Service Manual
41
begins counting. Compressor will not come on until this
delay expires. In the event of a power interruption, the time
guard will not allow another cycle for 5 minutes.
Compressor Operation
The basic scroll design has been modified with the addition
of an internal unloading mechanism that opens a by- pass
port in the first compression pocket, effectively reducing the
displacement of the scroll. The opening and closing of the
by- pass port is controlled by an internal electrically
operated solenoid. The modulated scroll uses a single step
of unloading to go from full capacity to approximately 67%
capacity.
A single speed, high efficiency motor continues to run while
the scroll modulates between the two capacity steps.
Modulation is achieved by venting a portion of the gas in the
first suction pocket back to the low side of the compressor,
thereby reducing the effective displacement of the
compressor.
Full capacity is achieved by blocking these ports, thus
increasing the displacement to 100%. A DC solenoid in the
compressor controlled by a rectified 24 volt AC signal in the
external solenoid plug moves the slider ring that covers and
uncovers these ports.
The port covers are arranged in such a manner that the
compressor operates at approximately 67% capacity when
the solenoid is not energized and 100% capacity when the
solenoid is energized. The loading and unloading of the two
step scroll is done ”on the fly” without shutting off the motor
between steps.
NOTE: 67% compressor capacity translates to
approximately 75% cooling or heating capacity at the indoor
coil.
The compressor will always start unloaded and stay
unloaded for five seconds even when the thermostat is
calling for high stage capacity.
Quiet Shift
Quiet shift is a field selectable defrost mode (factory set to
OFF), which will eliminate occasional noise that could be
heard at the start of defrost cycle and restarting of heating
cycle. It is selected by placing DIP switch 3 on defrost board
(see Fig. 31) in the ON position.
When Quiet Shift switch is placed in ON position, and a
defrost is initiated, the following sequence of operation will
occur. Reversing valve will energize, compressor will turn off
for 30 seconds, and then turn back on to complete defrost.
At the start of heating after conclusion of defrost, reversing
valve will de- energize, compressor will turn off for another
30 seconds, and the fan will turn off for 40 seconds, before
starting in the heating mode.
FAST# 1185790 Defrost
Control
This defrost control is used in some non- communicating
heat pumps and has all the same functionality, speedups,
and troubleshooting as the FAST# 1174185 except for the
forced defrost timing when Quiet Shift- 2 is enabled.
Quiet Shift- 2 (non- communicating)
Quiet shift- 2 is a field selectable defrost mode (factory set to
OFF), which will reduce the occasional noise that could be
heard at the start of defrost cycle and restarting of heating
cycle. It is selected by placing DIP switch 3 on defrost board
in the ON position.
When Quiet Shift- 2 switch is placed in ON position, and
defrost is initiated, the following sequence of operation will
occur: The compressor will be de- energized for
approximately 1 minute, then the reversing valve will be
energized. A few seconds later, the compressor will be
re- energized and the normal defrost cycle starts. Once
defrost termination conditions have been met, the following
sequence will occur: The compressor will be de- energized
for approximately 1 minute, then the reversing valve will be
de- energized. A few seconds later, the compressor will be
re- energized and the normal heating cycle starts.
Defrost
The defrost control is a time/temperature control which has
field selectable settings of 30, 60, 90, or 120 minutes,
factory set to 90 minutes. These settings represent the
amount of time that must pass after closure of the defrost
thermostat before the defrost sequence begins.
The defrost thermostat senses coil temperature throughout
the heating cycle. When the coil temperature reaches the
defrost thermostat setting of approximately 32_F/0_C, it will
close, which energizes the DFT terminal and begins the
defrost timing sequence. When the DFT has been
energized for the selected time, the defrost cycle begins.
Defrost cycle is terminated when defrost thermostat opens
at approximately 65_F/18.3_C, or, or automatically after 10
minutes.
Defrost Speedup
To initiate a forced defrost, speedup pins (J1) must be
shorted with a flat head screwdriver for 5 seconds and
RELEASED. If the defrost thermostat is open, a short
defrost cycle will be observed (actual length depends on
Quiet Shift switch position). When Quiet Shift is off, only a
short 30 second defrost cycle is observed. With Quiet Shift
ON, the speedup sequence is one minute; 30 second
compressor off period followed by 30 seconds of defrost
with compressor operation. When returning to heating
mode, the compressor will turn off for an additional 30
seconds and the fan for 40 seconds.
If the defrost thermostat is closed, a complete defrost cycle
is initiated. If the Quiet Shift switch is turned on, the
compressor will be turned off for two 30 second intervals as
explained previously.
OF2
O
F1
ON
QUIET
SHIFT
120
30
60
60
30
90
INTERVAL TIMER
OFF
P3
DFT
O R W
2
Y C
T2 C C O
DFT
T1 Y
P1
J1
SPEEDUP
Speedup
Pins
Defrost interval
DIP switches
Quiet
Shift
A05378
Fig. 31 – Defrost Control
CHECK CHARGE
(See Charging Tables 10 & 12)
Factory charge amount and desired subcooling are shown
on unit rating plate. Charging method is shown on
information plate inside unit. To properly check or adjust
charge, conditions must be favorable for subcooling
charging. Favorable conditions exist when the outdoor
temperature is between 70_F/21.11_C and 100_F/37.78_C,
and the indoor temperature is between 70_F/21.11_C and
80_F/26.67_C. Follow the procedure below:
Unit is factory charged for 15ft (4.57 m) of lineset. Adjust
charge by adding or removing 0.6 oz/ft of 3/8 liquid line
above or below 15ft (4.57 m) respectively.