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ECONOMIZER — The economizer control loop will be de-
layed 2 minutes after the supply fan is turned ON, to allow sys-
tem and temperatures to stabilize before starting control. When
coming out of STANDBY or Heating mode, a 4-minute delay
will occur before the economizer damper is controlled. During
this delay, damper position is limited to CLOSED or MINI-
MUM position (depending on current unit occupancy status).
If fan status is OFF, the outdoor air dampers will remain
closed (return air dampers will be open). If fan status is ON,
the outdoor air dampers will normally be at minimum damper
position.
Economizer operation is permitted if the system is not in
Heating mode, if outdoor air enthalpy (via switch or humidity
differential) is acceptable, and if outdoor-air temperature is less
than space temperature
If economizer operation is permitted, the economizer con-
trol loop checks for Cooling System operation. If ON, the out-
door air dampers will be driven to maximum position.
If cooling is not on, for VAV units, the economizer will
modulate to satisfy the supply air set point.
If cooling is not on, for CV units, the economizer will mod-
ulate to satisfy the space temperature set point.
If Economizer operation is not permitted, the outdoor air
dampers will be driven to minimum position (during Occupied
period) or closed (during Unoccupied period).
For VAV units, Economizer operation is not permitted when
Occupied Heating is enabled and the Return Air Temperature
is LESS THAN (OHSP + 1º F).
COOLING (All Units) — The controls try to control the
supply-air temperature (SAT) to the value specified by the
supply-air temperature set point by cycling the compressors
and the unloader(s). Both the supply- and return-air tempera-
ture sensors are used to adjust the cycling deadband to match
the actual load. The control system provides cooling capacity
control of cooling stages to maintain supply-air temperature
(VAV) or space temperature (CV) to an occupied or unoccu-
pied set point. Automatic lead-lag circuit switching occurs (if
configured) to equalize run times per circuit for increased total
service life. The compressor to start first is changed every time
stage equals zero.
NOTE: Automatic lead/lag should be disabled if optional hot
gas bypass is employed because the unit only contains hot gas
bypass on one circuit.
The VAV control system sequence uses the modified
supply-air set point (MSAT = supply-air set point + reset value)
as the supply-air temperature required to satisfy conditions
(submaster reference value [CSSR]) and outputs this value to
the submaster loop.
The submaster loop uses the modified supply-air set point
compared to the actual supply-air temperature to determine the
required number of capacity stages to satisfy the load. The log-
ic for determining when to add or subtract a stage is a time-
based integration of the deviation from the set point plus the
rate of change of the supply-air temperature.
The CV control system sequence reads the space sensor and
performs a calculation to determine the supply-air temperature
required (a cooling coil submaster reference [CCSR] value) to
satisfy conditions and outputs this value to the submaster loop.
OCCUPIED COOLING
General
— Economizer cycle must not be usable or outside air
damper position must be open to 90% or higher.
VAV Units
— Supply fan must be ON for cooling control to
operate. Sequence is as follows:
1. Unit must not be in heating mode.
2. Master Loop will survey occupancy status, Supply-Air
Set Point (SASP), and any Supply Air Temperature
Reset command, then issue Cooling Coil Submaster
Reference (CCSR) to Cooling Submaster Loop (CSL).
3. The CSL surveys actual SAT, then calculates number
of capacity stages required to produce the CCSR leav-
ing the unit.
4. Stages of cooling capacity are initiated. From zero
stages, there will be a 1.5 to 3 minute delay before the
first stage is initiated. The time delay between stages
in increasing demand is 90 seconds.
5. As actual SAT approaches CCSR value, stages are
released. The minimum time delay between stages on
decreasing demand is 90 seconds.
NOTE: Demand for heating has priority and Master Loop will
either terminate existing or prevent initiation of Cooling Cycle
by issuing a CCSR at the maximum limit. This will cause the
CSL to select zero stages of cooling capacity, initiating a stop-
page of an existing cooling cycle.
CV Units
— Supply fan must be ON for cooling control to op-
erate. Sequence is as follows:
1. Master Loop will survey space temperature and space
temperature offset inputs, then calculate CCSR value.
2. The CSL surveys actual SAT, then calculates number
of capacity stages required to satisfy space load.
3. Stages of cooling capacity are initiated. (From zero
stages, there will be a 1.5 to 3 minute delay before first
stage is initiated.)
UNOCCUPIED COOLING — The unoccupied cooling se-
quence of operation is similar to Occupied Cooling (see above)
except for the following:
1. Supply Fan will be OFF as demand is initiated.
2. The Master Loop will start Supply Fan and cooling
cycle. Fan status must be proved as ON within 2 min-
utes to continue with cooling operation.
3. Control set point will be Unoccupied Cooling Set
Point (UCSP).
4. At end of cooling cycle, Supply Fan will be turned
OFF.
OVERRIDES
First Stage and Slow Change Override
— The first stage
override reduces cycling on the first stage of capacity, and the
slow change override prevents the addition or subtraction of
another stage of capacity if the SAT is close to the set point and
gradually moving towards the set point.
Low Temperature Override
— This override function protects
against rapid load decreases by removing a stage every 30 sec-
onds when required based on temperature and the temperature
rate of change.
High Temperature Override
— This override function pro-
tects against rapid load increases by adding a stage once every
60 seconds as required, based on temperature and temperature
rate of change.
ADAPTIVE OPTIMAL START — Optimal start is used to
heat up or cool down the space prior to occupancy. The pur-
pose is to have the space temperature approach and then
achieve the occupied set point by the time of occupancy. The
control utilizes outdoor-air temperature, space temperature, oc-
cupied set point, and a “K” factor. The “K” factor is expressed
in minutes per degree, and calculates a start time offset, which
is the time in minutes that the system shall be started in ad-
vance of the occupied time. The control monitors its results and
adjusts the “K” factor to ensure that the occupied set point is
achieved at time of occupancy rather than too early or too late.
ADAPTIVE OPTIMAL STOP (CV Applications Only) —
Optimal stop is used to allow space temperature to drift to an
expanded occupied set point during the last portion of an
occupied period. The control calculates a stop time offset, (the
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