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mizer damper and using more outdoor air. However, if the
economizer algorithm can not prevent the SAT from
increasing above the top control band limit by closing the
economizer damper to its programmed minimum position,
one or more compressors may need to be turned on. Simi-
larly, if the economizer algorithm can not prevent the SAT
from dropping below the bottom control band limit by
opening the economizer damper, one or more heating
stages may need to be turned on.
• If the OAT is within the SAT control band, i.e. between the
programmed "Comfort Ventilation Upper Setpoint" and
"Comfort Ventilation Lower Setpoint", the economizer
damper is driven to the fully open position. In this case, no
other control action needs to be taken to maintain the SAT
within the specified control band.
STAGING CONTROL DURING COMFORT VENTILATION
The economizer control alone may not be able to maintain the
SAT within the specified control band. A separate staging
control algorithm supplements the economizer control and will
stage heating, or mechanical cooling as necessary. If the SAT
increases above the "Comfort Ventilation Upper Setpoint" for
more than 5 minutes, the control will destage a heating stage (if
any heating stages running), or add a compressor. The control
will repeat this process every 5 minutes until the SAT gets back
to within the control band.
If the SAT drops below the "Comfort Ventilation Low Supply Air
Setpoint" for more than 5 minutes, the control will destage a
compressor (if any compressors running), or add a heating
stage. The control will repeat this every 5 minutes until the SAT
gets back to within the control band. As the heating stages, or
compressors are staged up, or destaged, the economizer
controls continues using the economizer damper to "trim" the
effect of the staging and to maintain the SAT as near the middle
of the SAT control band as possible.
If the unit is using hydronic heat, the control modulates the
water valve to maintain SAT at the programmed "Comfort
Ventilation Low Supply Air Setpoint". At this setpoint, the
hydronic heat control does not conflict with the economizer
control, and will prevent SAT from dropping outside of the
specified control band when the economizer can no longer
maintain the SAT at its middle-of-the-band setpoint.
HYDRONIC HEAT
The Hydronic Heat option requires relocating the SAT sensor
downstream of the heating coil.
During hydronic heating, the SAT control to the selected
"Hydronic Heat First Stage SAT setpoint", or "Hydronic Heat
Second Stage SAT Setpoint" is performed using a Proportional /
Integral control algorithm. The control algorithm includes
special provisions (sometimes referred to as a "bumpless
transfer" feature) that allow it to resume control of SAT as fast
as possible when it is re-activated during the ON / OFF cycling
of the unit (cycling between a satisfied and a heating state
under control of a zone thermostat, or zone sensor). In its OFF-
state, the control algorithm continues monitoring the SAT and
Johnson Controls Unitary Products
performs calculations necessary to set its output, that controls
the hot water valve, to have the same value on starting the ON-
cycle as it had when it went into the OFF-cycle (in spite of some
decrease in the SAT during the OFF-cycle).
The "bumpless transfer" feature of the hydronic heat control
algorithm essentially acts similar to a "memory" that holds the
algorithm output during the OFF-cycles. This "memory" is reset
only on unit shutdown. The effect of this "memory" can be
observed on the action of the hot water valve. After unit startup
(during the first ON-cycle of the hydronic heat), there is no
"memorized" output value and the valve moves relatively slowly
into a control position required to maintain the SAT setpoint. On
subsequent ON cycles, the valve moves into a control position
much faster as dictated by the output value stored in control
algorithm "memory".
COOLING LOCKOUT ON OAT
This is the Outside Air Temperature Setpoint that the control
uses to lock out mechanical cooling when the OAT is below this
setpoint. There is a one-degree hysteresis on each side of the
setpoint. Adjustable from 0 °F to 100 °F, the default is 45 °F.
The change to 0 °F applies only to compressors # 1 and # 2
when head pressure control is installed. Cooling Lockout on
OAT may occur while the control is in an Economizer mode and
there is a demand for compressors.
WATER COIL FREEZE STAT (FSI)
This option is used only on rooftop units with hydronic heat
(Hydronic heat option is turned ON). When the FSI terminal
sees 24 VAC, the control will open the Hot Water valve to
100%. The control will continue to drive the valve to 100% until
five minutes after the switch has closed. If the control is
operating the Fan, it will close the Economizer fully until the
freeze condition is over. If the fan is off and the RAT drops
below 40 °F, the Hot Water Valve will turn on 100%. This control
sequence takes place regardless of the supply fan status (it is
expected to be used/ needed mostly in situations when the
supply fan is not running). A Hot Water Coil Freeze Alarm is
generated. The alarm is written to the Error History Buffer. In
networked applications, the alarm flag is readable by the
network.
After the valve override sequence described above is
completed, the valve will revert to normal operation and the
alarm is automatically reset. The same sequence is repeated
every time the Hot Water Coil Freeze Switch opens (infinite
retries).
CV OPERATION
The paragraphs below identify control modes of CV operation
and provide an overview of control methods in all modes. The
modes include Occupied and Unoccupied Heating and Cooling,
and differ depending on the method used to control zone
temperature (thermostat, or a space sensor). The heating vs.
cooling modes are entered under control of the thermostat, or,
in case of space sensor, control algorithm detecting zone
cooling or heating demand. The "occupied" vs. "unoccupied"
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