YORK INTERNATIONAL
60
Sequence of Operation
FORM 100.50-NOM1 (604)
COOLING DESTAGING
COOLING STAGED ON
UNOCCUPIED COOLING
SETPOINT
SPACE TEMPERATURE (F)
COOLING ON
COOLING OFF
-0.5˚F
+0.5˚F
FIG. 28 –
VAV UNOCCUPIED COOLING
TEMPERATURE CONTROL
Occupied Heating In Standalone Operation
The following conditions must be met to initiate heat-
ing operation:
• A RAT (Return Air Sensor) must be installed in the
unit and the
"RAT Sensor Enable"
must be ON.
• The RAT must be 2 degrees less than the
"Warm-up
RAT"
setting.
Unoccupied Heating In Standalone Op er a tion
• In this mode, the unit remains shut down, unoc-
cu pied heat ing is never used.
Occupied Cooling In Standalone Op er a tion
• The programmable parameter
VAV with Ther mo stat
must be set to OFF.
• Space Sensor signal must be un re li able or not de-
tect ed.
• The SAT setpoint for occupied standalone cool ing
when the economizer is active is
"VAV Cool High
Temp"
and when the economizer is not active the
SAT setpoint is
"VAV Cool Low Temp"
.
Unoccupied Cooling In Standalone Op er a tion
• In this mode, the unit remains shut down, un oc -
cu pied cooling is never used.
SAT Control Algorithm in VAV Cooling
This section describes the control of compressors to
maintain the SAT setpoint for:
• Economizer Operation when me chan i cal cool ing
is re quired to maintain the SAT setpoint
or
• Non-Economizer Operation or Econ o miz er Op er a-
tion when economizer is not suitable is as de scribed
in Figure 29.
LD06559
LD06560
Compressors staged OFF
ON
OFF
Compressors staged ON
-VAV Control
Offset
Selected SAT SeVAV Control
Offset
SUPPLY AIR TEMPERATURE (˚F)
FIG. 29 –
COMPRESSOR STAGING FOR VAV
COOLING
Sequence of Operation
The setpoint is as determined for the space tem per a ture
condition and is either the
"VAV Cool High Tem p"
or
the
"VAV Cool Low Tem p"
setting.
The SAT will be controlled at the above set/-
"VAV Con trol Offset"
.
The compressors will stage ON after the SAT has been
greater than (SAT Se VAV Control Offset) for
5 minutes.
Compressors will be staged OFF individually when
the SAT drops below the (SAT Setpoint – VAV Control
Offset).
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 con trol al go rithm will be used to control
the supply duct static pres sure by mod u lat ing 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 pro grammed
“VAV SP for
SAT Re set”
and request heating, or cooling, using the
fol low ing control al go rithm (Figure 30):
• The transition from cooling to oc cu pied heat ing
occurs at 2°F below the
“VAV SP for SAT Re set”
• The reverse transition at 1.5°F below the
“VAV SP
for SAT reset”
. There is no “satis
Þ
ed”, or “idle” state
of the unit be tween cooling and oc cu pied heating.
