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49
Loca te the enthalpy control in the economizer hood.
Locate two GRA leads in the factory harness and connect
these leads to enthalpy control sensors 2 and 3. (See Fig.
49.) Connect the enthalpy control power input terminals to
economizer actuator power leads RED (connect to TR)
and BLK (connect to TR1).
The outdoor enthalpy changeover setpoint is set at the
enthalpy controller.
The enthalpy control receives the outdoor ai r enthalpy
from the outdoor air enthalpy sensor and provides a dry
conta ct switch input to the RTU--MP controller. A closed
contact indicates that outside air is preferred to the return
air. An open contact indicates that the economizer should
remain at minimum position.
Differential E nthalpy Control — Differential enthalpy
control is provided by sensing and com paring the outside
air and return air enthalpy conditions. Install the outdoor
air enthalpy control as described above. Add and install a
return a ir entha lpy sensor.
Return Air Enthalpy Sensor — Mount the return--air
entha lpy se nsor (HH57AC078) in the return--air duct. The
return air sensor is wired to the enthalpy controller
(HH57AC077). (See Fig. 50. )
To wire the return air enthalpy sensor, pe rform t he
following:
1. Use a 2 --conductor, 18 or 20 AWG, twisted pair cable
to connect the return air enthalpy sensor to the
enthalpy controller.
2. At the enthalpy control remove the factory --installed
resistor from the (SR) and (+) terminals.
3. Connect the field--supplied RED wire to (+) spade
connec tor on the return air enthalpy sensor and the
(SR+) terminal on the enthalpy controller. Connect
the BLK wire to (S) spade connector on the return air
entha lpy sensor and the (SR) terminal on the enthalpy
controller.
NOTE: The enthalpy control must be set to the “D”
setting for differential enthalpy control to work properly.
The enthalpy c ontrol receives the indoor and return
enthalpy from the outdoor and return ai r enthalpy sensors
and provides a dry contac t switch input to the RTU--MP
controller. A closed contact indicates that outside air is
preferred to the return air. An open contact indicates that
the economizer should remain at minimum position.
Indoor Air Quality (CO
2
sensor) — The indoor air quality
sensor accessory monitors space carbon dioxide (CO
2
)
levels. This information is used to monitor IAQ levels.
Several types of sensors are available, for wall mounting
in the space or in return duct, with and without L CD
display, and in combination with space temperature
sensors. Sensors use infrared technology to measure the
levels of CO
2
present in the space air.
The CO
2
sensors are all factory set for a range of 0 to
2000 ppm and a linear mA output of 4 to 20. Refer to the
instructions supplied with the CO
2
sensor for electrical
requirements and terminal locations. See Fig. 51 for
typical CO
2
sensor wiring schematic.
To accurately monitor the quality of the air in the
conditioned air space, locate the sensor near a return--air
grille (if present) so it senses the concentration of CO
2
leaving the space. The sensor should be mounted in a
location to avoid direct breath contact.
Do not mount the IAQ sensor in drafty are as such as near
supply ducts, open windows, fans, or over heat sources.
Allow at least 3 ft (0.9 m) between the sensor and any
corner. Avoid mounting the sensor where it is influenced
by the supply air; the sensor gives inaccurate readings if
the supply air is blown directly onto the sensor or if the
supply air does not have a chance to mix with the room air
before i t is drawn into the return airstream.
Wiring the Indoor Air Quality Sensor —
For each sensor, use two 2--conductor 18 AWG (American
Wire Gage) twisted--pair cables (unshielde d) to connect
the separate isolated 24 vac power source to the sensor
and to connect the sensor to the control board terminal s.
To connect the sensor to the control, identify the positive
(4 to 20 mA) and ground (SIG COM) t erminals on the
sensor. See Fig. 51. Connect the 4--20 mA terminal to
RTU--MP J4--2 and connect the SIG COM terminal to
RTU--MP J4--3. See Fig. 65.
SEN
COM
J4-2
J4-3
IAQ Sensor
24 VAC
C08462
Fig. 65 -- RTU--MP / Indoor CO
2
Sensor
(33ZCSENCO2) Connections
Outdoor Air Quality Sensor (PNO 33ZCSENCO2 plus
weathe rproof enclosure) — The outdoor air CO
2
sensor is
designed to monitor carbon dioxide (CO
2
) levels in the
outside ventilation air and interface with the ventilation
damper in an HVAC system. The OAQ sensor is packaged
with an outdoor cover. (See Fig. 53.) The outdoor air CO
2
sensor must be located in the economizer outside air hood.
Wiring the Outdoor Air CO
2
Sensor — A dedicated power
supply is required for this sensor. A two--wire cable is
required to wire the dedicated power supply for the sensor.
The two wires should be connected to t he power supply
and terminals 1 and 2.
48TC