Low pressure air-handling system leakage in large commercial buildings: Diagnosis, prevalence, and energy impacts

Abstract

Air-handling system leakage reduces the amount of air delivered to conditioned spaces and in most cases wastes energy and money. Standards exist for where and how to measure system airtightness, but they tend to focus on new construction, and only on the high-pressure (1500–2500 Pa [6–10 in. w.c.])/ medium-pressure [500–1500 Pa (2–6 in. w.c.]) portions of the system. This article investigates air leakage in the low-pressure (≤500 Pa [≤2 in. w.c.]) portions of large commercial-building air-handling systems (i.e., downstream of variable-air-volume box inlet dampers). A simplified diagnostic protocol for measuring low-pressure leakage that can be used during normal system operation in an existing building is presented and utilized for this investigation. A validation of the protocol using a calibrated leak in a field installation is also presented, as are the results of applying this protocol in nine other buildings around the United States. The validation results indicate that normalized leakage can be measured to within 10 L/s at 25 Pa (20 cfm at 0.1 in w.c.), with and without the existence of significant flow through the minimum opening of the box inlet damper. The field test results indicate that low-pressure leakage varied considerably from system to system (standard deviation of 50% of the mean value), and that the average value was approximately 10% of the flow entering the low-pressure system sections. The variability of the measured results, combined with a simplified analysis of the impacts of this leakage, suggest that testing of low-pressure system leakage in commercial buildings should be economically justifiable.

Notes

Note that some systems employ a “duct static pressure reset” strategy, which involves changing the pressure set point so that one or more VAV box inlet dampers are as wide open as possible.

If the amount of primary airflow entering a fan-powered VAV box is lower than a specified threshold (e.g., for the test building, less than 40% of the box's cooling maximum airflow), the control system turns on the fan in the VAV box. Except when dampers are commanded to minimum position during reheat periods, increased downstream leakage causes the powered VAV boxes to operate at higher primary airflows to compensate for the leakage and the fans do not need to run as often.

Note that some of the leaked air is likely exhausted as relief air, which would also result in a thermal load that is not being considered here.

The building's relief fans operated with an irregular pattern to maintain room pressure set points (ran as needed to balance outdoor airflow supplied for ventilation, economizer use, and pre-cooling). There was no discernible correlation between system leakage and relief fan operation.

In this leakage testing mode, both supply fans serving the system were operated at full-speed and each VAV box inlet damper was adjusted until the design duct static pressure of 1 in. w.c. (250 Pa) upstream of VAV box inlet dampers was achieved (in this case, each damper was adjusted to 75% of cooling maximum flow for that box). In addition, return dampers were fully open, outdoor air dampers were fully closed, relief fans were off, and VAV box fans and reheat coils were off. The air-handling system never operated in this mode during normal operation, however. The specified leakage test conditions were used only to provide an operation-independent reproducible condition for determining system leakage flows.