Mis-diagnostics of Time/Temperature Defrost Boards in Split Heat Pumps


In a previous post, I mentioned that control boards are replaced and there is really nothing wrong with them. In recent months, there has been an increased number of defrost boards from split system heat pumps returned for warranty.  In-house testing of some returned boards has found that a majority of the boards were not defective.

The following information will help you better understand the operation of the board, and give some insight to other possible sources that may lead a technician to condemn the board when it is actually not defective.

These defrost boards have an internal five-minute compressor time delay that will be initiated anytime the room thermostat satisfies or the high pressure switch opens. In regards to the time delay, one thing that has been seen is that if there is a problem with the compressor contactor coil (such as being shorted or pulling in rough, or sticking occasionally) the board will have a tendency to go through the five-minute time delay, attempt to energize the compressor contactor, but will immediately return to another five-minute time delay and continuing repeating the time delay.

Before diagnosing the board as defective,  use the following troubleshooting procedures .

Troubleshooting Steps:

  • Ensure that the indoor unit transformer is tapped for the correct line voltage on the job.  Don’t assume it is 240 volt. If the primary voltage is 208 volts and you leave the transformer set for 240 volt primary — you are starting off with low voltage to the control and this, in itself, will cause problems, especially with the contactor pulling in.
  • Remove the wire attached to the “M” terminal on the board (see  below).

defrost board

  • Give unit a call for cooling (measure  for 24 v between Y and C).
  • With a screwdriver, short between the two test pins on the board until you hear the board click. (see below)

defrost board 2

  • Place voltmeter leads on the “M” terminal and “C” and you should read 24 volts.
  • If  you read 24 volts after completing the above steps, the board is good and you should check your compressor contactor coil and wires.
  • If  you do not read 24 volts at the above steps, ensure that you have continuity through the high pressure switch.
  • If  there is continuity through the high pressure switch, the board is likely  defective.
  • Make sure the defrost curve jumper is not left at the P position (see picture above)

This applies to both heating and cooling since the “Y” call on a heat pump is the COMPRESSOR CALL.  Heating and cooling is determined by the reversing valve.  Hopefully this will help you correctly  diagnose a problem and not condemn a control board that may still be good.

Thanks to  Alan Dukes  Technical Service Manager  Virginia Air Distributors, Inc. Richmond/Fredericksburg  for the information in this post.

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About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
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30 Responses to Mis-diagnostics of Time/Temperature Defrost Boards in Split Heat Pumps

  1. Kevin Kerch says:

    Thank you for posting the very helpful info, it’s very much appreciated. I’m not a tech but a very knowledgeable person in understanding how things work. I happen to own the LP9C + HL8B + TTSCC01 setup as a communicating hybrid duel fuel system. My whole entire system was replaced including correctly sized supply/return ductwork & resized registers. My tech/installer is very good at installing, troubleshooting & paying attention to detail.
    The system worked great until he replaced the originally installed LX THSU32HP7S with the TTSCC01 Comm stat. During a system defrost cycle, at the very end, the system would intermittently throw a High Pressure Fault and eventually Lock Out. Everything always checked out when he tested the system. Although his calls to UPG always resulted in informing him to swap out parts he knew this was not the answer. He’s not a person that just starts replacing unneeded parts when he knows they’re good.
    I came across your tech website in searching for the related problem & showed him this article which he was very impressed as he said it related to his way of thinking. This article let him think over what could be wrong & led him into solving the defrost fault issue. As I watched him he very lightly (with an electricians long screwdriver) moved a single wire at a time with the HL8B running in second stage heat. A clicking from the contactor opening & closing started as he neared a transformer. What he found was a poor connection with a crimped female spade connector on a blue wire to the bottom right of the transformer. Power shut down, wire stripped, crimped on a new spade connector & all is well!!! We both thank you very much for your professional work ethics & taking time in creating your site!!!

