TAB Journal Summer 2019 - 8

FIELD

VS
FACTORY

Casey McElwee, TBE
Professional Systems Analysis, Inc.

"

We were directed to use the manufacturer's method. As professionals, we could
not knowingly set up a building incorrectly. We discussed the issue directly with the
mechanical contractor and told them how we planned on balancing these boxes.

W

e recently did a high-rise project that used fan-powered
boxes. These boxes were series boxes that were served
by a dedicated outside air unit. The speed control
for these particular fan-powered boxes was controlled by a
voltage input from the building automation system. The project
specified that the manufacturer's voltage/cubic feet per minute
(CFM) table was to be used to set the primary airflow. After
setting the voltage to get the required fan CFM, we were then
directed to put a piece of cardboard with a hole in it over the
return opening where the filter usually sits. We were to then
hang a lightweight string over the hole in the cardboard, and by
adjusting the primary air, make it so the string was neutral to the
opening. At this point the primary airflow would be equal to the
fan airflow and at design.
We tried this technique on multiple boxes and each time we
checked the primary air with a pitot tube traverse on the inlet
of the primary air. We found that some boxes were perfect and
spot-on while others were found to be as much as 50 percent
higher than what the chart was saying it should be. We
presented this data to all of the project leads and explained

8

"

that the way they wanted us to test these boxes was not correct
and would not deliver the result they expected. Furthermore,
the units would also waste a lot of energy and not provide
optimal heating/cooling when needed. The system may work,
but it would not be at optimum performance.
After debating the issue over the phone multiple times with
multiple people, a meeting was scheduled to test these boxes
in front of the appropriate personnel to demonstrate that the
method specified to test the variable air volume(VAV) boxes
would yield results with errors. We spent a full day retesting
these boxes. Everyone agreed that something was wrong.
The general contractor suggested that we might be testing a
bad sample. He asked that we take one of the boxes that was
spot-on, switch the program card with the worst box that had
issues and retest. A day or two later the mechanical and control
contractor had the card switched on the box that was having
issues. So now we had a box that was having issues but with a
program card that was spot-on when it was in a different box.
We retested the troubled box and - what a surprise - we

TAB Journal Summer 2019



TAB Journal Summer 2019

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

TAB Journal Summer 2019 - Cover1
TAB Journal Summer 2019 - Cover2
TAB Journal Summer 2019 - 1
TAB Journal Summer 2019 - 2
TAB Journal Summer 2019 - 3
TAB Journal Summer 2019 - 4
TAB Journal Summer 2019 - 5
TAB Journal Summer 2019 - 6
TAB Journal Summer 2019 - 7
TAB Journal Summer 2019 - 8
TAB Journal Summer 2019 - 9
TAB Journal Summer 2019 - 10
TAB Journal Summer 2019 - 11
TAB Journal Summer 2019 - 12
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TAB Journal Summer 2019 - 15
TAB Journal Summer 2019 - 16
TAB Journal Summer 2019 - 17
TAB Journal Summer 2019 - 18
TAB Journal Summer 2019 - 19
TAB Journal Summer 2019 - 20
TAB Journal Summer 2019 - Cover3
TAB Journal Summer 2019 - Cover4
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