Transcript No Slide Title

The outdoor section
Now that we’ve checked our
thermostat and air handler, we’re
ready to check the outdoor unit.
The outdoor section
• Setting the outdoor unit on the shipped with
ISO pads, will help reduce sound
transmission through the ground.
• The concrete pad should not be touching the
structure, for this same reason.
• The unit should be set to prevent air recirculation across it’s coil. See the
installation instructions for that unit.
HEAT PUMP
REVERSING VALUE
ILLUSTRATION
SUCTION LINE
OUT DOOR COIL
TP-4
INDOOR COIL
. . .
TP-2
TP-3
.
TP-1
COMPRESSOR DISCHARGE
TO
ACCUMULATOR
TO
INDOOR COIL
FROM
OUTSIDE COIL
TP-4
. . .
TP-2
TP-3
FROM COMPRESSOR
DISCHARGE LINE
HEATING MODE
.
TP-1
5F
MAX TEMP.
DIFFERENCE
TO
ACCUMULATOR
TO
OUTDOOR COIL
TP-4
FROM
INDOOR COIL
. . .
.
TP-3
TP-2
TP-1
FROM COMPRESSOR
DISCHARGE LINE
COOLING MODE
5F
MAX TEMP.
DIFFERENCE
COOLING CONDITION
INDOOR COIL
SAT. SUCT. T. 41F
ENT. AIR T. 76F
4-WAY
VALVE
METERING
DEVICE
SUBCOOLING
10F
SUCT. P. 70 PSIG
SUCT. T. 52F
SUPERHEAT 11F
DISCHARGE
PRESSURE
260 PSIG
OUTDOOR COIL
SAT. COND. T. 120F
ENT. AIR T.
90F
COMPRESSOR
HEATING
CONDITIONS
INDOOR COIL
SAT. COND. T. 95F
ENT. AIR T. 70F
4-WAY
VALVE
METERING
DEVICE
SUBCOOLING
10F
SUCT. P. 43 PSIG
SUCT. T. 35F
SUPERHEAT 10F
DISCHARGE
PRESSURE
182 PSIG
OUTDOOR COIL
SAT. SUCT. T. 20F
ENT. AIR T.
45F
COMPRESSOR
HEAT PUMP
• TOTAL HEAT REJECTED EQUALS
HEAT ABSORBED +
HEAT OF COMPRESSOR
HEAT PUMP
• METERING DEVICES
– HEATING CYCLE, REFRIGERANT IS
METERED TO THE OUTDOOR COIL
– COOLING CYCLE, REFRIGERANT IS
METERED TO THE INDOOR COIL
HEAT PUMP
• METERING DEVICES
• REFRIGERANT METERED TO COIL
WHICH ABSORBS HEAT
• HEATING CYCLE REFRIGERANT
METERED TO OUTDOOR COIL
• COOLING CYCLE REFRIGERANT
METERED TO INDOOR COIL
METERING DEVICES
• CAPTUBE
• FIXED ORFICE
• TXV
METERING DEVICES
FLOW BYPASSES METERING DEVICE
CHECK VALVE
OPEN
METERING
DEVICE
FLOW IS METERED
CHECK VALVE
SHUT
METERING
DEVICE
The outdoor coil
Not in your books
• Spine fin coil is used for it’s efficiency.
• The majority of all heat transfer is done on
the leading edge (the tip, where the air and
coil first come into contact) of the fin.
• A spine fin coil is much more efficient than
a plate fin coil. So much so that it can do
the same job with much less refrigerant.
• Spine fin coils can easily be repaired.
The outdoor coil
Not in your books
• As mentioned earlier the outdoor coil will
produce run off during defrost.
• This water is normal and has not been
known to cause damage in the past!
• This water must drain freely from the unit.
Any sort of drain/collection system would
need to have some form of a heater to
keep from freezing up and damaging the
outdoor coil.
The role of the defrost control
• To extract heat from the outdoor air, the
heat pump must lower it’s outdoor coil
temperature below that of the outdoor
ambient.
