Vav Operation; Vav Operation With A Thermostat; Vav Operation With A Zone Sensor; Standalone Vav Operation - York MILLENNIUM Y22 Installation Manual

102777-YIM-E-0206
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 Set-
point the control will turn on the first compressor of stage two
and wait 5 minutes. The control will continue to turn on com-
pressors 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 compres-
sors if more than one is on.

VAV OPERATION

The paragraphs below identifies control modes of VAV opera-
tion and provides an overview of control methods in all
modes. The modes include Occupied and Unoccupied Heat-
ing 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 algo-
rithm detecting zone cooling, or heating demand. The "occu-
pied" 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 out-
put 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 occu-
pied 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 possi-
bility of cooling turned off while other zone(s) in the system
still have a cooling demand.
Unitary Products Group

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 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 ther-
mostat 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 temper-
ature 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 insuffi-
cient cooling while other zone(s) in the system still have a
high cooling demand. In unoccupied modes, the Space Sen-
sor controls the unit based on unoccupied heating and cool-
ing setpoints programmed in the unit controller.
The use of a Space Sensor is enabled by setting a program-
mable 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 occu-
pied/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 unoccu-
pied 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 set-
point. The thermostat's occupied heating setpoint should be
set well below the zone thermostat's cooling setpoint, to rep-
resent 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").
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