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Carrier 48 50PM C16-28 Manual

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18
Cooling Operation
The 48/50PG and 48/50PM unit’s cooling operation consists of:
demand, mode determination, staging request to satisfy the
demand, and handling a request with the unit’s resources. These
resources can include compressors, an economizer, and a reheat
coil. This section covers mechanical cooling. For economizer and
reheat cooling, refer to the Economizer and Humidi−mizer
sections, respectively. The unit enters a cooling mode based on a
demand, decides how to satisfy the demand, executes its plan, and
then leaves the cooling mode.
Cooling Mode Control
The cooling HVAC mode (Run Status→MODE→HVAC=3) has
six different expandable texts: Cooling, Free Cooling, Unoccupied
Free Cooling, Reheat1, Reheat2, or Reheat1/Reheat2. These are all
part of a general cooling mode and resemble the specific type of
cooling that is being performed at any given time. All types of
cooling are still performed under the general cooling function, and
the expanded text is for user reference only. The control will
display if it is ok to select the cooling mode (Operating Modes
→COOL→OK.CL= Yes).
Thermostat Control
For the unit to enter cooling mode, three things must be true: the
indoor fan must be ok to use, the mode changeover time guard
must be expired, and there must be a cooling demand (Y1, Y2, or
reheat demand). The unit will remain in cooling until the cooling
demand is dropped or if any of the above conditions turn false. If
only a reheat demand exists and a heat demand (W1, W2) occurs,
the unit will end cooling. The cooling mode can not officially end
until all compressors are off.
Space Sensor Control
For the unit to enter cooling mode, four things must be true: the
indoor fan must be ok to use, the mode changeover time guard
must be expired, the unit must have a valid space temperature, and
there must be a cooling or reheat demand. The unit will remain in
cooling for at least one minute and until the cooling demand drops
below −0.5F or if any of the above conditions turn false. If only a
reheat demand exists and the heat demand becomes greater than the
Reheat Heat Setpoint Deadband (Setpoints→RH.HB), the unit will
end cooling. The cooling mode can not officially end until all
compressors are off.
Staging Control
Once the unit is in a cooling mode, it must decide what the demand
is and how to satisfy it. If an economizer is installed and can be
used for cooling (Operating Modes→COOL→OK.EC= Yes), the
unit will use it first (see economizer section for its operation). If
the economizer can not be used or additional cooling is needed, a
mechanical cooling check is performed. OK to use Compressors?
(Operating Modes→COOL→OK.MC), will be set to yes when
compressors are enabled and not locked out. Based on the unit
control configuration, requested cooling stages (Run Status
→COOL→REQ.C) will be determined then passed to compressor
control to actually add the cooling stages.
Thermostat Control
There are two ways of requesting stages when thermostat control is
enabled, Traditional Thermostat control or Adaptive control.
Traditional Thermostat control is used if the Thermostat Control
Type (T.CTL) is set to 1, 2, or 3 and the economizer is not
available for free cooling. If Thermostat Control (T.CTL) is set to
0 or any time the economizer is available for free cooling, the unit
will use Adaptive control for staging.
T.CTL = 0 (Adaptive Control)
Stage timers, Supply air trend, and supply air temperature limits
apply when determining the request for stages. The first request
(REQ.C=1) comes immediately when the Y1 input is active. The
Cool Stage Increase Time (Configuration→COOL→C.INC) or
the Cool Stage Decrease Time (Configuration→COOL→C.DEC)
has to expire before another stage can be added or a stage can be
subtracted. The Supply−Air Trend (Operating
Modes→COOL→SA.TR) decides if the next stage can be
requested or should be subtracted based on the Y2 input status.
For 48/50PG16 units, the supply air trend will allow up to 2
requested stages for just the Y1 input and allow up to 3 requested
stages for an Y1 and Y2 input. If the Y1 and Y2 inputs are
dropped, the supply air trend is not considered because cooling is
no longer needed. If at any time the Supply−Air Temperature
(SAT) falls below the Minimum Supply Air Temperature Upper
Level (Configuration→COOL→SAT→SAT.U), the requested
stages will not be allowed to increase. If at any time the SAT falls
below the Minimum Supply Air Temperature Lower Level
(Configuration→COOL→SAT→SAT.L), the requested stages will
be reduced by one without honoring C.DEC. If SAT.L and SAT.U
are configured so that they are close together, the last stage of
compressor might cycle rapidly, slowed only by its minimum on
and off−time requirements.
T.CTL = 1 (1 Stage Y1)
Stage timers, Supply air trend, and supply air temperature limits do
not apply when determining the request for stages. Request staging
will follow the thermostat inputs directly. Y1 will request one
stage. Y2 will request all stages.
T.CTL = 2 (2 Stage Y1)
Stage timers, Supply air trend, and supply air temperature limits do
not apply when determining the request for stages. Request staging
will follow the thermostat inputs directly. Y1 will request two
stages on 48/50PG16 units and one stage for all other units. Y2
will request all stages.
T.CTL = 3 (Digital)
Stage timers, Supply air trend, and supply air temperature limits do
not apply when determining the request for stages. Request staging
will follow the thermostat inputs directly. Y1 will request one
stage. Y2 will request two stages. Y1 and Y2 will request three
stages on 48/50PG16 units and two stages for all other nits.
48/50PG and PM
… 18 …


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