• Depending on the humidity and
temperature, frost may form on the outdoor
coil.
• This frost will insulate the coil from the
outdoor air, reducing it’s ability to absorb
the heat from the outdoor air.
•
•
•
•
•
•
•
•
•
-
HEAT PUMP DEFROST
CONTROLS
CONTROLS THE DEFROST CYCLE IN A HEAT PUMP
DURING HEATING OPERATION.
THE DEFROST CONTROL CONTROLS THE
FOLLOWING FUNCTIONS:
1SWITCH OVER VALVE OR REVERSING VALVE
2OUTDOOR FAN MOTOR
3ELECTRIC OR GAS AUX. HEAT
4INDICATES A FAULT HAS OCCURRED
(OPTIONAL)
CONTROL THAT ESTABLISHES THE NEED FOR A
DEFROST
BOTH TIMED AND DEMAND CONTROLS ARE USED
TODAY
ELECTRO MECHANICAL TIMERS AND PRESSURE
SWITCHES USED IN THE PAST
Heat pump defrost controls
• Several types of defrost controls have been
used through the years.
• We will discuss their function and
diagnostics of each type.
• The newer solid state and Demand Defrost
controls will be the main focus of our time
today.
DEFROST CYCLE
• SYSTEM IN COOLING MODE-ENERGIZE REVERSING VALVE
– DIRECTS HOT GAS TO OUTDOOR COIL
TO MELT THE FROST
As the frost accumulates on the outdoor coil,
the systems capacity is reduced.
Ö To remove the frost/ice from the outdoor coil,
the system will shift itself into a variation of
the cooling mode.
Ö During a defrost cycle the outdoor fan will
stop, leaving the heat in the coil to remove the
frost.
Ö The type of defrost control system will
dictate how much frost/ice is allowed to
accumulate prior to initiating a defrost.
DEFROST CYCLE
• TEMPERING INDOOR AIR
– ELECTRIC HEAT IS TURNED ON TO
TEMPER THE AIR DURING DEFROST
CYCLE.
DEFROST CONTROLS
• THE MUCH OLDER EQUIPMENT
UTILIZED ELECTRO-MECHANICAL
TIME CLOCKS.
• ELECTRONIC TIMER - TIME &
TEMPERATURE DEFROST
• SOLID STATE - DEMAND DEFROST
DEFROST CYCLE
• OUTDOOR FAN OFF
– ENHANCES DEFROST
Electro-Mechanical systems
ç All electro-mechanical systems are a
“time/temperature” based control.
ç A predetermined amount of time must pass
before a defrost cycle can be initiated.
ç As mentioned earlier, the outdoor
conditions have major impact on the
amount of defrost needed.
ç These systems are temperature activated as
well.
Large amounts of frost may accumulate
before the “Time” has elapsed.
• This results in lower seasonal efficiency.
• The reduced capacity will result with a greater
dependence on a secondary heat source.
ELECTRONIC TIME-TEMPERATURE
DEFROST CONTROL
• 50 - 70 - 90 MINUTE COMPRESSOR RUN TIME
SELECTION
• 10 MINUTE TIME OVERRIDE IN DEFROST
CYCLE
• TWO TEST PINS - ADVANCES ELECTRONIC
TIMER AND PUTS SYSTEM INTO DEFROST
CYCLE FOR TESTING
• ON BOARD DEFROST RELAYS FOR OUTDOOR
FAN, SOV VALVE AND AUXILIARY HEAT
• DEFROST TERMINATED ON TIME OR
TEMPERATURE
• COMPRESSOR RUN TIME IS KEPT ONLY WHEN
COIL THERMOSTAT IS CLOSED
• The defrost thermostat is set to close at 25º.
• When the DT is closed, supplying 24 VAC
to the D terminal, jumpering the test pins
will speed up the internal clock.
• If you are testing the system, and have a
jumper from R to D, remove the jumper as
soon as the system shifts into defrost to
prevent excessive refrigerant pressures.
• These controls will default to the 90 minute
setting if the selector jumper is not connected.
