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: Service Manual
Manufacturer reserves the right to change, at any time, specifications and designs without notice and without obligations.
11
Check Defrost Thermostat
There is a liquid header with a brass distributor and feeder tubes going
into outdoor coil. At the end of 1 of the feeder tubes, there is a 3/8-in.
OD stub tube approximately 3 in. long. (See
.) The defrost
thermostat should be located on stub tube. Note that there is only 1 stub
tube used with a liquid header, and on most units it is the bottom circuit.
A97517
Fig. 13 – Defrost Thermostat Location
NOTE:
The defrost thermostat must be located on the liquid side of the
outdoor coil on the bottom circuit and as close to the coil as possible.
Defrost Control Board
Troubleshooting defrost control involves a series of simple steps that
indicate whether or not board is defective.
NOTE:
This procedure allows the service technician to check control
board and defrost thermostat for defects. First, troubleshoot to make sure
unit operates properly in heating and cooling modes. This ensures
operational problems are not attributed to the defrost control board.
Defrost Control
The demand defrost control is used in all Mobile Home series heat pump
models. Its features include a selectable initial defrost interval of either
30, 60, 90 or 120 minutes and an automatic interval adjustment that is
based off the previous defrost cycles. This section describes the
sequence of operation and troubleshooting methods for this control.
Cooling Sequence of Operation
On a call for cooling, thermostat makes R-O, R-Y, and R-G. Circuit R-O
energizes reversing valve switching it to cooling position. Circuit R-Y
sends low voltage through the safeties and energizes the contactor,
which starts the compressor and energizes the T1 terminal on the circuit
board. This will energize the OF2 fan relay which starts the outdoor fan
motor.
When the cycle is complete, R-Y is turned off and compressor and
outdoor fan should stop. With corporate thermostats, the O terminal
remains energized in the cooling mode. If the mode is switched to heat
or Off, the valve is de-energized. There is no compressor delay built into
this control.
A05332
Fig. 14 – Defrost Control
Heating Sequence of Operation
On a call for heating, thermostat makes R-Y, and R-G. Circuit R-Y sends
low voltage through the safeties and energizes the contactor, which starts
the compressor and energizes the T1 terminal on the circuit board. The
T1 terminal energizes the defrost logic. This will energize the OF2 fan
relay start the outdoor motor. The T1 terminal must be energized for
defrost to function.
When the cycle is complete, R-Y is turned off and the compressor and
outdoor fan should stop. There is no compressor delay built into this
control.
Defrost Sequence
The defrost mode is factory set to an initial 60-minute time interval. It
may also be adjusted to an initial interval of 30, 60, 90 or 120 minutes.
During operation, the control may vary based on the previous defrost
interval and previous defrost period. If the previous defrost period is less
than 2 minutes for two consecutive defrost cycles the control will
lengthen the defrost interval by 15 minutes, up to a maximum of 120
minutes or 30 minutes greater than the original setpoint, whichever
comes first. If the previous defrost period is more than 5 minutes for two
consecutive defrost cycles the control will shorten the defrost interval by
15 minutes, down to a minimum of 30 minutes or 30 minutes from the
original setpoint, whichever is first.
The defrost thermostat senses coil temperature throughout the heating
cycle. When the coil temperature reaches the defrost thermostat setting,
it will close, which energizes the DFT terminal and begins the defrost
timing sequence. When the DTF has been energized for the selected
time, the defrost cycle begins, and the control shifts the reversing valve
into cooling position, and turns the outdoor fan off. This shifts hot gas
flow into the outdoor coil which melts the frost from the coil. The
defrost cycle is terminated when defrost thermostat opens, or
automatically after 10 minutes.
FEEDER TUBE
DEFROST
THERMOSTAT
A97517
STUB TUBE
COIL
OUTDOOR FAN
RELAY
Y OUTPUT TO PRESSURE
SWITCHES AND CONTACTOR
THERMOSTAT INPUTS
T1 - ENABLES DEFROST
TIMER.MUST BE
ENERGIZED FOR
DEFROST TIMER
TO START
C - COMMON
O - REVERSING VALVE
SPEEDUP
DEFROST THERMOSTAT
MUST BE CLOSED BEFORE
DEFROST TIMER BEGINS