ASHRAE Journal - June 2020 - 67

COLUMN ENGINEER'S NOTEBOOK

what is going on. Once you have identiﬁed and corFIGURE 6 The proof of operation DP switches serving the pumps will not provide
accurate data, as discussed in the article under data integrity.
rected a problem like an unstable control loop, you
can slow the trends down for data archiving purposes.
But having the ability to sample fast for initial and
ongoing commissioning is a huge beneﬁt, so don't
give it up by accepting a network architecture that
can't deliver it.
Similarly, the controllers and operator work station
Pump 2
Pump 1
DP
DP
(OWS) need to have the memory required to store data
for both the short- and long term. This capability can be
quite important for a project seeking incentives from a
utility, or a sustainability program that requires maintenance and veriﬁcation. A system that has not been
designed for this may not deliver, compromising the
owner's ability to perform the meaFIGURE 7 Mixed air plenum temperature profile vs. the temperature at the sensor location. (For more details and
surement and veriﬁcation required
a video that shows how the temperature pattern varied over time, visit https://tinyurl.com/yd73sr7b, "Economizer
Stratification." ) Note that the cooling coil was off at the time captured by this image.
to claim the incentive offered by the
utility program.
Entering Filter Temps. (°F) Leaving CHW Coil Temps. (°F)
For systems facing these chal57.6
54.9 52.9 52.5 55.7
7
6
2
62.6
56.5
1 2 3 4
lenges, data loggers can provide
54.9 52.9 52.5 55.7
60.0
1
a solution that bridges the gap
55.8 63.3 60.9 64.6
57.8
64.4
5
6
7
8
5
4
3
by providing the data granular55.8 63.3 60.9 64.6
62.6
ity necessary for a retrocommisMixed Air Plenum Sensor
Chilled Water Coil Sensor
Average Values from Monday 04/20/15 08:30 a.m. to
Monday 04/20/15 08:30 a.m.
Locations; the Grid is the
Locations; the Orientation
sioning project, a troubleshooting
Maximum to Minimum Temperature Span = 12.1°F;
Filters and Orientation is
is Facing Toward the Fan
Average Outdoor Temperature = 52.2°F; Average Mixed Air
Facing Toward the Fan (Air
(Air Coming from Behind
effort or even a semipermanent
Temperature = 57.6°F.
Coming from Behind You);
You) and the Sensors are on
installation that gets you by
The Key Below Correlates the Temperature Gradient Colors
Sensors are on the Face of
a Grid Downstream of the
with the temperatures it Represents.
the Filters
Leaving Face of the Coil
until a controller upgrade can be
52.2 54.0 55.5 57.0 58.5 60.0 61.6 63.1 64.6
implemented.

Data Integrity
It's easy to forget that what you are seeing on the OWS
may not actually be what is going on out there. If the
sensing network has not been thoroughly commissioned, the OWS graphics might be a fairy tale.
For instance data reported by a differential pressure
switch that is being used as proof of operation for one
of two parallel pumps would provide misleading data if
the high port of the switch was connected to the header
downstream of the check valve associated with the pump
it served instead of upstream of the check valve (Figure 6).
Consider what happens if both pumps are running
and Pump 1 is shut down. When this happens, Pump
1's check valve will close due to the loss of pressure
it was creating and the pressure created by Pump 2.
Because the differential pressure (DP) switch is sensing the pressure on the header side of the check valve,
it will still see a pressure difference because Pump 2

is operating and pressurizing the header. Thus, the
switch will report Pump 1 as running, even though it
isn't.
If the DP switch was connected on the pump side of
the check valve (between the pump discharge and the
check valve), when Pump 1 shuts down (and the check
valve closes), the pressure across it will equalize and the
switch will correctly report that Pump 1 is off.
The mixed air temperature reported by a single point
sensor vs. an averaging sensor is another example of the
potential for misleading information. Because mixed air
plenums can be quite stratiﬁed, it is unlikely the data
from a single point sensor will be at all representative
of the actual mixed air conditions. It is not unusual for
economizer diagnostics to incorrectly report economizer
functionality due to this issue.
Figure 7 illustrates the phenomenon for an economizer
equipped air-handling unit in a facility in Monterey, Calif.
J U N E 2020

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ASHRAE Journal - June 2020

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