Which is how the equipment is shipped.
• I recommend one of the shorter time
settings, 50 or 70 minutes.
• The water run off from a unit in defrost is
pure water, and has never been reported to
caused roof damage to date!
• Trying to catch the water could damage the
outdoor coil (if the collected water froze and
backed up under the coil).
DFC
K2
CNT1152 1
OR
CNT1642
K1
M1
OD
FAN
M2
90 70 50
TST EDR R
R B B
X2 Y
24V AC
24V COMMON
INDOOR
THERMOSTAT
MS
SC
1
CNT1152 may be replaced
by CNT 1642
DT
O
O
D
T
K2
DFC
CNT1642
K1
M1
OD
FAN
M2
90
70
50
TST
EDR R
R
B
B
X2 Y
O
O
DT
T
X2
INDOOR
THERMOSTAT
Y
O
T
SC
DT
EDR
EDR
24V AC
24V COMMON
ECONOMIZER
(OPTIONAL)
EDC OR LPCO
MS
INDOOR
THERMOSTAT
G
TR TR1
F
COMMON FOR
ELECTRIC
HEATER
TIMER DEFROST CONTROL
TST EDR
R
R
B
B
X2
Y
O
O
D
T
TST EDR
R
R
B
B
X2
Y
O
O
D
T
TST EDR
R
R
B
B
X2
Y
O
O
D
T
TST EDR
R
R
B
B
X2
Y
O
O
D
T
TST EDR
R
R
B
B
X2
Y
O
O
D
T
TST EDR
R
R
B
B
X2
Y
O
O
D
T
TST EDR
R
R
B
B
X2
Y
O
O
D
T
There are several versions of these DFC boards.
Many feature the exact same functions, with the
only difference being the terminal for the defrost
thermostat being labeled “D” or “DT”.
TST EDR
R
R
B
B
X2
Y
O
O
D
T
There are several versions of these DFC boards.
Many feature the exact same functions, with the
only difference being the terminal for the defrost
thermostat being labeled “D” or “DT”.
TST EDR
R
R
B
B
X2
Y
O
O
D
T
There are several versions of these DFC boards.
Many feature the exact same functions, with the
only difference being the terminal for the defrost
thermostat being labeled “D” or “DT”.
TST EDR
R
R
B
B
X2
Y
O
O
D
T
There are several versions of these DFC boards.
Many feature the exact same functions, with the
only difference being the terminal for the defrost
thermostat being labeled “D” or “DT”.
TST EDR
R
R
B
B
X2
Y
O
O
D
T
There are several versions of these DFC boards.
Many feature the exact same functions, with the
only difference being the terminal for the defrost
thermostat being labeled “D” or “DT”.
TST EDR
R
R
B
B
X2
Y
O
O
D
T
There are several versions of these DFC boards.
Many feature the exact same functions, with the
only difference being the terminal for the defrost
thermostat being labeled “D” or “DT”.
TST EDR
R
R
B
B
X2
Y
O
O
D
T
DEMAND
DEFROST
CONTROLS
DEMAND DEFROST
• FEATURES
–
–
–
–
–
–
ADAPTABILITY
LEARNING PROCESS
DEFROST ON DEMAND
DIAGNOSTICS
TIME OVERRIDE
SOFT SWITCHOVER
DEMAND DEFROST CONTROLS
HEAT PUMP DEMAND DEFROST
CONTROLS
• -
THE DEMAND DEFROST CONTROL WAS
DEVELOPED BY TRANE IN 1984 AND IS PATENTED.
• HEAT PUMP SYSTEMS WITH DEMAND DEFROST
CONTROLS BENEFIT FROM A 6% HEATING SEASONAL
PERFORMANCE FACTOR (HSPF) ADVANTAGE.
• THERMISTOR SENSORS SENSE THE OUTDOOR
AMBIENT TEMPERATURE AND OUTDOOR COIL
TEMPERATURE.
• THE DIFFERENCE BETWEEN THE OUTDOOR
TEMPERATURE AND THE OUTDOOR COIL
TEMPERATURE IS CALLED THE DELTA TEMPERATURE
OR JUST DELTA-T.
