York W)036 THRU 060 Specifications download pdf (Page 43)

542300-YIM-A-1009

Johnson Controls Unitary Products 43

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 maximum 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 operating,

the second stage of mechanical cooling will not operate. 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 outdoor

air options with one exception. When the “R” to “G” circuit 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 compressor, initiate the ASCD

(Anti-short cycle delay), and stop the condenser fan. If the call

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

UCB will re-energize the compressor.

Should a high-pressure switch open three times within two

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

compressor and flash a code (see Table 25).

Low-Pressure Limit Switch

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

30 seconds of a cooling system's operation. For the following

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

compressor, initiate the ASCD, and stop the condenser fan.

Once the low-pressure switch has been proven (closed during 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 compressor, initiate the ASCD, and stop the condenser fan.

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

ASCD, the UCB will re-energize the compressor.

Should a low-pressure switch open three times within one hour

of operation, the UCB will lock-out the compressor and flash a

code (Table 25).

Freezestat

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

energize the compressor, initiate the ASCD, and stop the

condenser fan. 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 25).

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 operation.

Compressor minimum run time may extend the minutes of

compressor operation. The defrost cycle will begin immediately

following the elapse of the minimum run time.

When operating in low ambient mode, the UCB will not lockout

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 evaporator

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

discharge pressures due to a blocked condenser coil or a

condenser motor failure, (opens at 625 ± 25 psig).

3. A low-pressure switch to protect against loss of refrigerant

charge, (opens at 50 ± 5 psig).

The above pressure switches are hard-soldered to the unit. The

refrigeration systems are independently monitored and

controlled. On any fault, only the associated system will be

affected by any safety/preventive action.

The unit control board monitors the temperature limit switch of

electric heat units and the temperature limit switch and the gas

valve of gas furnace units.

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York W)036 THRU 060 Specifications Page 43

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