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351579-YTS-B-0608
first stage until the Space temperature reaches the Setpoint and
then it will turn off the entire stage one compressor.
If the unit starts with a demand greater than 2.0 °F, the control will
stage up first stage and wait 5 minutes. The control will continue
operating first stage until the Space temperature reaches the
Setpoint and then it will turn off all the stage one compressors
with a 30 second delay between them. If after 5 minutes the
Space temperature is not moving toward the Setpoint the control
will turn on the first compressor of stage two and wait 5 minutes.
The control will continue to turn on compressors until the Space
Temperature begins to move toward the Setpoint using the 5
minute trending method. The control will turn off all the second
stage compressors when the demand reaches .5 °F with a 30
second delay between them. The control will continue operating
first stage until the Space temperature reaches the Setpoint and
then it will turn off the entire stage one using a 30 second delay
between compressors if more than one is on.
VAV OPERATION
The paragraphs below identifies control modes of VAV
operation and provides 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, by control algorithm detecting zone
cooling, or heating demand. The "occupied" vs. "unoccupied"
modes are controlled by the internal "Occupied/unoccupied"
clock and the tables. If a thermostat is connected in addition to
the space sensor, it takes priority over the space sensor.
VAV OPERATION WITH A THERMOSTAT
The thermostat is placed in a selected, representative zone of a
VAV system and is used to reset SAT and cycle the VAV unit in
cooling and to control heating. It is expected that the thermostat
provide Yx outputs to control cooling and Wx output to control
heating. The thermostat is expected to provide an output to
control occupied/unoccupied mode (a contact closure that
supplies in the occupied mode 24 VAC to the "OCC" Input).
As the thermostat has a capability to turn off the unit in occupied
cooling mode, it is important to place it in a zone that is
representative of the remaining zones, or in a zone with the
highest occupied cooling demand.
The VAV zones are controlled by their own zone thermostats to a
temperature setpoint set independently of the setpoints of the
"system control" thermostat; the setting of the occupied cooling
setpoint on the "system control" thermostat should at least
roughly correlate to the zone temperature setpoint. For example,
the occupied cooling setpoint may be set at, or below the lowest
zone temperature setpoint selectable on the zone thermostat.
The setpoint setting should prevent a possibility of cooling turned
off while other zone(s) in the system still have a cooling demand.
VAV OPERATION WITH A ZONE SENSOR
The Space Sensor is placed in a selected, representative zone
of a VAV system. It is used to reset SAT in cooling and to control
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occupied heating based on comparison of zone temperature to
a programmed "VAV SAT Reset Setpoint". However, since the
zone is controlled by its own zone thermostat to a temperature
setpoint set independently of the VAV SAT Reset Setpoint, the
setting of the VAV SAT Reset Setpoint should at least roughly
correlate to the zone temperature setpoint.
For example, the VAV SAT Reset Setpoint may be set at, or
below the lowest zone temperature setpoint selectable on the
zone thermostat, in order to prevent a possibility of insufficient
cooling while other zone(s) in the system still have a high
cooling demand. In unoccupied modes, the Space Sensor
controls the unit based on unoccupied heating and cooling
setpoints programmed in the unit controller.
The use of a Space Sensor is enabled by setting a
programmable parameter "VAV Operation with Thermostat".
When the Space Sensor is enabled, the control self-configures
to a VAV operation with a Space Sensor if the sensor is
available, and to a Return Air Temperature operation if the
sensor is not available.
STANDALONE VAV OPERATION
In standalone operation the control unit receives an
occupied/unoccupied command from an external source (such
as a contact closure on the space sensor input). In occupied
cooling mode, it controls to the Low SAT setpoint. In
unoccupied mode, the unit shuts down. In unoccupied mode, an
external (network) command may be provided to start a
Morning Warm-up cycle. The MWU cycle (if enabled) will also
start upon a transition from Unoccupied to Occupied mode.
SUPPLY FAN OFF DELAY
Uses fan off delay.
OCCUPIED HEATING WITH A THERMOSTAT
When the unit enters the Occupied mode heating is started by
the thermostat turning ON its W1 output. In the Occupied mode
the thermostat will control to its occupied heating setpoint. The
thermostat's occupied heating setpoint should be set well below
the zone thermostat's cooling setpoint, to represent a heating
setpoint typical for all system zones.
In VAV heating, full heating (all heating stages ON) occurs when
W1=ON, and no heating (all heating stages OFF) occurs when
W1=OFF (note that in this case, the control will use all available
heating stages, regardless of setting of the programmable
parameter "Heat Stages").
The supply fan operates at 100% during heating. Depending on
the control setting, the fan will cycle On/Off with the W1 output,
or will remain On. On startup, the fan speed is ramped up, or
the inlet vanes ramped open (a "soft start").
During occupied heating, the economizer damper remains at
minimum position and all VAV boxes are fully open - box heat
relays are energized from Heat output of the controller. If
Hydronic heat option is used, the Heat output of the controller is
also actuated during heating, the SAT is controlled to the 2nd
stage hydronic heat setpoint.
Johnson Controls Unitary Products
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