HEAT PUMP DEMAND DEFROST
CONTROLS
• TRANE DEMAND DEFROST CONTROLS
OPERATE IN AN ADAPTIVE PROCESS TO
OPTIMIZE THE HEAT PUMP PERFORMANCE.
• THE DEMAND DEFROST CONTROL WILL
ADAPT TO THE HEAT PUMP IT IS INSTALLED
ON AND LEARN ITS OPTIMUM DEFROST
CONDITIONS AND STORE THEM IN MEMORY.
• THE DEMAND DEFROST CONTROL WILL
ADJUST THE TIME BETWEEN DEFROST
CYCLES TO MATCH THE WEATHER
CONDITIONS.
The Defrost Cycle is Initiated
• As ice builds on the coil, the delta-t of the system increases
until it reaches the initiate value, and the control starts the
defrost process.
• The initiate value is not constant, but instead, is a value that
the defrost control has learned will provide a fast, thorough
defrost at a given outdoor temperature.
• Low initiate values waste energy by defrosting too often, while
high initiate values fail to defrost the coils thoroughly.
• The control is continually varying the initiate value slightly to
learn the best value.
• It measures the success of each initiate value by measuring the
delta-t of the system during the next heating cycle.
DEFROST IS TERMINATED
• Once the temperature of the outdoor coil reaches the
termination value, the defrost control turns on the outdoor
fan. After waiting the soft-switch over time, it changes the
switch over valve to the heating mode.
• This soft-switch over delay time, 12 seconds, gives the
outdoor coil time to cool, reducing the refrigerant pressure
and reducing the surge that occurs when the switch over
valve is switched.
• If the compressor or “Y” cycles off during defrost, the
switchover valve remains energized but the “X2” output
will be OFF. If “Y” cycles off during the soft-switchover
time, the switchover valve is switched OFF immediately.
Normal operation
Demand Defrost Control Operation
•
•
•
•
•
•
•
Requirements for defrost initialization
1 - The outdoor temperature must be less than 52 F
2 -The coil temperature must be less than 33 F
3 -The Y line must be energized for a least 128
seconds
4 -The delta-T must be greater than the initiate
temperature
5 - It must be time for a timed defrost
6 -30 minutes of run time have passed since the unit
was first powered up
Demand Defrost Control
Operation
• If the unit is placed into a forced defrost by
placing the test jumper in the FRC DFT position
and Y is energized, none of the above conditions
need to be met.
Requirements for Defrost
Termination
• The coil temperature must exceed the terminate
value.
• If the outdoor temperature is greater than 22 F, the
terminate value is 47 F.
• If the outdoor temperature is less than 10 F, the
terminate value is 35 F.
• If the outdoor temperature is between 10 F to 22 F,
the terminate value is 25 F plus the outdoor
temperature.
OR
• The defrost control has a 15 minute override time.
The Test Mode
• The control may be placed into a test mode
by placing the RED jumper wire on the
TST pin.
• This will cause the LED to blink rapidly
and the operation of the control will be
sped up by a factor of 10.
• The controller will run normally once the
jumper is removed from the TST pin.
Fault Indications
• There are three types of faults the demand defrost control
can experience.
• All three faults can cause the defrost control to change
from a demand defrost control to a timed defrost control
which defrosts the unit every 30 minutes.
• When the control goes into a timed defrost mode due to a
FAULT A or FAULT C, on units that have the F line
feature, it alerts the home owner by energizing the F line
once every second which flashes a fault light on the
thermostat.
• If the thermostat is switched to the emergency heat
position, the F line will be energized, and the defrost
control will be cleared of all faults after a few seconds.
Fault Indications
•
•
•
•
•
FAULT A is indicated by 2 led flashes per second.
FAULT A is caused by one of two conditions:
1 - During heating the system's delta-T is unusually low.
(The capacity of the system is lower than it should be.)
2 - After 12 minutes of operation since the defrost
termination, delta-T is greater than the initiate value.
