Economizer And Power Exhaust Group - Carrier 48FP Controls Operation And Troubleshooting

Economizer and Power Exhaust Group —
group includes Economizer, Nighttime/Unoccupied Free Cool-
ing, and Modulating Power Exhaust.
ECONOMIZER — Economizer control is used to control the
outdoor and return air dampers of the unit, to satisfy space
cooling demand using all outdoor air (when permitted), and to
satisfy cooling in conjunction with compressor operation
(when conditions permit). During Occupied periods, the out-
door air dampers will be at the user-configured Minimum
Damper Position. During Unoccupied periods, the outdoor air
dampers will be closed. The Economizer function is also used
for Indoor Air Quality (IAQ), Outdoor Air Control (OAC), and
Building Pressurization modes. See Table 18. Economizer is
available as a factory-installed option only.
The user can install the following devices to enhance econo-
mizer control:
• Differential enthalpy sensor (field-supplied and
-installed)
• Outdoor air humidity sensor (field-supplied and
-installed)
• Return air humidity sensor (field-supplied and -installed)
• Freezestat (field-supplied and -installed)
Table 18 — Economizer Operation Definitions
ITEM
CV Constant Volume
DPSP Damper Position Set Point
ECSO Economizer Set Point Offset
ECONSR Economizer Submaster Reference
ESL Economizer Submaster Loop
NTLO Nighttime/Unoccupied Free Cooling Lockout
OAT Outdoor-Air Temperature
OCSP Occupied Cooling Set Point
OHSP Occupied Heating Set Point
SASP Supply Air Set Point Temperature (VAV only)
SAT Supply-Air Temperature
SPT Space Temperature
SPTRESET Space Temperature Reset
SRV Submaster Reference Value
VAV Variable Air Volume
Enthalpy Control — Outdoor air enthalpy control is standard
with factory-installed economizer option. Enthalpy is sensed
by a controller located behind the end outdoor air hood. Access
the controller by removing the upper hood filter. See Fig. 9.
The outdoor enthalpy controller permits selection of four
different enthalpy settings, reflecting different temperature-
humidity ranges. See Fig. 10 for available ranges. Adjust set-
ting on the enthalpy controller (see Fig. 11).
NOTE: Replace the outdoor air filter before restarting the unit.
Integrated Economizer with Differential Enthalpy Con-
trol — Integrated economizer operation can be enhanced by
adding a differential enthalpy control feature.
Differential enthalpy control adds a measurement for return-
air heat and moisture content conditions and compares these
conditions to those of the outdoor air. When the control
determines that the outdoor air conditions are cooler and drier
than those of the return air, it opens the outdoor air dampers on
a demand for cooling and permits integrated economizer oper-
ation (outdoor air with mechanical cooling stages) since using
the outdoor air at these conditions (instead of the warmer return
air) will result in more economical cooling operation.
The 48FP,JP,NP and 50FB,FP,NB,JB,JP,NP units have two
methods of accomplishing differential enthalpy control: En-
hanced enthalpy switch control operation and base unit control
logic enhancement.
Accessory 50DJ-902---321, Differential Enthalpy Sensor,
provides differential enthalpy sensing control via the existing
This
DEFINITION
enthalpy switch. This sensor (reference Part Number
HC57AC078) is installed in the return air duct and is wired di-
rectly to the factory-installed enthalpy switch (see Fig. 9).
When the enthalpy control determines that the outdoor air en-
thalpy is lower than the return air enthalpy, the enthalpy switch
closes (at Channel 10), signaling the base unit control to use the
economizer as first stage of cooling control immediately.
Adding two relative humidity sensors (one in the outdoor
air hood and one in the space or in the return air duct) allows
the base unit control to sense RH in both airstreams directly.
(See Fig. 12 for field wiring connections.) The base unit con-
trol calculates enthalpy in both air streams (using dry bulb tem-
peratures and RH at each sensor location). When the control
determines the outdoor air enthalpy is lower than the enthalpy
of the return air, the control will use the economizer as the first
stage of cooling control. (The addition of the RH sensors also
increases condition monitoring, possible alert messages and
permits enabling of Dehumidification mode and control of
field-installed Humidifiers.)
Sequence of Operation — The Master Loop will be delayed
2 minutes after the supply fan is turned ON, to allow all system
statuses and temperatures to stabilize before starting control.
When coming out of Standby or Heating mode, a 4-minute de-
lay 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 the fan status is OFF, the outside air dampers will remain
closed (return air dampers will be open).
If fan status is ON, the Master Loop will check for forced
status on the Damper Position Set Point (DPSP). If a forced
condition exists, the sequence is terminated.
Economizer operation is permitted if all of the following
conditions exist:
• System is NOT in Heating mode
• Outdoor air enthalpy (via switch or humidity differential)
is acceptable
• Outdoor air temperature is less than Space Temperature
If economizer operation is permitted, Master Loop checks
for Cooling System operation. If cooling is ON, the economiz-
er Submaster Reference (ECONSR) will be set to the mini-
mum position. The Economizer Submaster Loop (ESL) re-
sponds by driving outdoor air dampers to maximum position.
If Cooling is not on, in VAV operation, the Master Loop cal-
culates DPSP, compares it to SAT, computes ECONSR, and
outputs the value to the ESL. If Cooling is not on, in CV opera-
tion, the Master Loop calculates the DPSP, compares it to the
Space Temperature (SPT), computes ECONSR, and outputs
the value to the ESL. The ESL will compare ECONSR to the
actual supply air temperature, compute the required damper
position to satisfy ECONSR, and output the position require-
ment (at channel 14) to economizer motor. Economizer motor
will open Outdoor Air dampers (and close Return Air dampers)
and modulate to maintain supply air temperature at DPSP.
If economizer operation is NOT permitted, the ECONSR
will be set to maximum value. The ESL will respond by driv-
ing outdoor air dampers to minimum position (Occupied peri-
od) or closed position (Unoccupied period).
For VAV units, economizer operation is also not permitted
when Occupied Heating is enabled and Return Air Tempera-
ture is less than (OHSP + 1).
Economizer Configuration — To configure the economizer,
press to login. Enter the password. Press
enter the Economizer subfunction of the Service function.
Scroll down to Minimum Damper Position (MDP). The default
is 20%. The range of acceptable values is 0 to 100%. To
change the set point, enter the new number (example:
and press . See Table 19.
ENTER
20
to
)