACCA Manual J 8th Ed., ASHRAE Fundamentals 2021, IECC 2021 Climate Zone 2B (Phoenix)