Office Retrofit in Phoenix, AZ: Addressing Peak Summer Load

Scenario

A 3-story, 1980s-era office building in Phoenix, AZ is undergoing HVAC modernization. The building has minimal insulation, single-pane aluminum-framed windows, and high solar exposure. Local utility incentives require peak-load-optimized equipment sizing to avoid demand charges. Key constraints include limited roof space for new chillers and a strict budget cap that prohibits oversizing.

Given Data

• Surface area (exposed envelope): 2,450 m² (includes walls, roof, and west-facing glazing)
• CLTD (Cooling Load Temperature Difference): 28.5°C (based on ASHRAE RP-1167 data for July 21, 3 PM, Phoenix design day)
• CLF (Cooling Load Factor): 0.72 (elevated due to poor thermal mass, high infiltration, and aged envelope)

Calculation

The tool applies the standard conduction-based cooling load formula:

Cooling Load (W) = Surface Area × CLTD × CLF × 1000

(Note: The factor of 1000 converts kJ/(h·m²·°C) to W, assuming 1 W = 1 J/s = 3600 J/h → standard U-value scaling embedded in CLTD/CLF methodology)

Step-by-step:

• Surface Area = 2450 m²
• CLTD = 28.5 °C
• CLF = 0.72
• Cooling Load = 2450 × 28.5 × 0.72 × 1000
• First: 2450 × 28.5 = 69,825
• Then: 69,825 × 0.72 = 50,274
• Finally: 50,274 × 1000 = 50,274,000 W → 50.27 MW

Note: This value represents envelope-only conduction load. Internal gains and ventilation were calculated separately (adding +18.3 MW), yielding a total design load of 68.6 MW — but the tool’s output reflects only the envelope component used for façade-driven system sizing.

Result and Decision

The calculated envelope cooling load of 50.27 MW confirmed that the existing 40 MW chiller plant was critically undersized. Engineers selected two 30 MW variable-primary chilled water chillers (total 60 MW capacity with redundancy) with integrated heat recovery for domestic hot water — meeting both peak load and utility incentive requirements. Roof structural analysis verified support for the new units without reinforcement.

Lesson

Envelope-focused cooling load tools like this one must be applied with location-specific CLTD values — using default or generic values (e.g., 15°C) would have underestimated the Phoenix load by 90%, risking chronic system failure during monsoon-season peaks.