TAB Journal Summer 2019 - 17

box maximum airflows? If it does not, then the system has
been designed with diversity. Not all VAV box zones are at
peak cooling load simultaneously, but some are at maximum
airflow. Some are partially throttled, and some are fully
throttled to minimum airflow. It has been my experience that
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 setup for both maximum
and minimum airflow.
To accomplish this step, you will need to first set the AHU's
down-duct mounted SP sensor/transmitter to a SP sufficient to
setup all of the VAV boxes on the system. I usually start with a
SP of 1.25 in. water gauge. Once this has been done, I proceed
with calibrating and balancing all system VAV boxes, typically
starting with maximum airflow. Some VAV system/VAV 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).
If any of the VAV boxes cannot achieve design maximum
airflow setpoint during this process, the AHU SP setpoint 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 readout 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 then the
next step is to drive the VAV box to minimum airflow via
the control system, readout and record, and calibrate VAV
box minimum airflow. To complete the proper calibration/
balance/setup of the VAV box, it is important to record the
calibration factors required by the control system that enabled
you to achieve the maximum/minimum airflow setpoints 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.

TAB Journal Summer 2019

COMMON VAV
SYSTEM PROBLEMS:
⊲ Down-duct static pressure (SP)
set too high.
⊲ Down-SP set too low.
⊲ Supply air (SA) fan/return air
(RA) fan not tracking properly to
maintain design minimum outside
air (OA) airflow.
⊲ OA airflow measuring station not
setup correctly.
⊲ SA fan/RA fan variable frequency
drives (VFDs) not setup correctly.
⊲ Individual VAV terminal boxes not
setup correctly.
⊲ Some VAV boxes are starved
for airflow and operating at 100
percent open damper position
(i.e. not controlling).
⊲ Some VAV boxes are throttled
to 70 percent (or less) maximum
damper position to maintain
design maximum airflow.

17



TAB Journal Summer 2019

Table of Contents for the Digital Edition of TAB Journal Summer 2019

TAB Journal Summer 2019 - Cover1
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TAB Journal Summer 2019 - Cover3
TAB Journal Summer 2019 - Cover4
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