25TPB7: Installation Instructions
Manufacturer reserves the right to change, at any time, specifications and designs without notice and without obligations.
9
Sequence of Operation
NOTE: Defrost control board is equipped with 5 minute lockout timer
that is initiated upon any interruption of power.
Turn on power to indoor and outdoor units. Transformer is energized.
These models utilize a 2-stage indoor thermostat. With a call for first
(low) stage cooling or heating, the outdoor fan and low-stage
compressor are energized. If low-stage cannot satisfy cooling or heating
demand, high-stage is energized by the second (high) stage of the indoor
thermostat. After the second stage is satisfied, the unit returns to
low-stage operation until second stage is required again. When both,
first and second stage cooling or heating are satisfied, the compressor
will shut off.
Cooling
With first stage cooling, Y1 and O are powered on; and with second
stage cooling, Y2, Y1 and O are powered on. The O energizes the
reversing valve, switching it to cooling position. The Y1 signal sends
low voltage through the safeties and energizes the T1 terminal on the
circuit board. If the compressor has been off for 5 minutes, or power has
not been cycled for 5 minutes, the OF2 relay and T2 terminal will
energize. This will close the contactor and start the outdoor fan motor
and compressor. When the cycle is complete, Y1 is turned off, stopping
the compressor and outdoor fan. The 5 minute time guard 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.
Heating
With first stage heating, Y1 is powered on; with second stage heating,
Y2 and Y1 are powered on. The Y1 signal sends low voltage through
the safeties and energizes the T1 terminal on the circuit board. If the
compressor has been off for 5 minutes or power has not been cycled for
5 minutes, the OF2 relay and T2 terminal will energize. This will close
the contactor and start the outdoor fan motor and compressor.
When the cycle is complete, Y1 is turned off, stopping the compressor
and outdoor fan. The 5 minute time guard 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 vents, 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 vents.
The vent 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-2
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 60
minutes. These settings represent the initial time period that must pass
after closure of the defrost thermostat before the defrost sequence
begins, after which the defrost period varies with defrost length. If Quiet
Shift 2 is enabled, the variable defrost intervals will be disabled to
provide options where using a specific setting is desired. In this case the
30, 60, 90, or 120 minutes setting will not change unless the dipswitch is
changed and power is cycled.
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, 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- 2 switch position). When Quiet Shift- 2 is off,
only a short 30 second defrost cycle is observed. With Quiet Shift- 2 ON,
the speedup sequence is approximately 3 minutes; 1 minute 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 1 minute.
If the defrost thermostat is closed, a complete defrost cycle is initiated. If
the Quiet Shift- 2 switch is turned on, the compressor will be turned off
for two 1 minute intervals as explained previously.
A05378
Fig. 11 – Defrost Control
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