Major System Components; General; Factory-Installed Components - Carrier 48AJ Operating And Service Manual

A700 (Supply Air Temperature Sensor Failure) — This alarm
indicates a failure of the sensor supply air temperature sensor
or the leaving air temperature sensor (if using hydronic heat).
This alarm occurs when the temperature sensor (Tempera-
→ →
tures AIR.T SAT) is outside the range –40 to 240 F (–40 to
116 C). This alarm resets automatically. The cause of the alarm
is usually a faulty thermistor, a shorted or open thermistor
caused by a wiring error, or a loose connection.
T701 (Staged Gas 1 Thermistor Failure)
T702 (Staged Gas 2 Thermistor Failure)
T703 (Staged Gas 3 Thermistor Failure) — If any of the staged
gas thermistors (Temperatures
alert will be generated and the remaining thermistors will be
averaged together (Temperatures
the failed thermistor. Recovery is automatic. Reason for failure
may be incorrect wiring, faulty thermistor, or a damaged input
on the staged gas control board (SCB).
A704 (Staged Gas Leaving Air Temperature Sum Total Fail-
ure) — If all three staged gas thermistors (Tempera-
→ →
tures AIR.T S.G.L1-3) fail (the sensor is outside the range
of –40 F to 240 F), staged gas will be shut down and this alarm
will be generated. Recovery is automatic. Reason for failure
may be faulty wiring, faulty thermistors, or damaged inputs on
the staged gas control board (SCB).
T705 (Limit Switch Thermistor Failure) — A failure (the sen-
sor is outside the range of –40 F to 240 F) of this thermistor
→ →
(Temperatures AIR.T S.G.LM) will cause an alert to occur
and a disabling of the limit switch monitoring function for the
staged gas control board (SCB). Recovery is automatic.
Reason for failure may be due to faulty wiring, a faulty
thermistor, or a damaged input on the staged gas control board
(SCB).

MAJOR SYSTEM COMPONENTS

The 48/50A series package rooftop units with
General —
electric cooling and with gas heating (48A units) or electric
cooling and electric heating (50A units) contain the
ComfortLink™ electronic control system that monitors all
operations of the rooftop. The control system is composed of
several components as listed below. See Fig. 13-19 for typical
control and power component schematics. Figures 20 and 21
show the layout of the control box, unit, and thermistor and
transducer locations.

Factory-Installed Components

MAIN BASE BOARD (MBB) — See Fig 22. The MBB is
the center of the ComfortLink control system. The MBB con-
tains the major portion of the operating software and controls
the operation of the unit. The MBB has 22 inputs and 11 out-
puts. See Table 106 for the inputs and output assignments. The
MBB also continuously monitors additional data from the
optional ECB1, ECB2, SCB, and CEM boards through the
LEN communications port. The MBB also interfaces with the
Carrier Comfort Network through the CCN communications
port. The board is located in the main control box.
ECONOMIZER BOARD (ECB1) — The ECB1 controls the
economizer actuator and the power exhaust fans. The ECB1
→ →
AIR.T S.G.L1-3) fails, an
→ →
AIR.T S.G.LS) without
operates the economizer motor using a digital communication
signal that also provides status and diagnostics for the econo-
mizer motor. See Fig. 23. The ECB1 also controls the operation
of the power exhaust motors and provides up to 6 stages of dig-
itally sequenced power exhaust either based on the economizer
motor position or the building pressure. The board has 4 inputs
and 6 outputs. Details can be found in Table 107. The ECB1
board is located in an auxiliary box located at the end of the
unit behind the filter access door. The board also contains
a second LEN port than can be used with the accessory
Navigator™ display.
VAV BOARD (ECB2) — The VAV board (which is the same
hardware as the ECB1) is used to control the supply fan on
VAV units. See Fig. 23. It sends a 4 to 20 mA signal to the VFD
based on a supply duct pressure sensor connected to the board.
The board also accepts a signal from another pressure sensor
that monitors building pressure and controls the operation of
the optional modulating power exhaust motors. The board will
also be used on CV units with the optional building pressure
control feature and modulating power exhaust. This board is
also used to control a digitally controlled hot gas bypass sole-
noid with an integral orifice for use in low load applications.
This board is located in the auxiliary control box. Input and
output assignments are summarized in Table 108.
STAGED GAS HEAT BOARD (SCB) — When optional
staged gas heat is used on CV and VAV units, the SCB board is
installed and controls operation of the gas valves. See Fig. 24.
The SCB also provides additional sensors for monitoring of the
supply-air temperature. This board is located in the gas heat
section of the unit. The inputs and outputs are summarized in
Table 109.
CONTROL EXPANSION MODULE (CEM) — The optional
CEM (also available as an accessory) is used to accept inputs
for additional sensors or control sequence switches, including:
• smoke control mode field switches
• VAV Supply Air Temperature Set Point reset using an exter-
nal 4 to 20 mA signal
• outdoor air CO sensor (for supply duct pressure reset using
2
an eternal 4 to 20 mA signal)
• external fan status pressure switch input (CV units)
• demand limit sequence proportional signal or discrete
switches
The CEM board is located in the main control box. See
Fig. 25. The inputs and outputs are summarized in Table 110.
INTEGRATED GAS CONTROL (IGC) — One IGC is pro-
vided with each bank of gas heat exchangers (2 used on the size
020-050 units and 3 on size 051 and 060 units). The IGC
controls the direct spark ignition system and monitors the
rollout switch, limit switches, and induced-draft motor Hall
Effect switch. The IGC is equipped with an LED (light-
emitting diode) for diagnostics. See Table 111.
COMPRESSOR PROTECTION BOARD (CS) — This board
monitors the status of the compressor by sensing the current
flow to the compressors and then provides digital status signal
to the MBB.
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