TAB Journal Summer 2019 - 18

So as not to waste effort, I have found that if the VAV box
has a reheat coil, after the VAV box has been calibrated is a
good time to measure the coil 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 confi rm that 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 that 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 that you release all
overrides on the VAV box, before moving on to the next VAV
box. Continue this process until all the VAV boxes have been
calibrated, balanced, and tested for heating capacity.
Once VAV box calibration and setup has been completed for
all VAV boxes, the next step is to calibrate the OA airflow
sensor, and/or SA fan/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

"

Correct setup of a VAV
system is not rocket
science, but it does
require a deliberate,
methodical approach
combined with a
systematic protocol to
achieve correct results.

"

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 confi rm that design minimum OA airflow can still
be maintained (per the calibrated OA airflow sensor) with
all VAV boxes operating at minimum airflow. To ensure the
repeatability of the control system, next drive all VAV boxes
to maximum airflow again, and verify that design minimum
OA airflow is still correct.

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TAB Journal Summer 2019

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