Fig. 10 - Vav Unoccupied Cooling Temperature Control; Fig. 11 - Compressor Staging For Vav Cooling - York J Series Manual

Unit Sequence of Operation
COOLING DESTAGING
COOLING ON
COOLING OFF
UNOCCUPIED COOLING
-0.5 ˚ F
SPACE TEMPERATURE (F)
FIG. 10 – VAV UNOCCUPIED COOLING
TEMPERATURE CONTROL
• The SAT setpoint for unoccupied cooling is the
VAV Cool High Temperature Setpoint.
Occupied Heating In Standalone Operation
• In this mode, the unit remains shut down, occupied
heating is never used.
Unoccupied Heating In Standalone Operation
• In this mode, the unit remains shut down, occupied
heating is never used.
Occupied Cooling In Standalone Operation
• The programmable parameter VAV with Thermostat
must be set to OFF.
• Space Sensor signal must be unreliable or not de-
tected.
• The SAT setpoint for occupied standalone cooling
is the VAV Cool Low Temperature Setpoint.
Unoccupied Cooling In Standalone Operation
• In this mode, the unit remains shut down, unoc-
cupied cooling is never used.
SAT Control Algorithm in VAV Cooling
The control of compressors to maintain the SAT setpoint
for:
• Economizer Operation when mechanical cooling is
required to maintain the SAT setpoint and when
26
COOLING STAGED ON
+0.5 ˚ F
SETPOINT
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• Non-Economizer Operation or Economizer Opera-
tion when economizer is not suitable is as described
in Figure 12.
The setpoint is as determined for the space temperature
condition and is either the VAV Cooling High Tem-
perature Setpoint or the VAV Cooling Low Temperature
Setpoint.
The SAT will be controlled at the SAT Setpoint +/- VAV
Control Offset around the SAT setpoint.
The compressors will stage ON after the SAT has been
greater than (SAT Setpoint + VAV Control Offset) for
5 minutes.
Compressors will be staged OFF individually when
the SAT drops below the (SAT Setpoint – VAV Control
Offset).
Compressors staged OFF
ON
OFF
Selected SAT Setpoint
-VA V Control
Offset
SUPPLY AIR TEMPERATURE ( ˚ F)
FIG. 11 – COMPRESSOR STAGING FOR VAV
COOLING
Supply Duct Static Pressure Control Algorithm
The Supply Duct Static pressure will be controlled
to the duct static pressure setpoint. An appropriate PI
closed loop control algorithm will be used to control
the supply duct static pressure by modulating the sup-
ply fan VFD.
Occupied VAV Heating / Cooling Control
Algorithm With A Space Sensor
In the occupied mode, the Space Sensor will compare
the space temperature to the programmed "VAV Setpoint
for SAT Reset" and request heating, or cooling, using
the following control algorithm (Figure 12):
FORM 100.50-NOM7 (808)
Compressors staged
+VAV Control
Offset
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JOHNSON CONTROLS