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  1. #1
    Join Date
    Jan 2013

    Heat pump iced up. Interesting hybrid problem w/oil burner.

    I recently installed a Rheem heat pump to replace my existing AC unit. I had it mated to my existing Lenox oil furnace(about 8years old).

    The other night I went out and noticed my heat pump was a giant block of ice. I figured no problem since I have an oil furnace as my aux heat. I programed the thermostat to use the aux heat and what happened next was the oil burner would fire but then after about a minute it would shut down and power to the thermostat would be lost.

    I believe I have determined the furnace was tripping the limit switch because it was not turning on the blower after 45 seconds. I am able to turn on the blower using the thermostat(carrier digital). I did some checking on the Blower Control Board and determined it does not seem to be turning the blower on and have ordered a replacement. I set the thermostat so the blower runs 24/7 and the system seems to be working and I'll replace the board as soon as the new one arrives.

    Here is the part I am still wondering about. Why did my heat pump freeze up? Should I have it wired differently? I have a theory on what happened. The heat pump has a sensore to detect when it is freezing up and does not have a timer like some. I think when the heat pump tried to go into defrost mode it started the aux heat. Since the blower would not start it tripped the limit switch which causes it to loose all power and the thermostat goes dead too. After the limit switched cooled the system power was restored and the heat pump started to run without ever getting a chance to complete the defrost cycle. I'm guessing the defrost cycle somehow gets canceled when the thermostat looses power?

    After a while of continuing building up more frost the heat pump again tries to defrost and the cycle continues? This is my theory but I am not positive? I have both the heat pump and oil furnace running with the blower running continuesly and heat pump has not frosted up and neither has the limit switched be tripped. I have run both the furnace and the heat pump separatly for many hours each and both seem to be working(except blower function)

    My question is: Am I correct in my assumption of what is happening? Am I wired correctly in the first place that the thermostat should loose power when the limit switch is tripped? How long would it take the heat pump to realize it is a block of ice after power is restored to the thermostat and try to go into defrost?
    Thanks for reading and looking forward to your thoughts.

  2. #2
    Join Date
    Mar 2006
    The South
    Was this a DIY install?

    If so, you should go back and read this forum's policies concerning questions from guests.

    I would say one of two things. Either defrost function is faulty or bad wiring.

    Call a pro to diagnose and correct/repair.


  3. #3
    Join Date
    Oct 2006
    The Beach
    Call a pro. And ask him to fix what ever is wrong. This is not a DIY site.
    Blue Fox

  4. #4
    Join Date
    Jan 2013
    The heat pump was a professional install by a licensed HVAC contractor but unfortunatly he is a one man business and on vacation till Tues. The oil furnace was a previous install by another contractor that did oil.
    Sorry about the DIY post. Should the thermostat loose power if the oil furnace goes into an overlimit?

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