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Sequence of Operation
HEAT REJECTION — When the system pumps are operat-
ing and there is flow to the system, heat rejection control
compares the system supply water temperature to the high set
point in the system loop temperature set points.
If the system supply water temperature approaches the high
set point, the controller commands the heat rejection outputs to
hold the system supply water temperature as close as possible
to the high set point.
If the system return water temperature drops below the
system high water temperature set point minus a hysteresis
value the heat rejection, outputs are disabled.
When the cooling tower output control type is set to
“variable,” the heat rejection outputs are calculated using a PID
calculation based on the system high temperature water set
point. Both outputs will modulate together.
When the heat rejection control type is set to “staged,” the
outputs will be modulated in a lead/lag configuration. The
modulating signal of the lead output will control a transducer
of two or four stages.
When the lead output is at 100% and the set point is not
being maintained, the lag output will be modulated to produce
staging from another two or four stage output transducer.
The value output by each analog output is determined by a
PID calculation of leaving water temperature versus the system
high leaving water temperature set point.
In the event that the system high water temperature set point
cannot be maintained and is exceeded by a defined Hysteresis,
the heat rejection outputs will go to maximum output, the
system water source heat pumps will stop cooling, and the
system pumps will circulate water until the system tempera-
tures are returned to the normal range. System cooling will
then be allowed to continue.
HEAT ADDITION — When the system pumps are operating
and there is flow to the system, boiler control compares the
system supply water temperature to the low set point in the
system loop temperature set points.
If the system supply water temperature approaches the low
set point, then the controller commands the boiler outputs to
hold the system supply water temperature as close as possible
to the low set point.
If the system return water temperature rises above the
system low water temperature set point plus a Hysteresis value,
the boiler outputs are disabled.
When the boiler output control type is “variable,” the boiler
outputs will be calculated using a PID calculation based on the
system low temperature water set point. Both outputs will
modulate together.
When the boiler control type is set to “staged,” the outputs
are modulated in a lead lag configuration. The modulating
signal of the lead output controls a transducer (two or four
stages).
When the lead output is at 100% and the set point is not
being maintained, the lag output modulates to produce staging
from another two or four stage output transducer.
The value output by each analog output is determined by a
PID calculation of leaving water temperature versus the system
low leaving water temperature set point.
In the event that the system low water temperature set point
cannot be maintained and is exceeded by a defined Hysteresis,
the boiler outputs will go to maximum output, the system water
source heat pumps will stop heating, and the system pumps
will circulate water until the system temperatures are returned
to the normal range. System heating will then be allowed to
continue.
WATER CIRCULATION — When operational mode is de-
termined by one of the 18 occupancy schedules, one of the 18
network or global schedules, or occupancy overrides, then the
system pumps are put in operational mode. Also, if one of the
system enable outputs is forced on, then the system pumps are
put in operational mode.
If the remote occupancy input is detected, then the pumps
are put in operational mode and all 18 schedules overridden to
on. This energizes all 18 system enable outputs.
The pump control type is configured as either variable flow
or constant flow. When the pumps are in operational mode, the
pump speed output values are determined in one of two ways:
1. In constant flow mode, the desired output (one or two) is
ramped to 100% to turn the pumps on in a lead/lag
fashion, based on run time.
2. In variable flow mode, both outputs are determined by a
Proportional Integral Derivative (PID) calculation, which
controls the system pressure set point.
Whenever the runtime of the lead pump exceeds the
runtime of the lag pump by 50 hours or more, then the pump
with the lesser runtime will become the lead pump.
If the lead pump output is at 100% and the loop water flow
input detects no flow, then the lag pump will be put to 100% to
start the lag pump. Both outputs will stay at 100% as an indica-
tor that the lead pump did not start.
At each subsequent start-up, the lead pump will be tried
again. If both pumps are commanded to 100% and no flow is
detected, then the pumps will be forced off. To restart the sys-
tem, a pump must be energized manually or the flow switch
must be overridden to indicate flow. The lead pump will start if
there is a demand for system circulation.
In the event that the pumps have been commanded to
provide water flow but either the flow switch does not indicate
flow or the differential pressure sensor does not indicate pres-
sure is present, then the controller assumes that the pumps have
failed and commands the water source heat pumps to turn off
cooling or heat. The system fans will continue to circulate the
air.
In the event of a system shutdown command, the controller
commands the water source heat pumps to turn off all cooling
and heat sources and circulating fans.