Residential Air Conditioners and Heat Pumps Using R-22 and Puron® Refrigerant: Application Guideline and Service Manual
Manufacturer reserves the right to change, at any time, specifications and designs without notice and without obligations.
15
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. Defrost
thermostats are used in Legacy models, a coil temperature thermistor is
used in Preferred and Evolution series units. See
.
A97517
Fig. 8 – 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
.) 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.
HK32EA001/007 DEFROST CONTROL
The HK32EA001/007 defrost control is used in all Legacy Series heat
pump models with copper tube condenser coils. 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
trouble shooting methods for this control.
Cooling Sequence of Operation (HK32EA001/007)
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 Bryant 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 (KH32EA001)
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 (HK32EA001/007)
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°F, 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 at approximately 65°F, or automatically after 10 minutes.
A05332
Fig. 9 – HK32EA001/007 Defrost Control
Troubleshooting (HK32EA001/007)
If outdoor unit will not run:
1. Does the Y input has 24 volts from thermostat? If not, check
thermostat or wire. If yes proceed to #2
2. The Y spade terminal on the circuit board should have 24 volts if Y
input is energized. This output goes through the pressure switches
and to the contactor. If 24 volts is present on the Y spade terminal,
and the contactor is not closed, check voltage on contactor coil. If
no voltage is present, check for opened pressure switch.
3. If voltage is present and contactor is open, contactor may be
defective. Replace contactor if necessary.
4. If contactor is closed and unit will still not run, check wiring,
capacitor and compressor
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
