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Temperature Reset — If the unit is equipped with the ac-
cessory temperature reset package, and DIP switch 2 is in
the ON position, the unit will reset the supply-air tempera-
ture to a calculated value when necessary. When this con-
dition is in effect, a
will appear in the display.
21
Demand Limit — If the unit is equipped with the accessory
demand limit control module or the field-supplied, single-
step demand limit potentiometer, and DIP switch 5 is in the
ON position, the unit will limit the capacity stages to a pre-
determined value. When this condition is in effect, a
appear in the display.
Morning Warm-Up —If the morning warm-up heat routine
is enabled using DIP switch 4, and conditions of the occu-
pied space warrant, the unit will begin the morning warm-up
routine. When this condition is in effect, a
in the display.
Internal Failure — If the unit detects an internal fault (such
as a time measurement failure), or detects an incorrect volt-
age on an input channel, a
unit will shut down.
Quick Test — If the display button is pressed during the
initialization period of the processor, the unit will run its self-
diagnostic routine. When this is in effect, an
in the display screen.
CODES 51 THROUGH 87, OVERLOAD INFORMATION
— These codes indicate diagnostic information when there
is a unit problem such as a faulty thermistor, potentiometer,
or compressor fault. Refer to Service and Troubleshooting
section on page 21 for more details. Codes 52-54, 56-58, 61,
62, 65-69, 73, 74, and 77-80 are not used on these units.
Under normal operation, only the stage number is dis-
played when the display button is pressed. If a status or over-
load code is displayed, the display will rotate every 2 sec-
onds and will display up to 3 codes. Overload information
takes priority over all other codes. The codes are stored in
the microprocessor as long as the board remains energized.
Operating Sequence —
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.
A 20 will appear in the display screen, and the initializa-
tion 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.
22
will appear
26
will be displayed, and the
30
will appear
88
The sequence presented be-
Once the initialization period is complete, the supply and
return fans begin operation. While the fans are operating,
the economizer dampers are closed and return air from the
building is being circulated. After 2 minutes, the processor
checks the resistance value of thermistor T2. If T2 tempera-
ture sensed is 5° F or more below the set point of the morn-
ing warm-up potentiometer (P6), the unit will begin the morning
warm-up routine, and a
Unit heat will be energized through the heat interlock re-
lay (HIR), and all of the occupied space air terminals open.
will
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 leaving-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 was unable to meet the
load due to poor outdoor-air enthalpy, the dampers will re-
turn to the minimum position as determined by P5.
Compressors, unloaders, and condenser fans will be cycled
to maintain a leaving-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 180 sec-
onds (034-074 only) to prevent nuisance trips of the low-
pressure switch. After start-up, a low-pressure trip will be
ignored for 30 seconds by the processor.
SIZE 024-034 AND 054-074 UNITS — These units have 2
compressors and 2 unloaders on compressor 1. See Fig. 21-23
for compressor and condenser fan motor locations. The op-
erating 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
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
Stage 3 Relay K1 is energized. Compressor no. 1 is fully
loaded.
13
26
will be displayed.
1
⁄
capacity. The crankcase
3
2
⁄
capacity.
3
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