York MILLENNIUM Y22, Installation Manual

Page: 85 / 140

102777-YIM-E-0206
Unitary Products Group 85
of zone temperature to a programmed "VAV SAT Reset Set-
point". The "VAV SAT Reset Setpoint" would typically be set
well below the "Unoccupied Cooling Setpoint", causing the
unoccupied cooling to typically use a Low SAT Setpoint.
There are two different SAT control algorithms, one used
when free cooling is available (economizer operation) and the
other one for mechanical cooling only. These two SAT control
algorithms are described in the “Occupied Cooling in Standa-
lone Operation” section below. Since the economizer is not
active in Unoccupied mode, this will be restricted to mechani-
cal cooling.
UNOCCUPIED HEATING IN STANDALONE OPERATION
Unoccupied heating is never used in standalone operation.
OCCUPIED COOLING IN STANDALONE OPERATION
The unit controls cooling to a programmed Low SAT setpoint
("VAV Lower Cooling SAT Setpoint") during the occupied
mode. There are two different SAT control algorithms, one
used when free cooling is available (economizer operation)
and the other one for mechanical cooling only. First, when
free cooling is available ("economizer suitable"), the SAT con-
trol algorithm is as described in the Economizer Operation
section later in this document, except the programmed Econ-
omizer First Stage Setpoint and Economizer Second Stage
Setpoint are replaced by the programmed VAV Upper Cooling
SAT Setpoint and VAV Lower Cooling SAT Setpoint.
When free cooling is not available, the SAT control is per-
formed by the following control algorithm: Minimum compres-
sor off times (ASCD) of 5 minutes and minimum on times of
one to ten minutes [default is three] are maintained and the
cycling rate of the compressors will not exceed maximum 6
cycles/hour. The SAT is controlled in a ±5
°
F band around the
active SAT setpoint. A compressor is allowed to turn on only if
the current SAT minus the Predicted SAT Drop is more than
5
°
F.
SUPPLY DUCT PRESSURE CONTROL ALGORITHM
The Supply Duct pressure is controlled to a specified setpoint
(see paragraph Duct Pressure Setpoint, section Option Oper-
ation earlier in this document). An appropriate closed loop
control PI algorithm is used to control the supply duct static
pressure by modulating Inlet Guide Vanes, or the supply fan
VFD.
MORNING WARM UP / VAV OCCUPIED HEATING
CONTROL ALGORITHM
STARTING MORNING WARM UP
Non-networked systems: The MWU (Morning Warm Up)
option is enabled by setting the option parameter and estab-
lishing a start and end Occupied time period in the control’s
Occupied 1 weekly schedule.
MWU (Morning Warm Up) is initiated upon all transitions from
Unoccupied to Occupied mode. The system clock is used to
identify, for the Morning Warm Up, a 1 hour period before
actual occupancy begins (also see Intelli-Start). If the MWU
option is disabled, the occupancies should be scheduled at
their regular time.
An Occupied Override signal will initiate a warm up of up to
two hours (the setpoint value of the Unoccupied Override
Time Period). It is important to note that the unit is still operat-
ing in an Unoccupied mode for the first hour after the occu-
pied Override input is turned on. Reference the Weekly and
Holiday Schedule, Tables 44 and 45.
MORNING WARM UP / VAV OCCUPIED HEATING FUNC-
TION
Morning Warm Up controls the RAT to a preprogrammed
Morning Warm Up RAT Setpoint. An appropriate closed loop
control algorithm is used to control heat stages to accomplish
this. The algorithm uses a dead band (i.e. control RAT
between the MWU RAT Setpoint and two degrees below the
setpoint).
FIGURE 30 - SAT CONTROL BAND
Stage Up If SAT is Above Setpoint
By + 5.0
0
F, or More, For 5 Minutes
Active SAT Setpoint
Stage Down If SAT is Above Setpoint
By - 5.0
0
F, or More, For 5 Minutes
- 5.0
0
F
+ 5.0
0
F