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  1. #1
    Join Date
    Oct 2005
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    40
    I noticed this morning that my heat pump actually went into a defrost cycle when it was only 40 degrees outside. The humidity level outside was at 90%, so I am not sure if that made a difference. We had the winter maintenance done last week and the tech said everything check out ok. Do you think this is something I need to call the tech about or is this just a normal heat pump operation at 40 degrees and 90% humidity?

  2. #2
    Join Date
    Apr 2005
    Location
    NE PA
    Posts
    698
    In a heat pump the outside coil will be a number of degrees colder than the ambient. Given the high relative humidity, it is entirely possible that the unit was starting to frost up. As frost builds, the airflow is reduced and the coil will get colder, increasing frost buildup.

    The defrost control uses a sensor or switch to measure the coil temperature and initiate defrost. Since the coil is colder defrost is likely with such a high humidity. If it was not so humid, I would be more suspicious, but not undet the conditions you mentioned. Make sure that the outside coil is clean of buildup from grass, leaves, etc. Also there should be sufficient clearance around the compressor for free airflow. Otherwise you should be fine.

    paul

  3. #3
    Originally posted by mgfarmer
    I noticed this morning that my heat pump actually went into a defrost cycle when it was only 40 degrees outside. The humidity level outside was at 90%, so I am not sure if that made a difference. We had the winter maintenance done last week and the tech said everything check out ok. Do you think this is something I need to call the tech about or is this just a normal heat pump operation at 40 degrees and 90% humidity?
    On my Carrier heat pump, defrost is a function of time and outside coil defrost thermostat.
    The thermostat is set to close its contacts at 30*F (to initiate a defrost cycle at the appropriate time)but does not open contacts until the outside coil temp rises to 80*F.
    This means that the defrost cycle is initiated regardless of need to defrost so long as the defrost thermostat contacts are closed.

  4. #4
    Next time go outside and see if the coil is indeed frosted.

  5. #5
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    33,897
    With a timer, doesn't matter if it is frosted. Only matters is the coil is below 30 and it would be if 40 ambient

  6. #6
    Join Date
    Oct 2005
    Posts
    40
    Thanks for the replies. It was definitely a little frosted since I saw a tiny bit of water come from underneath the heat pump when defrost cycle was completed.

  7. #7
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,914
    Defrost is actually needed more often when the outdoor temperature is in the 35-45 range than when it is colder outside.
    There is generally a good deal of water in the air at those temperatures, and it is cold enough that the coil is operating below freezing, so the inevitable happens.

    At lower temperatures there is much less water in the air to frost up the coil because most of it tends to get frozen out of the air long before the air is pulled through the coil.

    Thats the main reason time/temp defrost controls are stupid. They cause the system to defrost most often when it is needed the least, wich is a significant waste of energy and a source of additional wear on the compressor and reversing valve. Unfortunatly some manufacturers choose to stick with dumb defrost controls, I personally would not have, or sell, heat pumps that rely on time and temperature controls for defrost.

    I like demand defrost because it will cycle the defrost mode more often if the conditions warrant, like if it is in the upper 30's and drizling, you likely need defrost quite frequently. I have seen a lot of cases where a time/temp control was working perfectly, but due to the weather conditions, the coil was building up more ice between defrost cycles than could be removed during the cycle, so the unit turned into an icecube, even though it was working perfectly.
    When it is very cold and dry, the system could go for days without going into defrost if it has a demand control, but a time/temp control will go into defrost every time the time interval comes up, usually every 30, 60 or 90 minutes.

    If more government is the answer, then it's a really stupid question.

  8. #8
    Join Date
    Oct 2006
    Location
    Emerald Coast, FL 30.1N 85.8W
    Posts
    681

    evaporator coil is usually 20 degrees lower than ambient

    If it's 40 degrees outside, your evaporator coil(outside in heating mode) is operating at 15 to 20 degrees, so the defrost cycle will kick in(usually by time & temperature control).

    Originally posted by mgfarmer
    I noticed this morning that my heat pump actually went into a defrost cycle when it was only 40 degrees outside. The humidity level outside was at 90%, so I am not sure if that made a difference. We had the winter maintenance done last week and the tech said everything check out ok. Do you think this is something I need to call the tech about or is this just a normal heat pump operation at 40 degrees and 90% humidity?

  9. #9
    Join Date
    Oct 2006
    Location
    Emerald Coast, FL 30.1N 85.8W
    Posts
    681
    If you're correct, then time & temperature control should be a candidate for the "horse & buggy" control award -- manufacturers will dump them in order to boost their efficiency ratings.

    Mark -- are all the higher SEER units demand defrost control(I would think so)?

    Thanks


    Originally posted by mark beiser
    Defrost is actually needed more often when the outdoor temperature is in the 35-45 range than when it is colder outside.
    There is generally a good deal of water in the air at those temperatures, and it is cold enough that the coil is operating below freezing, so the inevitable happens.

    At lower temperatures there is much less water in the air to frost up the coil because most of it tends to get frozen out of the air long before the air is pulled through the coil.

    Thats the main reason time/temp defrost controls are stupid. They cause the system to defrost most often when it is needed the least, wich is a significant waste of energy and a source of additional wear on the compressor and reversing valve. Unfortunatly some manufacturers choose to stick with dumb defrost controls, I personally would not have, or sell, heat pumps that rely on time and temperature controls for defrost.

    I like demand defrost because it will cycle the defrost mode more often if the conditions warrant, like if it is in the upper 30's and drizling, you likely need defrost quite frequently. I have seen a lot of cases where a time/temp control was working perfectly, but due to the weather conditions, the coil was building up more ice between defrost cycles than could be removed during the cycle, so the unit turned into an icecube, even though it was working perfectly.
    When it is very cold and dry, the system could go for days without going into defrost if it has a demand control, but a time/temp control will go into defrost every time the time interval comes up, usually every 30, 60 or 90 minutes.


  10. #10
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    33,897
    Great post, Mark, and my opinion too!

    And no, most units aren't demand. Rheem/Ruud, A-S/Trane, some of the York family are about all true demand. Everyone else that I know of is timed.

  11. #11
    Join Date
    Oct 2006
    Location
    Emerald Coast, FL 30.1N 85.8W
    Posts
    681

    Well what does that say?

    Old wise Bald One -- from a cost issue, time & temp is probably cheaper than demand, but the defrost cycle is ridiculously inefficient to a heat pump's SEER efficiency.

    Mark -- weigh in on this puppy.

    Thanks.

    Originally posted by BaldLoonie
    Great post, Mark, and my opinion too!

    And no, most units aren't demand. Rheem/Ruud, A-S/Trane, some of the York family are about all true demand. Everyone else that I know of is timed.

  12. #12
    Join Date
    Oct 2006
    Posts
    28
    Originally posted by BaldLoonie
    Great post, Mark, and my opinion too!

    And no, most units aren't demand. Rheem/Ruud, A-S/Trane, some of the York family are about all true demand. Everyone else that I know of is timed.
    Is there a work-around for those of us with Carrier Infinity heat pumps?

  13. #13
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,914
    Originally posted by robrecht
    Originally posted by BaldLoonie
    Great post, Mark, and my opinion too!

    And no, most units aren't demand. Rheem/Ruud, A-S/Trane, some of the York family are about all true demand. Everyone else that I know of is timed.
    Is there a work-around for those of us with Carrier Infinity heat pumps?
    I don't know much about the Infinity, but I would be absolutely shocked if Carrier doesn't use a demand defrost system for it.

    Lennox also uses some demand defrost controls on thier better equipment.
    If more government is the answer, then it's a really stupid question.

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