Carrier Heat Pump Not Defrosting - NEED HELP


Old 01-31-14, 08:46 AM
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Angry Carrier Heat Pump Not Defrosting - NEED HELP


I have a Carrier heat pump (model number 38YCC024340) which freezes up. I had a tech come out and check the system and diagnostic said could not get the unit to shift into forced defrost and the defrost control board needed to be replaced. I replaced the board and the coil thermostat sensor with brand new parts. The problem is still occurring.

What could the issue be? What is my next step to try?
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Old 02-01-14, 03:17 AM
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Step 9—Defrost-Control Board
Solid-state defrost boards used on heat pumps replace electromechanical
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 defrostcontrol
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, insulated
wires connected to DFT and R terminals on control
board. Resistance reading may be zero (indicating closed
defrost thermostat) or infinity (∞ for open thermostat) depending
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
9. Remove protective cover from TP1 and TP2 speed-up terminals.
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
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
have closed, energizing supplemental heat (W2) and reversing
valve solenoid (O).
11. Unit should remain in defrost no longer than 10 minutes.
Actual time in defrost depends on how quickly speed-up
jumper is removed. If it takes 3 sec to remove speed-up
jumper after unit has switched to defrost, only 7 minutes of
defrost cycle remains.
12. After a few minutes in defrost (cooling) operation, liquid line
should be warm enough to have caused defrost-thermostat
contacts to open. Check resistance across defrost thermostat.
Ohmmeter should read infinite resistance, indicating defrost
thermostat has opened at approximately 80°F.
13. Shut off unit power and reconnect fan lead.
14. Remove jumper wire from speed-up terminal and reinsert
cover on speed-up terminals. Failure to remove jumper causes
unit to speed up operating cycles continuously.
15. Remove jumper between DFT and R terminals. Reconnect
defrost thermostat leads.
16. Replace control-box cover. Restore power to unit.
Old 02-01-14, 08:59 AM
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Welcome to the forums.

Is the board changing status at all ? Like.... is the condensor fan shutting down, reversing valve operating or does the unit stay locked into heat mode ?
Old 02-06-14, 08:58 AM
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Thumbs up Fixed!

So I am reporting back because I have fixed the issue, but I would like to place what was done in writing so others searching for a fix will have access to this information.

After replacing the board and thermostat, I noticed the unit was still freezing up. I find the article posted below about how to test the defrost and begin my testing. I found the unit would now go into defrost but would only run for about 1 minute, even when the whole unit was covered in ice. The valve would shift, the fan cut off, but after a minute, it went back into heat mode. The thermostat tested to be closed when the ice was present before a forced defrost.

What it ended up being is the last person to work on the unit placed the thermostat on the wrong coil. It was the coil which heated up quickly before the rest of the unit warmed up, causing the board to think the unit was no longer frozen after a minute or so, but still leaving the unit frozen. When I replaced the thermostat, I used the same mounting position, assuming it was the correct one. This was a mistake.

I moved the thermostat to one of the coils which remained frozen the longest during the forced defrost and now the unit is working correctly.

I posted a photo for reference. Thanks for the help!
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Old 02-06-14, 11:11 AM
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Good find, I bet a tech would have over looked that for several calls.
Old 01-07-15, 07:35 PM
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I am having a similar problem with my Carrier heat pump.

I ran the Diagnostic that Houston204 wrote out, and when it gets to the point where you short out the speed up terminals, with the defrost thermostat terminals connected, it does not go into defrost mode no matter how long I keep the speedup terminals connected. The voltage between C and W2 never changes

I figured the board was bad, so I replaced the defrost control board with a new one and got the same result. No defrosting, just a ever growing outdoor unit, covered in ice/frost.

The unit will switch into cooling mode via the thermostat and melt the ice, so I am guessing the reversing solenoid is OK, but I'm not sure where to look next...

The thermostat is a Carrier TSTATCCNHP01-B which appears correct for a heat pump.
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