Carrier JK034-074 Air Conditioner User Manual


 
Operating Sequence 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 identification ‘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 field-installed timeclock or a
field-installed switch in the occupied space, the unit will be-
gin its initialization mode.
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A 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
F or more below the set point of the morning warm-up
potentiometer (P6), the unit will begin the morning
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warm-up routine, and a 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 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 NOTAVAILABLE 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 thermistorT1 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 ± 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 — 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 first 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. 2is 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 first 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 first 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 first 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.
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