• After running in a FAULT A condition for 30 minutes, the
defrost control will indicate it by flashing the LED. After
running 2 hours with this fault, the demand defrost control
will then change to a timed defrost operation, and the
homeowner will be notified via the F line.
Fault Indications
• FAULT A can be cleared by any one of the following
operations:
• 1 - If the control has two normal defrost cycles
•
(If the fault has been present long enough for the
Fault light to start Flashing, it will flash until the first
normal defrost.)
• 2 - If power is removed from the defrost control for
more than 30 seconds.
• 3 - If the F line is energized for a few seconds.
Fault A
Fault A
Fault A
Fault A
Fault A
Fault A
Fault A
Fault A
Fault A
Fault Indications
• FAULT B is indicated by 3 led flashes per
second.
• FAULT B is caused by 10 successive failures
of the heat pump to terminate on temperature
during the defrost cycle.
• The unit will stop defrosting after 15 minutes
due to the override timer.
• Once a FAULT B occurs, the control will
indicate it by flashing the led. After FAULT B
occurs 10 times, the control will act as a timer
defrost control.
Fault Indications
• FAULT B can be cleared by any one of the
following operations:
• 1 - If the control reaches the terminate value
during one of the timed defrost cycles.
•
(Since this fault is pretty common in cold
climates with high winds, this fault does not alert
the home owner via the F line.)
• 2 - If power is removed from the defrost control
for more than 30 seconds.
• 3 - If the F line is energized for a few seconds.
Fault Indications
• FAULT C is indicated by 3 led flashes per
second.
• FAULT C is caused by 15 successive faults
in which the unit has an unusually high
delta-T 15 minutes after a defrost
termination.
• (Poor air flow over outdoor coils - they are
probably blocked by ice.)
Fault Indications
• FAULT C can be cleared by any one of the
following operations:
• 1 - If the unit has a normal delta-T 15 minutes
after a defrost. (If the fault has been present
long enough for the Fault light to start flashing,
it will continue to flash until there is a defrost
cycle without a high delta-T fault.)
• 2 - If power is removed from the defrost control
for more than 30 seconds
• 3 -If the F line is energized for a few seconds
Fault B or C
Fault B or C
Fault B or C
Fault B or C
Fault B or C
Fault B or C
Fault B or C
Fault B or C
Fault B or C
Fault B or C
Fault B or C
Fault B or C
Fault B or C
Combination Fault
Indications
• FAULT A & FAULT B, indicated by 4 led flashes per second, is
caused by 60 or more FAULT A's occurring and one or more
FAULT B's occurring.
• FAULT A & FAULT C, indicated by 4 led flashes per second, is
caused by 60 or more FAULT A's occurring and one or more
FAULT C's occurring.
• Combination faults will notify the home owner by toggling the F
line.
• Combination faults can be cleared by any one of the following
operations:
• 1 - If the F line is energized for a few seconds
• 2 - If power is removed from the defrost control for more than 30
seconds
Power-up Initial Defrost
• After the defrost board is powered up, its first
defrost cycle will be a timed cycle.
• On the newer boards, the unit must run during
defrosting conditions for 30 minutes while some
older boards must run for 45 minutes.
• Assuming there are no faults, subsequent defrost
cycles will be performed on demand.
• The delay in each case can be reduced to 1/5 this
time by placing the red jumper lead on the TST
terminal.
Forcing the Unit to Defrost
• Placing the red jumper wire on the FRC DFT pin, the
defrost board can be forced to initiate a defrost
immediately even if outside temperature conditions are
not met.
• The only requirement is that the compressor is running
or the “Y” signal is present.
• The defrost will be terminated normally when the
outdoor coil becomes hot enough.
• If the jumper is still on the FRC DFT lead upon defrost
termination, it will initiate the defrost cycle again.
No Fault Found
• DON’T REPLACE IT!
– If LED blinks,
– & Sensors ohm out OK,
– & Forced defrost works.
