Pressure Switches; Defrost Thermostats; Defrost-Control Board - Carrier R-22 Application Manual And Service Manual

Step 7—Pressure Switches
Pressure switches are protective devices wired into control circuit
(low voltage). They shut off compressor if abnormally high or low
pressures are present in the refrigeration circuit. Depending on unit
model, you may find a low- and/or high-pressure switch in system.
LOW-PRESSURE SWITCH
Located on suction line of condensing unit only, the low-pressure
switch protects against low-suction pressures caused by such
events as loss of charge, low airflow across indoor coil, dirty
filters, and so forth. It opens on a pressure drop at about 27 psi. If
system pressure is above this, switch should be closed. To check
switch, turn off all power to unit, disconnect leads on switch, and
apply ohmmeter leads across switch. You should have continuity
on a good switch. Because these switches are attached to refrig-
eration system under pressure, it is not advisable to remove this
device for troubleshooting unless you are reasonably certain that a
problem exists. If switch must be removed, remove and recover all
system charge so that pressure gages read 0 psi.
Wear safety glasses and gloves when working with refriger-
ants.
Apply heat with torch to solder joint and remove switch. Wear
safety glasses when using torch. Have quenching cloth available.
Oil vapor in line may ignite when switch is removed. Braze in
1/4-in. flare fitting and screw on replacement pressure switch.
HIGH-PRESSURE SWITCH
Located on discharge line, the high-pressure switch protects
against high-discharge pressures caused by such events as over-
charge, condenser-fan motor failure, system restriction, and so
forth. It opens on pressure rise at about 435 psi. If system pressures
go above this setting during abnormal conditions, the switch
opens. Do not attempt to simulate these system abnormalities as
high pressures pose a serious safety hazard. High-pressure switch
is also checked with an ohmmeter similar to checking low-pressure
switch. If system pressure is below 435 psi, the switch shows
continuity. It is replaced in the same manner as low-pressure
switch. Observe all safety precautions.
LIQUID-LINE PRESSURE SWITCH
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 7 psi when the heating
piston is in the liquid valve or 27 psi when the heating piston is in
the liquid line. Troubleshooting and removing this switch is
identical to procedures used on other switches. Observe same
safety precautions.
Step 8—Defrost Thermostats
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 30° ± 3°F and
open at 80° ± 5°F.
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.
Step 9—Defrost-Control Board
Solid-state defrost boards used on heat pumps replace electrome-
chanical timer and defrost relay found on older defrost systems.
The defrost-control board can be field-set to check need for defrost
every 30, 50, or 90 minutes of operating time by connecting the
jumper (labeled W1 on the circuit board) to the terminal for the
defrost time desired. The board is set at factory for 90 minutes. The
defrost period is field-selectable, depending upon geographic areas
and defrost demands. Two types of defrost boards are used. Their
functions are described in the sections to follow.
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.
HK32FA003, 006 DEFROST CONTROL
This control board utilizes screw terminals for the low-voltage
field wiring. The board has a feature that allows the heat pump to
restart in defrost if room thermostat is satisfied during defrost. To
troubleshoot the board, perform the following items.
1. Turn thermostat to OFF. Shut off all power to outdoor unit.
2. Remove control-box cover for access to electrical components
and defrost-control board.
3. Disconnect defrost-thermostat leads from control board and
connect to ohmmeter. Thermostat leads are the black, insu-
lated wires connected to DFT and R terminals on control
board. Resistance reading may be zero (indicating closed
defrost thermostat) or infinity ( for open thermostat) depend-
ing on outdoor temperature.
4. Jumper between DFT and R terminals on control board as
shown in Fig. 19.
5. Disconnect outdoor fan motor lead from OF2. Tape lead to
prevent grounding.
6. Turn on power to outdoor unit.
7. Restart unit in heating, allowing frost to accumulate on
outdoor coil.
8. After a few minutes in heating, liquid-line temperature at
defrost thermostat should drop below closing set point of
defrost thermostat of approximately 30°F. Using ohmmeter,
check resistance across defrost-thermostat leads. Resistance of
zero indicates defrost thermostat is closed and operating
properly.
9. Remove protective cover from TP1 and TP2 speed-up termi-
nals. Install jumper wire on speed-up terminals. This reduces
the timing sequence to 1/60 of original time. (See Fig. 20.)
Since Fig. 20 shows timing cycle set at 30 minutes, unit
initiates defrost within approximately 30 sec; if setting is at 50
minutes, within 50 sec; 90 minutes, within 90 sec. When you
hear the reversing valve change position, remove protective
cover/jumper. Otherwise, control will terminate normal 10-
minute defrost cycle in approximately 10 sec.
Exercise extreme caution when shorting speed-up pins. If pins
are accidentally grounded, damage to the control board will
occur.
10. Unit is now operating in defrost mode. Using voltmeter, check
between C and W2 as shown in Fig. 21. Reading on voltmeter
should indicate 24v. This step ensures defrost relay contacts
18