Saving energy in variable air volume systems by optimizing minimum airflow rates.

Status: Current

Project Objective

This project aimed to develop a new control strategy for accurately meeting the minimum ventilation air flow rate setpoint in variable air volume (VAV) terminal units, in order to yield significant energy savings.

Project Results

These results show significant potential for reducing energy costs in existing VAV buildings with a very low first cost. Compared to the existing base case scenario using single-max VAV logic, this strategy reduced the mean zone airflow fraction from 0.44 to 0.27 during the intervention period. Outside (ventilation) airflow at the air handling unit remained the same during both periods. The corresponding reductions in average heating, cooling, and fan power were 41%, 23%, and 15% respectively.

This finding can be crucial for energy reduction and cutting costs, as VAV buildings make up the majority of large US office buildings. TAV may be applied as a low-cost retrofit strategy in any building that has direct digital control to each VAV terminal.

Significance to Industry

Typical variable air volume (VAV) terminals spend the majority of time at their minimum airflow set-points. These are often higher than the minimum ventilation requirements defined by code, resulting in excess energy use and a risk of over-cooling the spaces. Design guidelines recommend setting the minimum airflow setpoint to the larger of the minimum ventilation requirements and the lowest controllable airflow setpoint allowed by the box controller.

However, identifying this minimum controllable airflow setpoint can be a challenge as it depends on attributes of both the VAV terminal and the VAV controller, which are generally provided by separate manufacturers. This is problematic as the manufacturers may not be known until after a project is bid. Additionally, ASHRAE Standard 62.1 ventilation requirements can be complicated to understand and use, which leads many designers to opt for the default minimums listed in ASHRAE Standard 90.1. These are some of the reasons why, for most building systems, HVAC designers commonly set VAV minimum airflow setpoints to higher values than necessary when compared to the ventilation rates required by codes and standards.

Research Approach

To address this issue, a time-averaged ventilation (TAV) control strategy was developed and tested in an institutional building on UC Berkeley’s campus. This strategy allows a VAV box to accurately meet any defined minimum airflow setpoint. Whenever a zone does not require cooling, TAV alternates the VAV damper between partially open and fully closed so that the average airflow matches a predefined ventilation setpoint. This can be achieved by hiring a controls vendor to implement the strategy, or (as in our study) directly over BACnet using open source tools.

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