• Look Elsewhere For Cause(s)
• Package Unit Ambient Sensor Bracket
– Added for better ambient sensing
ODS-A Thermistor Failures
• Cause
– Field miss-wire during installation
• Solution
– X2 wire color changed from BR/X2 to BK
• DO NOT REPLACE CONTROL
– Use RES0118 ODS-A kit
– Needed ONLY for GE “blue light” thermostat
Temperature Sensors
• The temperature sensors are temperature dependent
resistors encased in a heat-shrunk plastic shroud and
sealed from air and moisture by a hot glue adhesive fill.
• The resistance of the sensor decreases as temperature
increases according to a specified curve. See the
temperature / resistance curve provided with the service
information.
• It is rare for a temperature sensor to fail, and most that
have been replaced in warranty were OK, or they had
been damaged by rough handling.
Temperature Sensors
• A common problem with sensors is damage
inflicted by forcing ohmmeter probes into their
connectors. This permanently bends the
contact causing an intermittent connection.
• On some package units, the sensor leads have
been pinched by the access panel. Checking
the sensor resistance to ground will detect this
problem.
Resistance
9500
6400
5400
4450
10000
3700
3200
2700
2400
9000
1950
1700
1450
1150
8000
1070
920
720
620
7000
545
480
420
370
330
6000
285
260
230
205
5000
185
165
145
130
4000
115
110
Defrost Thermister Resistance
DEFROST THERMISTOR RESISTANCE
10000
9000
8000
7000
5000
4000
3000
3000
2000
2000
1000
1000
-40
0
OUTDOOR TEMPERATURE
115
95
85
75
105
35
Outdoor Te mpe ra ture
75
65
55
45
35
20
10
0
-10
-20
-30
0
-40
Resistance
6000
115
Thermostat wire slid
in to openings for
control board pins
(18awg. MAX). Take
readings here.
To avoid damage to the connectors when
ohming out insert some short pieces of
thermostat wire into the sensor connectors,
NOT your meter leads.
Check Out Procedure
• The following tests are to be performed to check for
proper operation after installation of the demand
defrost control.
• Step 1: Apply power to the system and operate in
heating mode. The switchover valve should be off
(heating mode), and the outdoor fan should be on.
• Step 2: Check to be sure the led on the defrost board
is blinking once every second and the fault light on
the thermostat is off.
• Step3: Force the system to defrost by placing the
red jumper wire on the FRC DFT pin.
Check Out Procedure
• As the unit goes through the defrost process, the
switchover valve should be energized, the
auxiliary heat should be on and the outdoor fan
motor should be off.
• Step 4: Monitor the system to be sure it stops
defrost within 15 minutes.
• Step 5: At the end of the defrost cycle, the
outdoor fan should come on, and after a 12
second delay, the unit will switch back to the
heating mode.
Check Out Procedure
• Step 6: If the board has the fault light feature (G15-G17&
G19-G20), test this feature by placing the red jumper wire on
the FRC FLT pin for a few seconds then return the red
jumper wire to the NORM pin.
• This should cause the led on the defrost board to blink four
times a second, and the indoor fault indicator on the
thermostat should be blinking once a second.
• Step 7:
Switch the indoor thermostat to the emergency
heating position for at least 30 seconds and then back to the
heat pumps position.
• The indoor fault indicator should stop blinking and the LED
should blink once a second indicating that everything is
normal.
Helpful Hints in Case of
Difficulty
• Led fails to blink on the defrost board
•
If the led is off, check between R and B
to be sure 24 volts is present.
•
Remove power for one minute and reapply power. Check to see if the LED
starts flashing.
Helpful Hints in Case of
Difficulty
• LED flashes, but the unit fails the forced defrost test.
• 1. Be sure the red jumper wire was returned to the NORM
pin.
• 2. Make sure the thermostat is calling for heating (24 V on Y)
• 3. Verify that the sensors are connected and mounted in the
right places
• 4. Check the electrical connections to the defrost board (24V
on O while defrosting)
• 5. There is a 1 minute minimum time in defrost under
normal operation.
