Carrier 48FK, JK034-074, 50FK manual

Operating Sequence Ð The sequence presented be- low assumes that the unit is equipped with heat for morning warm-up and an economizer. If these items are not enabled with the appropriate DIP switches, the processor bypasses these subroutines. This sequence is also based on an EPROM (erasable, programmable, read-only memory) processor chip with the identi®cation `HT204485-1-XX,' where `XX' is re- placed by a 2-digit number representing the current software version. See Fig. 1 for EPROM chip location.

When power is applied to the occupied mode relay (OMR) through the closure of either a ®eld-installed timeclock or a ®eld-installed switch in the occupied space, the unit will be- gin its initialization mode.

A20 will appear in the display screen, and the initial- ization period will last approximately 2 minutes. During this time, the economizer dampers open and close to determine the resistance range for full economizer operation of the econo- mizer position potentiometer (P2). The processor loads the necessary constants for unit operation, and also checks the thermistors and other potentiometers for their values and va- lidity. After the initialization period, the screen goes blank until the display button is pressed.

Use caution during this time (after initialization when the screen is blank), because the unit supply and return fans could start at any time. Personal injury could result from contact with rotating fans.

Once the initialization period is complete, the supply fan begins operation. While the fan is operating, the economizer dampers are closed and return air from the building is being circulated. After 2 minutes, the processor checks the resis- tance value of thermistor T2. If T2 temperature sensed is 5° F or more below the set point of the morning warm-up potentiometer (P6), the unit will begin the morning

warm-up routine, and a 26 will be displayed.

Unit heat will be energized through the heat interlock re- lay (HIR), and all of the occupied space air terminals open. The unit will continue heating the space until the return-air temperature is within 2° F of set point. The unit will then shut off the heat and continue to circulate air. The unit will cycle in and out of the heating mode until the return-air tem- perature reaches the morning warm-up set point (P6). Once morning warm-up has been terminated, the unit cannot re- turn to morning warm-up until the unit is powered down and restarted. This action signals a return to the Occupied mode.

NOTE: Occupied heat is NOT AVAILABLE on these units.

Once out of the morning warm-up routine, the unit will begin its cooling routine based on the supply-air set point (P1). At step zero, the unit has no mechanical cooling on, and the economizer may or may not be operational. The econo- mizer will move to the minimum position determined by po- tentiometer P5 if no cooling load is detected. Once a cooling load is detected by thermistor T1 sensing a temperature higher than the cooling demand set point (P1), the economizer will begin modulating to meet the load if the outdoor enthalpy is good. The processor will attempt to maintain a supply- air temperature of P1 ± 2° F by modulating the economizer dampers.

No mechanical cooling will take place until the econo- mizer dampers are fully open (if the outdoor-air enthalpy permits). If the economizer is unable to meet the cooling demand, then mechanical cooling is used in conjunction with the economizer. If the economizer is unable to meet the load due to unacceptable outdoor-air enthalpy, the dampers will return to the minimum position as determined by P5.

Compressors, unloaders, and condenser fans will be cycled to maintain a supply-air temperature 2° F below the poten- tiometer P1 set point once the mechanical cooling stages be- gin. Each unit's cycling is slightly different, and is based on the number of compressors and unloaders. The operational loading sequence of compressors is as follows:

During the start-up of the lead compressor for each cir- cuit, the low-pressure switch will be bypassed for 120 sec- onds to prevent nuisance trips of the low-pressure switch. After start-up, a low-pressure trip will be ignored for 30 sec- onds by the processor.

SIZE 034,038 AND 048-088 UNITS Ð These units have 2 compressors and 2 unloaders on compressor 1. See Fig. 42 and 43 for compressor and condenser-fan motor locations. The operating sequence is as follows:

Stage 1 Relays K1, K2, and K3 are energized. Compressor no. 1 starts with both unloaders energized. Com- pressor no. 1 runs at 1¤3 capacity. The crankcase heater for this compressor has been deenergized, and the ®rst stage of condenser fans have been en- ergized. Outdoor (condenser) fan motor no. 1 (OFM1) has started on all units.

Stage 2 Relays K1 and K3 are energized. Compressor no. 1 is running with unloader 1 (U1) energized. The compressor is now operating at 2¤3 capacity.

Stage 3 Relay K1 is energized. Compressor no. 1 is fully loaded.

Stage 4 Relays K1, K2, K3, and K5 are energized. Com- pressor no. 1 is running at 1¤3 capacity, and com- pressor no. 2 is running at full capacity. The crankcase heater for compressor no. 2 has been deenergized.

Stage 5 Relays K1, K3, and K5 are energized. Compressor no. 1 is running at 2¤3 capacity, and compressor no. 2 is running at full capacity.

Stage 6 Relays K1 and K5 are energized. Both compres- sors are running fully loaded.

Size 034 and 038 units have 2 condenser fans, one of which is controlled by the microprocessor. The OFM1 is energized with compressor no. 1. The OFM2 is controlled by the pro- cessor and is cycled based on input from circuit thermistor T3 or T4.

Size 048 units have one fan that can be controlled by the processor. The other 2 are controlled by the compressors. The OFM1 is energized by compressor no. 1, and OFM3 is energized by compressor no. 2. The OFM2 is cycled by the processor based on input from either circuit (thermistors T3 and T4).

On size 054,064 units, the ®rst 2 condenser fans energize with the compressors; compressor no. 1 controls OFM1, and compressor no. 2 controls OFM2. The OFM3 and OFM4 are staged by the microprocessor based on the condensing tem- perature input from thermistor T3 or T4.

On size 074-078 units, the ®rst 3 condenser fans energize with the compressors; compressor no. 1 controls OFM1, and compressor no. 2 controls OFM2 and OFM3. The OFM4 and OFM5 are staged by the microprocessor based on con- densing temperature input from either circuit's T3 or T4 thermistor.

On Size 088 units, the ®rst 4 condenser fans energize with the compressors; compressor no. 1 controls OFM1 and OFM3, and compressor no. 2 controls OFM2 and OFM4. The OFM5 and OFM6 are staged by the microprocessor based on condensing temperature input from either circuit's T3 or T4 thermistor.