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  1. #1
    Join Date
    Jan 2012
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    Freezing capillary coil in outdoor A/C unit.

    I have a residential split A/C unit with R22 refrigerant that it not cooling and shows ice forming on the high pressure line.

    (I have read the rules concerning DIY but I'm trying to learn the workings of an A/C system and I live on an island with no 'pros' so I hope someone will still give me some suggestions or an explanation were to look for.)

    Small description of my findings: When the A/C is switched on the pressure on the low pressure line almost goes into vacuum and the high pressure line shows ice forming. The ice forming starts directly after a small (capillary?) tube in a dual loop that sits in between the condenser and the connection point for the tubing to the indoor unit.

    What I have understood so far from Internet sources is that the flow from the condenser to the evaporator is metered by a thermostatic expansion valve (TXV) or a capillary tube to reduce the 100% sub-cooled liquid high pressure refrigerant to a low pressure liquid vapor mixture that subsequently enters the evaporator where it boils off and extracts heat.

    I have never read that the TXV or capillary tube is located in the outdoor unit, it is always located just before the evaporator in the indoor unit. But I have checked the indoor unit and found no metering device so the double looped capillary coil I found in the outdoor unit must be the metering device. Has anybody ran into a system like this and agrees with me?

    From the fact that the high pressure line starts forming ice just at the boundary between the capillary tube and the normal tubing that goes to the indoor unit I conclude that the refrigerant already starts boiling off at this boundary and therefore cooling the high pressure line. I can think of two reasons why this is happening: 1) the capillary tube is clogged restricting the liquid flow, or 2) there is not enough refrigerant in the system.

    I hope you can give me a suggestion what the problem might be or what to check or measure to rule out possibilities. As I said, there are no 'pro's' here and I'm very interested to learn A/C system.

    Thanks, Vincent

  2. #2
    Join Date
    Mar 2005
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    burlington county n.j.
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    9,699
    if this is a mini split system the metering device would be in the outdoor unit.
    that would put your diagnoses on the right track.

  3. #3
    Join Date
    Dec 2010
    Location
    New Orleans
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    1,012
    you are out of luck if you don't call a pro. I would not attempt to fix this problem on my own if I were you.

  4. #4
    jpsmith1cm's Avatar
    jpsmith1cm is offline Global Moderator/AOP Committee
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    Sep 2008
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    Western PA
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    Mini split systems, and this sounds like a mini split, are probably some of the most difficult equipment to work on.

    Not a project for someone who is "learning"

  5. #5
    Join Date
    Jan 2012
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    Kitts Hill Ohio
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    Have you tried to take the therm bulb loose and warm it what happens then? Have you found any oily spots in the sealed system?

  6. #6
    Join Date
    Dec 2010
    Location
    New Orleans
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    yea oil spots, yea, He doesn't even have gauges so what good would any troubleshooting attempt be worth.

  7. #7
    Join Date
    Jan 2012
    Location
    Montreal, Canada
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    When the A/C is switched on the pressure on the low pressure line almost goes into vacuum and the high pressure line shows ice forming.
    I was just wondering how you managed to get low pres ? And if you do have gauges whats your high pres?
    I will give you props for already figuering out so much on your own. And theres no point in having a site like this if folks interested in the trade cant ask questions. i undestand scaring off the cheapos who try to get a free ride but this boy did his homework.

  8. #8
    Join Date
    Dec 2003
    Location
    Sugar Land, Tx
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    123
    If this is a mini-split you will not be able to get a high pressure. must be charged by weight and verifying SH. and would be best left to a pro.
    Techson Mechanical LLC
    "We Shall Mount Up Wings As Eagles"

  9. #9
    Join Date
    Jan 2012
    Location
    Canada
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    You'll get ice on your cooling coils if you don't have sufficient airflow to get rid of heat fast enough.

    Is the cooling coil next to a fan on the unit? Is it possible that you simply aren't pushing air past the cooling coil fast enough?

  10. #10
    Join Date
    Dec 2010
    Location
    New Orleans
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    Quote Originally Posted by Waxy View Post
    You'll get ice on your cooling coils if you don't have sufficient airflow to get rid of heat fast enough.

    Is the cooling coil next to a fan on the unit? Is it possible that you simply aren't pushing air past the cooling coil fast enough?
    brilliant.

  11. #11
    Join Date
    Feb 2010
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    In a boiler room
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    Quote Originally Posted by Waxy View Post
    You'll get ice on your cooling coils if you don't have sufficient airflow to get rid of heat fast enough.

    Is the cooling coil next to a fan on the unit? Is it possible that you simply aren't pushing air past the cooling coil fast enough?


    Please don't tell me you're an engineer.

  12. #12
    Join Date
    Jan 2012
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    Canada
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    Yep.

  13. #13
    Join Date
    Jan 2012
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    Thanks guys for your support and ideas. From what t527ed, jpsmith1cm and techsonac wrote it is clear that I'm dealing with a mini-split system. This is confirmed by the fact that it's only possible to measure the pressure on the low pressure side, just as techsonac said.

    @DeniV: I measured the pressure with a gauge set and as soon as the system is switched on the pressure drops to just above 0 psi. Thanks for your supportive comments also. If by any change you know a good Internet resource or book please let me know.

    I have an identical unit that's working without difficulties and to test whether I'm doing the right thing I have measured the superheat as suggested by techsonac. I found 10.6 F and I understood that this value should be between 8 and 12 F, so no problem here (as expected from an operational unit).

    So far to me the only possibilities seem to be 1) the capillary tube is clogged restricting the liquid flow or 2) there is not enough refrigerant in the system.

    The easiest thing (and less risky I think) to do is to add refrigerant and see what will happen. I cannot try this earlier than this weekend. I'll keep you posted.

    Does somebody know what happens to the pressure on the low side when adding refrigerant bit by bit? Will the pressure also increase bit by bit or is there some critical quantity at which the pressure suddenly increases?

    Thanks again, Vincent


    @keeplearnin: you seems to be the wise guy here judging to your ability to discern "brilliant" suggestions by other members, so what were you thinking when I wrote "the low pressure line almost goes into vacuum", that I measured it with my nose? Yes I have gauges...

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