  2. Keith says:

    Hello sir on a Lennox parts(46K67)Defrost board were to jump out test operation put it in defrost mode because I jump it out it didn’t go in to defrost ,I tri return it back to supple store they want take back . Please respond back to me it would be very helpful . Thank You

    • you need to work with your distributor.. Not all sequence of operation are alike. You would need to contact Lennox for info on their specific board,

      • Keith says:

        Yes they did allow me to return the board back found that the new board want in to defrost either I place the pin cause board test mode foe defrost did work what there’s show me. So I decide to go with ICBM defrost board it work I just return Lennox back for refund the were not happly they claimed when temperature were 54degree it want go in defrost that is crazy should be able to still jump test pin to make it in defrost mode with in 5 to 21 second if please I formed me with more information , thank You

      • Keith says:

        Yes they did allow me to return the board back found that the new board want in to defrost either Iplace the pin cause board test mode go into defrost did work for what they show me. So I decide to go with ICM defrost board it work I just return Lennox back for refund the were not happy say when temperature were 54degree it want go in defrost that is crazy should be able to still jump test pin to make it in defrost mode with in 5 to 21 second if please I formed me with more information , thank You

  3. Jason says:

    Hi, thanks for the info. I am actually going to replace my defrost control board tomorrow morning. Tech told me it was bad and wanted to charge $700 to replace it so I bought it for $70 and will do it myself. I was already skeptical as to whether this would fix the problem or not. There is the mentality these days to just throw parts at things instead of finding the actual problem. We have a York E4FD030S06A. It is flashing a fault code 2 on the EM heat light which I believe means the high pressure is getting above 400psig, or at least it thinks it is. Freon levels were checked, air filter replaced, they said they checked the indoor coil. I don’t know whether they checked the contactor or the pressure switch so I will try those tomorrow. When the thermostat is turned off and then back on again it will work for a while but then eventually fault out again. Of course this would happen during the coldest winter I can remember, probably more likely to surface when it is working so hard I guess. Any other suggestions would be greatly appreciated… Thanks for any help.

    • flash code 2 is high discharge PRESSURE (over 400 psi) I would look to see if you have a problem with the OUTDOOR metering device if it is a TXV. If itis sticking shut, the pressure will climb. Also, indoor air flow is important

      • Jason says:

        Thanks again, when swaping the board out today I did notice that one of the indoor return air vents was pretty much closed so I opened it up. I also noticed that the brown and white wires (as indicated on your diagram above) were switched. Brown was going to W and White was going to W1/66. I don’t know if this could have caused the problem. Is the TXV the thing that the blue wires come from? How can I check it? Any help is appreciated.

  4. Jason says:

    I just went and checked the system again. As expected changing the defrost control board didn’t do anything, the system was flashing code 2 again. I wish I could find a technician that could properly diagnose & fix this

  5. Jason says:

    I’m not sure that my system is a TXV, it’s a 10 seer York (E4FD030S06A). There is a thing called a refrigerant control, or a pressure control (S1-025-33305-048) does this perform the same basic function?

    • the part number you mention is the HIGH PRESSURE Control that is opening and giving you the flash code 2. Now you just need to determine what the cause is. You could go to the corporate web site — http://www.york.com —- click on residential, and in the new screen on the upper right will be a drop down box that you can put your zip code into. This will bring up the York Dealers in your area. These usually are participating dealer who attend training and know most of the products. You could see if one of them can help solve your problem

    • Keith says:

      Hey Jason I came across a problem like that I found out you got to determent the pressure low side and high if your low side drop in pressure then you have a fault Txv replace Txv. That one solution the next solution is check indoor coil from being block air flow clear or clean the fins,the next attempt to see it if being over charge recover some the freon.one these 3 solution should clear you up .

      • Keith — thank you or your input. You are correct about indoor air flow causing high pessure trips. His unit does not have a txv but has an orifice metering device in the outside coil.
        I appreciate the helpful comments — they help all who reasd them. Thnks

  6. Dave Freeman says:

    A new thread on this subject matter: demand defrost. I have a new 16 SEER, 2-1/2 ton Luxaire (made by York as I understand it) heat pump that, since the heating season started, is defrosting every 40 minutes (the minimum wait time according to the owner’s manual) without fail, as long as the ambient temperature is below 50 degrees. The defrost time lasts 2-3 minutes. Many times there is no evidence of any frost on the unit when it goes into the defrost mode. Last week the tech replaced the liquid line temperature sensor which changed nothing; I doubted it would as the sensor would have to have been working to terminate the defrost cycle prior to the maximum allowed 8 minutes. The ambient temperature sensor also seems to be functioning properly. There are no fault codes displayed.

    Any suggestions – the constant defrosting is expensive and annoying! The unit is set to the factory-specified defrost initiation curve (Pin 1).

