ASHRAE Journal - April 2021 - 19

COLUMN SOLVING PROBLEMS

Too often Benson said he sees probe-style temperature
sensors selected for air-handling units' (AHU) mixed
air sections. Systems don't always control with this, but
if you see a need for active heating or cooling from the
AHU when an economizer should be able to do all the
work, this may be part of the issue. Stratiﬁcation is likely
to occur and mixing sensors can be a much better ﬁt
for control logic to maximize efﬁciency for climates in
which outside air temperature widely varies.
Outdoor air temperature sensors are often installed
where they are exposed to direct sunlight or where
nearby thermal mass, such as concrete slabs, can create
microclimates, according to Benson. This can cause the
temperature to swing by several degrees. The locations
can reduce the number of hours when outdoor air is
used and unnecessarily increase loads, he said.
" A comparison between the values of site and local
weather sensors can sometimes identify issues or conﬁrm where microclimates are notable. When referenced
by an AHU, consider placing these sensors within outdoor air intakes so you have a more accurate representation of the actual air temperature available for economizing, " Benson said.

Overloaded Subnetworks
For Joseph Kilcoyne, P.E., Member ASHRAE, vice chair
of TC 1.4, most performance issues are due to overloaded
subnetworks. In existing facilities-particularly larger
buildings or campuses-new controllers are frequently
added to existing networks, rather than running new
communication cable back to a supervisory controller
in an IT closet, he said. Every manufacturer has different recommendations on the maximum number of ﬁeld
controllers to be located on each subnetwork. Usually,
fewer than 50 controllers is acceptable, while fewer than
30 is best practice, according to Kilcoyne.
" We've found individual subnetworks containing over
100 controllers. It took over ﬁve minutes for information
on this network to pass from the controller to the main
BAS server, " said Kilcoyne, a principal at SC Engineers.
To remedy overloaded subnetworks, Kilcoyne advises
adding additional supervisory controllers. " If you've
got an overloaded subnetwork, the only fundamental
ﬁx is to split it into smaller subnetworks. You will have
to add supervisory controller(s) somewhere, and just
make sure you don't exceed the length limitation on the
subnetwork cabling, " he said.

Most challenges Kilcoyne encounters are in the supervisory level-the network supervisors that host one or
more serial subnetworks of ﬁeld controllers and report
directly to the BAS server usually over an IP connection.
These may also act as an advanced application controller
for a plant or AHU, he said.
" The challenges are found here because these controllers are frequently the bottlenecks for communication.
All communication points and trend points pass through
these supervisory controllers, " Kilcoyne said. " They usually have limited memory and can easily become overwhelmed. These controllers are also where most of the
network conﬁguration settings are programmed. "

Accurate As-Builts and Sequences of Operations
Del Monaco, an associate principal at P2S Inc., said the
most common issues he sees when diagnosing issues
in an existing BAS are having accurate as-builts and
sequences of operations.
" Oftentimes, the system has evolved over time, and
operators have made changes to the sequences and
setpoints that deviate from the original design intent, "
he said. " This is usually in response to occupant complaints, or a system does not appear to be working as
intended. These changes are made, and there is no formal record of it anywhere. "
He suggested ﬁnding any available as-builts and
sequences. Because the system may have been modiﬁed,
Del Monaco suggests engineers compare the general network architecture from the as-builts to the installed conditions via ﬁeld surveys, which can give the engineers a
solid foundation to design around moving forward.
He said engineers should understand how they want to
design the system, what parts and pieces they will need
to upgrade and modify on the existing system, and know
the ramiﬁcations to the upstream and downstream
equipment if they replace components/controllers
in-between.
" I would recommend an assessment of the equipment prior to modiﬁcations. Are the sensors, devices
and controllers operating? Can we identify any failures
in the current system and operation? I've worked on
renovations where the owner intended to keep some of
the components, and when we went to test them at the
end of construction, they were not functioning. These
resulted in change orders that would have been part of
the base bid, " he said.
APRI L 2021

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ASHRAE Journal - April 2021

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