•
If defrost is forced there is no minimum time in defrost.
Helpful Hints in Case of
Difficulty
• 1- The indoor fault indicator goes on and then goes off.
• During extreme weather conditions, the unit may have
difficulty defrosting. This will cause a fault and alert the
owner, but when the weather is more temperate, the fault
will clear and the indicator will stop blinking.
• 2 - The outdoor sensor (ODS) is burnt on the defrost
board.
• Check control wiring “T” and “X2” .
• On the newer units the X2 lead is black and this is not as
likely to happen.
Helpful Hints in Case of
Difficulty
• On Equipment with ICM-2 indoor fan
motors if unit changes indoor fan speed
during defrost, check dip switch 7 & 8
settings.
• If system has supplemental strips installed,
X2 is energizing them and the blower is
responding to it’s dip switch settings.
Helpful Hints in Case of
Difficulty
• On systems that do not have supplemental
heat strips, do not connect thermostat wiring
to W1, W2, or W3 connections.
• On package equipment verify unused
control wires are individually isolated.
• Verify sensors are making proper contact to
the pins on the DFC.
PRESS DOWN ON TAB TO
RELEASE THE SPRING
CONNECTOR
USE SCREWDRIVER HERE
AND HERE TO LIFT UP SO IT
LOOKS LIKE THIS
Demand Defrost Control
Problems
Excessive Ice Built-up on OD Coil
• Low refrigerant charge
• Defrost Control will not Initiate
A. No 24 VAC between R&B at defrost control
B. No 24 VAC between B&Y at defrost control
with system running
C. Verify correct sensor location, mounting &
their resistance
D. Verify ambient sensor is connected to AMB
position on defrost control
E. Verify coil sensor is connected to coil position
on defrost control
Demand Defrost Control
Problems
Excessive Ice Built-up on OD Coil
• SOV Inoperative
A. Stuck in heating mode
B. Open switchover valve (SOV) coil
C. Defective defrost control
• Defrost control contacts to OD fan fail to
open during defrost cycle
Demand Defrost Control
Problems
Excessive Ice Built-up on OD Coil
• Defrost control terminates, but does not
remove ice
A. Windy conditions
B. Outdoor unit located under eaves
C. Lack of proper drainage
D. Night setback operation
Ice Build Up on Lower Part of
Outdoor Coil
• Low refrigerant charge
• Coil sensor connected to wrong pass of
outdoor coil, or poor contact
• Leaking check valve (outdoor unit)
• Distributor tube restricted
• One pass of OD coil restricted
• Lack of proper drainage
Defrost Initiates, but Terminates Only
on a Time Override
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Low refrigerant charge
Outdoor fan on during defrost
Windy conditions
Night setback operation
Unit location
Coil sensor in contact with ice
Coil sensor circuit open or reading very
high resistance.
Defrost Cycle Initiates, but Will
Not Terminate
• Switch-over valve stuck in cooling
mode
• Sensors not making proper contact
to DFC pins.
• Defrost Control
Unit Goes Into Defrost In
Cooling Mode
• Defective Sensors
• Defective Defrost Control
Control Terminates Defrost
Before Frost Is Gone
• Coil sensor mounted in wrong
location or has incorrect resistance
reading
• Refrigerant overcharge
Defrost Initiates About Every 15
Minutes
• Coil sensor
• Ambient sensor
• Defrost control
Defrost Initiates About Every 30 Minutes.
Fault Light on Indoor Thermostat Will Be
Flashing If Wired
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Coil sensor
Ambient sensor
Weather conditions
Night setback operation
Outdoor fan on during defrost
System refrigerant charge
SOV operation
Two-Compressor Controllers
• Tyler Version For V.S. OD Fan Motor
– Cooling model
• 21C150624G02, CNT2275
– Heat pump model
• 21C150625G02, CNT2276
Two-Compressor Controllers
• Ft. Smith Version for PSC Fan Motor
– Cooling model
• 21C150624G01, CNT1858
– Heatpump model
• 21C150625G01, CNT1859
Heat Pump Control