The Importance of Design Criteria Review

Working in existing buildings is always a challenge. It is not always clear what criteria was used to design the existing HVAC systems. When it comes time to replacing existing equipment due to age or underperformance, the designer should always verify the current criteria the system needs to meet.

For example, we recently designed the replacement of a roof mounted dedicated outdoor air handling unit to provide ventilation to a child care center. It turned out that the existing unit was designed to meet the minimum ventilation rates required by ASHRAE 62.1. On most jobs, this fact would not require a second thought and the new unit could be sized to match the existing unit’s capacity. However, this building was a childcare center owned by GSA and should have been designed to meet the special ventilation requirements for child care centers required by PBS-140.

The design required a new air handling unit twice the size of the existing unit (existing units shown in photos) in order to meet the increased ventilation requirements of PBS-140, and what began as an exercise in determining proper design criteria, cascaded into the need for an intricate engineered solution to fit the larger unit in the limited available roof space. The design required high levels of coordination between the mechanical, roofing, and structural engineers.

The moral of the story is never assume existing systems were built to the correct criteria, and always perform your criteria review as soon as possible. This will help determine if coordination will be required with other disciplines in order to make the upgrades necessary to meet the clients criteria.