Carrier 48DL Installation Manual download pdf (Page 62)

264975-BIM-B-1208

62 Johnson Controls Unitary Products

completed. Upon the final compressor de-energizing, the

blower is stopped following the elapse of the fan off delay for

cooling, and the economizer damper goes to the closed posi-

tion. If the unit is in continues fan operation, the economizer

damper goes to the minimum position.

ECONOMIZER WITH DUAL ENTHALPY SENSORS -

The operation with the dual enthalpy sensors is identical to

the single sensor except that a second enthalpy sensor is

mounted in the return air. This return air sensor allows the

economizer to choose between outdoor air and return air,

whichever has the lowest enthalpy value, to provide maxi-

mum operating efficiency.

ECONOMIZER WITH POWER EXHAUST -

A unit equipped with an economizer (single or dual enthalpy)

and a power exhaust operates as specified above with one

addition. The power exhaust motor is energized 45 seconds

after the actuator position exceeds the exhaust fan set point

on the economizer control. When the power exhaust is oper-

ating, the second stage of mechanical cooling will not oper-

ate. As always, the "R" to "G" connection provides minimum

position but does not provide power exhaust operation.

MOTORIZED OUTDOOR AIR DAMPERS -

This system operation is the same as the units with no out-

door air options with one exception. When the "R" to "G" cir-

cuit is complete, the motorized damper drives open to a

position set by the thumbwheel on the damper motor. When

the "R" to "G" circuit is opened, the damper spring returns

fully closed.

COOLING OPERATION ERRORS

Each cooling system is monitored for operation outside of the

intended parameters. Errors are handled as described below.

All system errors override minimum run times for compressors.

HIGH-PRESSURE LIMIT SWITCH

During cooling operation, if a high-pressure limit switch

opens, the UCB will de-energize the associated compressor,

initiate the ASCD (Anti-short cycle delay), and, if the other

compressor is idle, stop the condenser fans. If the call for

cooling is still present at the conclusion of the ASCD, the

UCB will re-energize the halted compressor.

Should a high-pressure switch open three times within two

hours of operation, the UCB will lock-out the associated com-

pressor and flash a code (see Table 80). If the other com-

pressor is inactive, the condenser fans will be de-energized.

LOW-PRESSURE LIMIT SWITCH

The low-pressure limit switch is not monitored during the ini-

tial 30 seconds of a cooling system's operation. For the fol-

lowing 30 seconds, the UCB will monitor the low-pressure

switch to ensure it closes. If the low-pressure switch fails to

close after the 30-second monitoring phase, the UCB will de-

energize the associated compressor, initiate the ASCD, and,

if the other compressor is idle, stop the condenser fans.

Once the low-pressure switch has been proven (closed dur-

ing the 30-second monitor period described above), the UCB

will monitor the low-pressure limit switch for any openings. If

the low-pressure switch opens for greater than 5 seconds,

the UCB will de-energize the associated compressor, initiate

the ASCD, and, if the other compressor is idle, stop the con-

denser fans.

If the call for cooling is still present at the conclusion of the

ASCD, the UCB will re-energize the halted compressor.

Should a low-pressure switch open three times within one

hour of operation, the UCB will lock-out the associated com-

pressor and flash a code (Table 80). If the other compressor

is inactive, the condenser fans will be de-energized.

FREEZESTAT

During cooling operation, if a freezestat opens, the UCB will

de-energize the associated compressor, initiate the ASCD,

and, if the other compressor is idle, stop the condenser fans.

If the call for cooling is still present at the conclusion of the

ASCD, the UCB will re-energize the halted compressor.

Should a freezestat open three times within two hours of

operation, the UCB will lock-out the associated compressor

and flash a code (Table 80). If the other compressor is inac-

tive, the condenser fans will be de-energized.

LOW AMBIENT COOLING

To determine when to operate in low ambient mode, the UCB

has a pair of terminals connected to a temperature-activated

switch set at 45ºF. When the low ambient switch is closed

and the thermostat is calling for cooling, the UCB will operate

in the low ambient mode.

Low ambient mode operates the compressors in this manner:

10 minutes on, 5 minutes off. The indoor blower is operated

throughout the cycle. The 5-minute off period is necessary to

defrost the indoor coil.

Low ambient mode always begins with compressor opera-

tion. Compressor minimum run time may extend the minutes

of compressor operation. The defrost cycle will begin immedi-

ately following the elapse of the minimum run time.

When operating in low ambient mode, the UCB will not lock-

out the compressors due to a freezestat trip. However, a

freezestat trip will de-energize the associated compressor. If

the call for cooling is still present at the end of the ASCD and

the freezestat has closed, the unit will resume operation.

SAFETY CONTROLS

The unit control board monitors the following inputs for each

cooling system:

1. A suction line freezestat to protect against low evapora-

tor temperatures due to a low airflow or a low return air

temperature, (opens at 26 ± 5 °F and resets at 38 ± 5°F).

2. A high-pressure switch to protect against excessive dis-

charge pressures due to a blocked condenser coil or a

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Carrier 48DL Installation Manual Page 62

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