    BTW, next day after the tech was here, I began to hear a rattle seemingly coming from the compressor or very near by that can be heard at the air handler inside as the noise is transmitted along the refrigerant line.

    • Dave — refer to publication 501062-UAI-D-0814 which you can get from your contractor or local distributor. This will explain everything about the defrost control. The control regulates the defrost operation of the heat pump based on accumulated compressor run time, outdoor coil temperature,
      and outdoor ambient temperature. The control will cause the unit to operate in the normal heating mode until it determines that a defrost cycle is needed.
      All defrost timings are based on accumulated compressor run time.
      Pin 1 is the correct position for defrost.
      The control initiates a defrost cycle every 6 hours (accumulated compressor run time) to recirculate refrigerant lubricants. This forced defrost timer resets and restarts following the completion or termination of a defrost cycle.

      • Dave Freeman says:

        Thank you for your reply and reference to the publication; I downloaded it this morning and found it to be a very informative update to an older version I had.

        I confirmed from the pub that defrost initiates no sooner than 40 minutes of accumulated compressor run time after the previous defrost cycle and ONLY IF the difference in outdoor coil temp and outdoor ambient temp calls for defrost. My concern is that each time the 40 minutes of accumulated run time passes, the unit defrosts without fail, even with little or no frost buildup, even when the ambient temperature is just below 50 degrees. (It refrains from defrosting if the ambient temp is above 50 as the manual says it should).

        My just replaced unit had the old style timer defrost mechanism and was set at 90 minute defrost intervals and worked fine that way for years. My concern stems from that experience; my understanding is that demand defrost is more efficient implying I would be experiencing less frequent defrost cycles with this new unit, not twice as many!

        Bottomline, would you conclude this new unit is operating incorrectly defrosting every 40 minutes or is that typical behavior? I can’t find reference anywhere to a “typical” number of defrost cycles daily (I know it can vary widely) and my tech doesn’t seem to have a feel for this either.

        I thank you very much for your insight.

      • if you look in the publication, there is a graph showing the algorithm for how defrost should work. I would suggest you contact your local distributor and have them work with your contractor to see if everything is functioning properly. Since I don’t know your area and operatring conditons, this would be best to work with your local distributor.

      • Dave Freeman says:

        Thank you – I will continue to work with them – I appreciate your time on this topic…

  7. Mark Kenney says:

    What does th 1,2,3,4 defrost setting mean on the New York defrost boards

    • those are the “set points” for defrost timing based on the model number/size of the unit. That should be matched to you unit and it will be explained in the installation instructions that came with the unit

  8. Glenn says:

    I have a very old York model EICPO30AO-6a and it quit defrosting, after inspecting I noticed the tube that goes into the evap coil is missing off the defrost timer , can you supply some info on how this set up works . Had a new main board about 5 years ago.

    • on that heat pump — thetube from the defrost control was in between the condenser fan and coil. If too much Ice built up on the coil, it created a negative pressure which would move the switch initiating defrost. If the control was updated, did they keep this defrost initiation or is that now built into the board based on “time/temp” which is why the tube was removed?

  9. Glenn says:

    The main board replacement was a direct cross , no mods ,no time temp adjustment , the defrost control is mounted on the front panel section with the tube port on the switch. So I can test the defrost control with a vacuum pump ? Dealer did say way back this was a weak link . is there an update for this . The tube is probably inside the condenser somewhere .

  10. Steven Friedrich says:

    Hi Mike. I was just reading this post from Glenn, about the E1CP not defrosting. The E1CP enters a defrost based upon 2 conditions. The vacuum switch must be closed, NOT OPEN for at least 12 seconds AND the liquid line sensor has to show a resistance equivalent to a temp of 39 degrees or less. You have to have both conditions in order to get a defrost. As frost and ice accumulate on the coil the vacuum increases and CLOSES the switch. If you want to check out the board to see it go into a defrost, remove the liquid line sensor and jump out the defrost switch. The unit should enter a defrost within 12 seconds.–Steve Friedrich New York Branch

    • STeve — You guys sure “dodged a blizzard” in NYC.
      THANK YOU for the E1CP defrost info. Since that was a home owner — I always try to refer them to a dealer. Always appreiate anyone sharing info on the site — Take Care!

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