Correct Setup and Balancing of a VAV Air-Handling System
In 16-plus years of experience working in the testing and balancing (TAB) and commissioning industries on testing and/or commissioning variable air volume (VAV) systems, I've come across numerous instances where it is clearly evident that a VAV system has not been set up correctly.
All of these issues have a common denominator: They are either energy wasteful and/or fail to maintain the project document required minimum outside air (OA) ventilation airflow under all operating conditions. Stated another way, incorrect VAV system setup inflicts higher operating costs and/or potential IAQ problems on building owners.
The correct setup of a VAV system is not rocket science, but it does require the TAB technician to be deliberate, methodical, and to follow an established, systematic protocol until all components are operating as designed. Toward that goal, this article will outline the steps I take to properly set up a VAV system. Method
The first step in achieving a successful VAV air-handling system setup is to thoroughly understand the system as designed. Has the system been designed with diversity in mind or not such that the air-handling unit (AHU) scheduled supply air (SA) airflow equals the summation of all the individual VAV box maximum airflows? If it does not, then the system has been designed with diversity. Not all VAV box zones operate at peak cooling load simultaneously. Some are at maximum airflow, some are partially throttled, and some are fully throttled to minimum airflow. Most VAV systems have been designed with diversity, so it is therefore important to determine what percent diversity has been established by the engineer of record. Once this has been done, the next step is the same for AHUs with or without diversity: All the individual VAV boxes on the system need to be systematically set up for both maximum and minimum airflow.
To accomplish this, set the AHUs down-duct mounted static pressure (SP) sensor/transmitter to an SP sufficient to set up all of the VAV boxes on the system. I usually start with an SP of 1.25 inches water gauge (wg). Once this has been done, I proceed with calibrating and balancing all system VAV boxes, typically starting with maximum airflow. Some VAV system/box controls only have one point of calibration — maximum airflow; however, some have two points of calibration (one at maximum airflow and one at minimum airflow).
Common VAV System Problems
- Down-duct SP is set too high or too low
- SA fan/return air (RA) fan not tracking properly to maintain design minimum outside air (OA) airflow.
- The OA airflow measuring station is not set up correctly.
- The SA fan/RA fan variable frequency drives (VFDs) are not set up correctly.
- Individual VAV terminal boxes are not set up correctly.
- Some VAV boxes are starved for airflow and operating at 100% open damper position (i.e., not controlling).
- Some VAV boxes are throttled to 70% (or less) maximum damper position to maintain the design maximum airflow.
If any of the VAV boxes cannot achieve design maximum airflow set point during this process, the AHU SP set point will need to be increased to a level that enables the VAV box to achieve maximum airflow per the control readout. Once the VAV has been set for the correct maximum airflow per the controls, the next step is to read out and record all downstream SA outlet airflows and utilize the total measured airflow obtained to calibrate the VAV box controller for maximum airflow. If two points of calibration are required, the next step is to drive the VAV box to minimum airflow via the control system, read out and record, and calibrate the VAV box minimum airflow. To complete the proper calibration/balance/setup of the VAV box, it's important to record the calibration factors required by the control system that enabled you to achieve the maximum/minimum airflow set points in the control system that matched the airflow values obtained by direct airflow measurement. This process should next be systematically applied to all VAV boxes served by the AHU.
So as not to waste effort, if the VAV box has a reheat coil, after the VAV box has been calibrated is a good time to measure the coil's heating capacity. If the VAV box has electric reheat, open the heater compartment or the electrical disconnect box at the VAV cabinet for access to the heater wires to obtain the electrical measurements necessary for determining electric coil heating capacity. Some VAV box control systems are wired such that if you have already overridden the heater to “on,” turning the local disconnect “off” will release the override.
