Office Retrofit in Chicago: Balancing IAQ and Energy Efficiency

Scenario

Retrofit of a 1980s Class-B office building in downtown Chicago (ASHRAE Climate Zone 5A). The project aimed to modernize HVAC while complying with ASHRAE 62.1-2019 for tenant occupancy post-pandemic. Key constraints included limited rooftop space for new equipment, existing ductwork with high static pressure losses, and a strict energy-use intensity (EUI) target of ≤ 75 kBtu/ft²/yr.

Given Data

• Number of People: 62
• Outdoor Airflow Rate per Person: 15 cfm/person (standard for offices per Table 6-1)
• Area of the Space: 12,400 ft² (two open-plan floors, 6,200 ft² each)
• Outdoor Airflow Rate per Unit Area: 0.06 cfm/ft² (per Table 6-1 for offices)

Calculation

The Ventilation Rate Procedure requires calculating both components and selecting the greater value:

• People-based ventilation = 62 people × 15 cfm/person = 930 cfm
• Area-based ventilation = 12,400 ft² × 0.06 cfm/ft² = 744 cfm

Total required ventilation rate = max(930, 744) = 930 cfm

Note: This is the minimum total outdoor air (OA) flow required at peak occupancy — not supply air. Duct leakage and fan safety factors were added separately during system design.

Result and Decision

The design team selected a variable-air-volume (VAV) air-handling unit with integrated demand-controlled ventilation (DCV) using CO₂ sensors and occupancy scheduling. To meet the 930 cfm OA requirement without exceeding EUI limits, they specified a 75% effective enthalpy wheel ERV — reducing heating/cooling load by ~210,000 Btu/hr annually. A dedicated outdoor air system (DOAS) was rejected due to roof space limitations; instead, OA was introduced via mixed-air dampers with precise modulation.

Lesson

When retrofitting aging buildings, the ventilation rate is only the starting point — real-world success hinges on integrating the calculated OA requirement with system-level efficiency strategies (e.g., ERVs + DCV) before equipment selection, not as afterthoughts.