Carrier 48/50AJ specifications Major System Components, Factory-Installed Components

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

General — The 48/50A Series package rooftop units with 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. 15-23 for typical control and power component schematics. Figures 24 and 25 show the layout of the control box, unit, and thermistor and transducer locations.

Factory-Installed Components

MAIN BASE BOARD (MBB) — See Fig 26. 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 115 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® system through the CCN communi- cations 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 operates the economizer motor using a digital communication signal that also provides status and diagnostics for the econo- mizer motor. See Fig. 27. 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. Additionally, ECB1 provides an output that will send a 4 to 20 mA signal to a field-installed VFD power ex- haust accessory. Details can be found in Table 116. 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. 27. 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 117.

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. 28. 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 118.

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 CO2 sensor (for supply duct pressure reset using 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. 29. The inputs and outputs are summarized in Table 119.

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 120.

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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