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Thread: Central A/C pipe freezes

  1. #1
    DIY Junior Member phoenixphyre's Avatar
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    Default Central A/C pipe freezes

    You will all have to pardon me as I do not know anything at all about central air conditioners or the proper names of the components.

    Therefore, I will post the photos. The bottom, thin pipe always freezes over (even in hot weather) and we have to turn the unit off for a few hours until all of the encased ice melts.

    If anyone could shed some light as to what we can do to eliminate this issue, it would be most appreciated.

    Thank you in advance.




  2. #2
    In the Trades Bob NH's Avatar
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    The ice on the outside doesn't in itself affect performance of the A/C but it could indicate a problem with the A/C.

    The unit outside is the condenser of the A/C. The hot gas from the compressor should be going through the large tube where it is condensed (becomes liquid). The liquid moves through the small pipe to the evaporator which is the cold unit somewhere inside. The evaporation should occur in the evaporator. The ice on the tube indicates that you are getting evaporation in the liquid-return tube. That could mean that you have too little refrigerant in the system.

    The ice means that the suction of the air conditioner is operating below the freezing point, which will cause loss of capacity and high electrical consumption. It can also cause ice to form on the evaporator which will further decrease capacity and increase electrical consumption.

    I would have the system checked and serviced if required.

  3. #3
    Retired Defense Industry Engineer jadnashua's Avatar
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    Normally, the larger pipe is insulated the entire distance into the compressor. The smaller one normally doesn't need insulation.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Junior Member phoenixphyre's Avatar
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    Quote Originally Posted by Bob NH View Post
    The ice on the outside doesn't in itself affect performance of the A/C but it could indicate a problem with the A/C.

    The unit outside is the condenser of the A/C. The hot gas from the compressor should be going through the large tube where it is condensed (becomes liquid). The liquid moves through the small pipe to the evaporator which is the cold unit somewhere inside. The evaporation should occur in the evaporator. The ice on the tube indicates that you are getting evaporation in the liquid-return tube. That could mean that you have too little refrigerant in the system.

    The ice means that the suction of the air conditioner is operating below the freezing point, which will cause loss of capacity and high electrical consumption. It can also cause ice to form on the evaporator which will further decrease capacity and increase electrical consumption.

    I would have the system checked and serviced if required.
    Thank you very much; I'm going to have someone come and check the unit out as I do not want this to go on and have the entire unit go out.

  5. #5
    DIY Junior Member phoenixphyre's Avatar
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    Quote Originally Posted by jadnashua View Post
    Normally, the larger pipe is insulated the entire distance into the compressor. The smaller one normally doesn't need insulation.
    I will have my husband insulate the larger pipe.

    I thank the both of you for your responses as we did not have any idea as to what the problem may be.

  6. #6
    DIY Member D.Smith's Avatar
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    Check your air filter first before you call somone. Blocked airflow will deminish your system.

  7. #7
    Moderator & Master Plumber hj's Avatar
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    Default ac

    usually a blocked air flow will freeze up the inside coil. Ice on the outside piping usually indicates a low refrigerant level.

  8. #8
    DIY Junior Member phoenixphyre's Avatar
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    Thanks guys; we'll check the air filter and the refrigerant levels before we call the unit in for service.

  9. #9

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    Also, if you can, take a look at the heater side of the evaporator coil. I have seen them plug up with dust, which has the same effect as a clogged air filter.

    The evaporator coil is the unit inside, at the heater, where the tubes you showed in your photo connect.

  10. #10
    Moderator & Master Plumber hj's Avatar
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    Default coil

    The photo showed the connections to the evaporator coil outside.

  11. #11

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    I was trying to make the association of the pipes shown and the evap.
    Last edited by PEW; 07-08-2008 at 08:20 AM.

  12. #12
    Aspiring Old Fart, EE, computer & networking geek Mikey's Avatar
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    Normally the small pipe is warm or hot to the touch, and the large pipe is cold. It's somewhat counterintuitive, because the stuff in the hot pipe actually flows into the house, and the stuff in the cold pipe flows out of the house. The large (cold) pipe is insulated in interior spaces to prevent condensation; it's not that important to insulate it once the pipe leaves the house, if you don't mind the dripping. Both Wikipedia and the "How stuff works" websites have simplified explanations of what's going on:

    http://en.wikipedia.org/wiki/Air_conditioner

    http://home.howstuffworks.com/ac3.htm

  13. #13
    DIY Junior Member phoenixphyre's Avatar
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    Quote Originally Posted by Mikey View Post
    Normally the small pipe is warm or hot to the touch, and the large pipe is cold. It's somewhat counterintuitive, because the stuff in the hot pipe actually flows into the house, and the stuff in the cold pipe flows out of the house. The large (cold) pipe is insulated in interior spaces to prevent condensation; it's not that important to insulate it once the pipe leaves the house, if you don't mind the dripping. Both Wikipedia and the "How stuff works" websites have simplified explanations of what's going on:

    http://en.wikipedia.org/wiki/Air_conditioner

    http://home.howstuffworks.com/ac3.htm
    Thanks Mikey; we will take a look at the Wikipedia links.

  14. #14
    In the Trades Bob NH's Avatar
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    The HOW STUFF WORKS site has information that will help in understanding the problem. The description includes:

    "The cold side, consisting of the expansion valve and the cold coil, is generally placed into a furnace or some other air handler. The air handler blows air through the coil and routes the air throughout the building using a series of ducts. The hot side, known as the condensing unit, lives outside the building. In most home installations, . . . "

    When you combine that with the Wikipedia picture, you can see what should be cold and what should be hot.

    The original post describes the unit as a "Central A/C unit"; not a reversible heat pump.

    The compressor is outside. It takes cold gas from the evaporator (inside) and compresses it, after which it is condensed to liquid. The large pipe is supposed to carry the vapor from the evaporator (low density, therefore larger pipe) and it is cold but should not be below freezing. It has just come through the evaporator coil with air blowing over the coil, which should make it well above freezing.

    The small pipe returns the liquid refrigerant to the evaporator where it goes through the expansion valve. The liquid should be at high pressure and should never be below freezing under any imaginable circumstances if the equipment is working.

    Since the expansion valve is usually inside with the evaporator, it may be that the expansion valve has failed open or is open by command of the control system. Another possiblity is that there is something in the condenser that is plugging the liquid refrigerant return. It is also possible that much of the refrigerant has leaked out of the system but there is still enough left to get some cooling. If the evaporator is calling for more refrigerant from the expansion valve it could expose the liquid return pipe to low pressure on the suction side of the compressor.

    Those are the only things that would produce frost on the small pipe.

    Freezing temperatures and frost on the pipes can only occur when the compressor pressure is so low that the evaporation temperature of the refrigerant is below the freezing point.

    The one thing that is certain is that the unit must be serviced.

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