Service Manual

12
To replace
switch:
1. Apply heat with torch to solder joint and remove
switch.
PERSONAL INJURY HAZARD
Failure to follow this caution may result in personal
injury.
Wear safety glasses when using torch. Have
quenching cloth available. Oil vapor in line may ignite
when switch is removed.
CAUTION
!
2. Braze in 1/4- in. flare fitting and replace pressure
switch.
Loss of Charge Switch (HP
Only)
Located on liquid line of heat pump only, the liquid line
pressure switch functions similar to conventional
low- pressure switch.
Because heat pumps experience very low suction
pressures during normal system operation, a conventional
low- pressure switch cannot be installed on suction line.
This switch is installed in liquid line instead and acts as
loss- of- charge protector. The liquid- line is the low side of
the system in heating mode. It operates identically to
low- pressure switch except it opens at 23 (+/- 5) psig for
R- 410A and 7 (+/- 5) psig for R- 22 and closes at 55 (+/- 5)
psig for R- 410A and 22 (+/- 5) psig for R- 22. Two- stage
heat pumps have the low- pressure switch located on the
suction line. The two- stage control board has the capability
to ignore low- pressure switch trips during transitional
(defrost) operation to avoid nuisance trips. Troubleshooting
and removing this switch is identical to procedures used on
other switches. Observe same safety precautions.
Defrost Thermostat
Defrost thermostat signals heat pump that conditions are
right for defrost or that conditions have changed to terminate
defrost. It is a thermally actuated switch clamped to outdoor
coil to sense its temperature. Normal temperature range is
closed at 32_ +3_F and open at 65_ +5_F. De fros t
thermostats are used in non- communicating models, a coil
temperature thermistor is used in Communicating units.
FEEDER TUBE
DEFROST
THERMOSTAT
STUB TUBE
A97517
Fig. 7 – Defrost Thermostat Location
Check Defrost
Thermostat
There is a liquid header with a distributor and feeder tube
going into outdoor coil. At the end of 1 of the feeder tubes,
there is a 3/8- in. OD stub tube approximately 2 in. (50.8
mm) long. (See Fig. 7.) 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.
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. For a copper stub tube, the DFT will
have a copper cup. For an aluminum stub tube, the DFT will
have an aluminum cup. Don’t interchange material types.
Defrost Control Boards
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.
FAST # 1173636 / 1177927 / 1190281
DEFROST CONTROL
The FAST # 1173636 / 1177927 / 1190281 defrost control is
used in all Performance heat pump models. Its features
include selectable defrost intervals of 30, 60, 90 minutes,
and standard defrost speed up capability. This section
describes the sequence of operation and troubleshooting
methods for this control.
Cooling Sequence of Operation (FAST # 1173636
/
1177927 /
1190281)
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 (ODF relay for 1190281).
When the cycle completes, R- Y is turned off. Compressor
and outdoor fan should stop. With ICP 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.
Heating Sequence of
Operation
(FAST # 1173636 / 1177927 /
1190281)
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 (ODF relay for
1190281). 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 (FAST # 1173636 / 1177927 / 1190281
)
The defrost control is a time/temperature control that has
field selectable settings of 30, 60, and 90 minutes. These
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_Fitwillclose,
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 at approximately
65_F, or automatically after 10 minutes.