Carrier 48/50PD05 manual Cooling Speed Demand Window, Cooling Supply Air Set Point Reset

48/50PD

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.

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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 Down 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.

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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.

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