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variable or Unoccupied Cool Set Point (Setpoints→UCSP) plus
the Fan Speed Control Demand
(Configuration→COOL→FS.CD) configuration variable the
indoor fan will run at 100%. When the temperature in the
conditioned space is between the Occupied Cool Set Point
(Setpoints→OCSP) and Occupied Cool Set Point
(Setpoints→OCSP) plus the Fan Speed Control Demand
(Configuration→COOL→FS.CD) configuration variable the
indoor fan modulates to satisfy the Occupied Cool Set Point
(Setpoints→OCSP) or Unoccupied Cool Set Point
(Setpoints→UCSP). When the temperature in the conditioned
space falls to 0.5F below the Occupied Cool Set Point
(Setpoints→OCSP) or Unoccupied Cool Set Point
(Setpoints→UCSP) for 5 minutes the controls will run unit in
ventilation mode. See ventilation mode sequence of operation.
C09143
Fig. 6 -- Cooling Speed Demand Window
Cooling Supply Air Set Point
Reset
The PD unit can be configured to allow for reset of the Cool
Supply Air Set Point (Setpoint→SASP). This is needed for
applications where a high Cool Supply Air Set Point
(Setpoint→SASP) is required or where dramatic load changes
occur over short time periods. In these situations a high supply air
temperature may not provide enough cooling to reduce the Space
Temperature (Temperatures→AIR.T→SPT) to the Occupied Cool
Set Point (Setpoints→OCSP) or the Unoccupied Cool Set Point
(Setpoints→UCSP) over a reasonable time period.
The compressor or economizer operation supplying cooling to the
conditioned space controls to the Supply Air Control Point (Run
Status→COOL→SA.CP). The Supply Air Control Point (Run
Status→COOL→SA.CP) which is a calculated value that is equal
to the Cool Supply Air Set Point (Setpoint→SASP) plus the
calculated amount of reset required up to the values set by
configuration variables SASP Maximum Reset Down
(Configuration→COOL→SAT→SA.MD) and SASP Maximum
Reset Up (Configuration→COOL→SAT→SA.MU).
Reset of the Cool Supply Air Set Point (Setpoint→SASP) is
limited by the configuration variables SASP Maximum Reset
Down (Configuration→COOL→SAT→SA.MD) and SASP
Maximum Reset Up (Configuration→COOL→SAT→SA.MU).
The SASP Maximum Reset Down
(Configuration→COOL→SAT→SA.MD)
configuration variable
can be set from --20 to 0 F and will allow the Cool Supply Air Set
Point (Setpoint→SASP) to be reset to as much as 20 F below the
Cool Supply Air Set Point (Setpoint→SASP). The SASP
Maximum Reset Up (Configuration→COOL?SAT→SA.MU)
configuration variable can be set from 0 to 20 F and will allow the
Supply Air Temperature to be Reset to as much as 20 F above the
Cool Supply Air Set Point (Setpoint→SASP). The SASP
Maximum Reset Down (Configuration→COOL→SAT→SA.MD)
and SASP Maximum Reset Up
(Configuration→COOL→SAT→SA.MU) configuration variables
can be set independently and do not need to be equal values above
and below the Cool Supply Air Set Point (Setpoint→SASP).For
example, the SASP Maximum Reset Down
(Configuration→COOL→SAT→?SA.MD) configuration variable
can be set to --10 and the SASP Maximum Reset Up
(Configuration→COOL→SAT→SA.MU) configuration variable
can be set to 0 giving the range of Cool Supply Air Set Point
(Setpoint→SASP) reset of +0 and --10 F.
In normal operation when the Space Temperature is within the
speed demand window the unit control will modulate the indoor
fan speed and supply air temperature along a defined curve to bring
the Space Temperature to Occupied or Unoccupied Cool Set Point.
Since this curve is independent of the conditioned space load the
control scheme will adapt to any load present in the conditioned
space by adjusting the fan speed and supply air temperature and
move the space temperature along the defined time temperature
curve to Occupied or Unoccupied Cool Set Point.
When the space temperature rises above the Occupied Cool Set
Point (Setpoints→OCSP) + Fan Speed Control Demand
(Configuration→COOL→FS.CD) for over 2 minutes or is outside
the speed demand window and there is some amount of SASP
Maximum Reset Down (Configuration→COOL? SAT→SA.MD)
configured the indoor fan will be locked at 100% and the unit
controls will subtract a calculated amount of temperature reset to
the Cool Supply Air Set Point (Setpoint
→SASP). The indoor fan
speed will be locked at 100% as long as reset is applied. The
Supply Air Temperature will now adjust to this new Supply Air
Control Point (Run Status→COOL→SA.CP). The controls will
continue to subtract a calculated amount of temperature reset
cumulatively to a value equal to the Supply Air Temperature Set
Point (Setpoint→SASP) + SASP Maximum Reset Down
(Configuration→COOL→SAT→SA.MD). When the reset is no
longer needed the control will remove Do wn Reset until all reset is
removed and the Supply Air Control Point (Run
Status→COOL→SA.CP) is back to the original Supply Air Set
Point (Setpoint→SASP). The indoor fan will then modulate lower
than 100% to move the Space Temperature (Temperatures→SPT)
along the defined time temperature curve to Cool Supply Air Set
Point (Setpoint→SASP) or Unoccupied Cool Set Point
(Setpoints→UCSP). See Fig. 6 and 7.
C09144
Fig. 7 -- Cool Supply Air Set Point
Cooling Mode with Economizer Using Space
Temperature Sensor T55, T56, T58 or T59
If an economizer is installed, the Economizer Installed
configuration (Configuration→ECON→ EC.EN) should be set to
YES. The economizer is controlled by the Econo Commanded
Position (Outputs→ECON→ EC.CP) on the Economizer Control
Board. If the indoor fan is off or the building is unoccupied, the
economizer position is zero.
When the space temperature is above the Occupied Cool Set Point
(Setpoints→OCSP) and the economizer is installed, configured
and working correctly the unit controls will always try to meet
space cooling demand using the economizer before turning on the
compressor. The economizer will provide free cooling when the
following outside air conditions are true.
48/50PD