Reducing Maximum Airflow During Deadband Operation for Variable Air Volume HVAC Systems

Measure Overview

This proposed code change would modify prescriptive requirements for multi-zone variable air volume (VAV) HVAC systems in Section 140.4(c), such that the central air handler is required to be capable of turning down as much as the terminal units in individual zones during deadband and economizer operation to avoid wasting fan energy.  

Multi-zone VAV systems control space temperature by modulating airflow to the space. The systems also provide a minimum flow of outdoor air to the space based on the outdoor airflow requirements in Section 120.1(c)3. Section 140.4(d)(2)(ii) currently requires terminal units in the individual zones for these systems to be capable of turning down airflow to no greater than the design outdoor airflow during deadband operation, where there is no demand for heating or cooling. In many applications, this would be 15 percent or less of the air handler’s design airflow. However, there is no requirement that the central air handler be able to turn down flow to any level.   

Some direct expansion (DX) air conditioners do not turn down below 50 percent airflow. Designers have two ways to handle the mismatch: (1) they can set the minimum zone airflow to match the higher airflow if they use a modeling compliance path or (2) they can specify a bypass duct that returns the excess airflow directly to the unit. In either case, the central air handler’s fan uses more energy than necessary.  

With the proposed change, while in deadband or economizer operation, the maximum airflow rate through an air handler would not exceed the greater of 15 percent of the design airflow or the sum of the design primary airflow of the associated terminal units. For new construction, per Section 140.4(d)(2)(ii), the design primary airflow is not to exceed the values in Section 120.1(c)(3) minimum requirements in Section 120.1(c)(3). A higher airflow would be permitted if more than the minimum is required for cooling during economizer operation.   

This proposed code change would apply to new construction, to additions where the addition is served by a new DX or hydronic unit, and to alterations where a new DX or hydronic air handler unit is installed. Note that this proposal only applies to multi-zone VAV systems and does not apply to single-zone VAV systems that control the capacity of mechanical cooling directly based on space temperature. The proposed code change will apply to all climate zones, and to all building and space types that have multi-zone VAV systems.  

The compliance software already allows entry of the minimum airflow; the CBECC baseline will be updated as needed to reflect the code change. This proposed code change will require modification of the current acceptance test.