Once you have completed your readings, record the heater data/amps/volts. For VAV boxes with electric reheat, this is also a good time to confirm the electric heater will energize at minimum airflow and not be inhibited by the airflow-proving switch that protects the heater. I have found, in some instances, the placement of the airflow-proving switch may need to be changed or the minimum airflow increased to enable the electric heater to energize. If the VAV box has hot water (HW) reheat, and the HW system has already undergone balancing, then the HW coil heating capacity can be obtained. Once all of the aforementioned steps have been completed, it is very important to release all overrides on the VAV box before moving on to the next one. Continue this process until all the VAV boxes have been calibrated, balanced, and tested for heating capacity.
Once VAV box calibration and setup have been completed for all VAV boxes, the next step is to calibrate the OA airflow sensor and/or SA fan/return air (RA) fan airflow offset. This should be done with the AHU operating near design airflow with all VAV boxes operating under control and measured OA airflow reading steady and at, or near, design minimum OA airflow.
Compare the measured OA airflow (by traverse) to the control system airflow display and calibrate the sensor as needed until the two airflow values match. After calibration of OA airflow, and with the AHU operating at design airflow, re-traverse the OA airflow to verify calibration. Once calibration is verified, drive all VAV boxes to minimum airflow (via the control system) to confirm the design minimum OA airflow can still be maintained (per the calibrated OA airflow sensor) with all the VAV boxes operating at minimum airflow. To ensure the repeatability of the control system, drive all the VAV boxes to maximum airflow again and verify the design minimum OA airflow is still correct.
The next step, after OA airflow calibration has been completed, is to drive all VAV boxes to maximum airflow to determine the “worst case scenario” VAV box. This VAV box will be the one with the lowest percentage of design maximum airflow or the highest percentage of damper open per the control system display. Once the worst-case scenario box has been determined, increase the AHU's VFD speed or close the surrounding VAV boxes until you achieve design maximum airflow at a 90% open, per the control system display. In going through this process, it is understood that the worst-case scenario VAV box is operating under control. The next step is to verify what AHU down-duct SP was required to satisfy the worst-case scenario VAV box. This SP value then becomes the AHU SP set point.
Once the AHU SP set point has been established, the next step is to drive all VAV boxes to maximum airflow and allow the AHU system to settle out (reach steady state). Once steady state operation has been achieved, verify that the OA airflow is still at the specified design minimum airflow value. With the AHU operating at maximum airflow, and the OA airflow correct, the RA airflow system can now be balanced. In the last step, only drive all VAV boxes to maximum airflow if the system has no diversity.
After the RA airflow system has been balanced, the next step is to finalize the AHU setup. Prior to final setup, check the condition of the AHU filters (prefilters and final filters, if applicable) and have the installing contractor install a clean set of filters. Set the AHU for maximum airflow and wait for the VAV boxes to settle out. For final airflow setup, if the AHU has been specified with diversity, determine which VAV boxes need to be set at maximum airflow, which need to be at minimum airflow, and set the VAV boxes accordingly, such that the total airflow equals the AHU's scheduled maximum airflow. If maximum airflow is not achieved at this point, adjust the AHU VFD speed to get back to maximum airflow. Please note that if any speed changes are required, the system's SP set point needs to be reevaluated and the minimum outdoor airflow reverified before AHU testing is completed. Measure, read, and record all final AHU airflows (i.e., SA, OA, and RA) and take an SP profile across all unit components. The VAV AHU setup is now complete.
The correct setup of a VAV system is not rocket science, but it does require a deliberate, methodical approach combined with a systematic protocol. This summary has presented such a protocol, and I have used this same protocol to accomplish the successful setup of numerous VAV systems. Feel free to use it, and I would welcome any feedback. It should be understood that the protocol presented here is the most basic and straightforward for setting up a simple VAV system. With today's more sophisticated control systems, more bells and whistles have been added, and many more control strategies now exist, such as a VAV box SP reset. In addition, there are also other energy-saving strategies, such as optimizing the AHU SP set point for minimum airflow, like what was done for maximum airflow.