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  1. #1
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    Split System. Low pressure but no leak



    Hello All,

    We are working on a residential split system AC in south florida. 10 year old carrier split 3.5 ton on R-22.

    Here's the history.

    We had a dirty coil, causing high head and low suction pressure and freeze ups. Pumped down system, removed coil and did a coil cleaning. Reinstalled coil, pressure tested, vacuumed to 500 microns, released refrigerant charge and all was ok. Pressures were normal and system operated fine in cooling for 3 days.

    We then received a call from the client for no cooling. Found liquid line cold and suction line luke warm. Put on gauges and found 120 psi oin high side and 20 on low side. Onsite tech figured he had a leak, so charged a couple of pounds of refrigerant but saw NO CHANGE on high or low side. Recuperated the charged refrigerant. Found discharge line between the compressor and the condenser coil hot, the condenser was cold. the only way that he was able to get the pressures normal were by turning off the condenser fan. The outdoor temperature was mid 70's. Indoor temperature was about 80

    So, here's what I think the possible culprits may be, but am looking for some help, suggestions, expertise...

    1. non condensibles somewhere in the capillaries of my condenser or in the compressor?
    2. Restriction in the condenser?
    3. Bad discharge valve on the compressor (reciprocating-hermetic)?

    Please give help advice.

    JBW



  2. #2
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    Re: Split System. Low pressure but no leak

    .

    Non condensables causes high pressures not low.

    Blockages, moisture would be the obvious, but I would
    check airflow over the evap to make sure there is no problems
    there. Then check for contamination. You say you took
    it to 500 microns? How long did it stay there for?

    Coolrunnings.

    .

  3. #3
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    Re: Split System. Low pressure but no leak

    If you had bad valves in the comp then i expect the suction pressure would be higher, when valves go bad its like having a much smaller compressor in the system so the high side presssure goes down because the comp can't move enough refrigerant to keep the condenser happy ( unless it has fan speed control) and the suction preeure is higher than normal as the comp can't keep up with the evap.
    You could pump the system down and see how quick it does and how low a vac the comp will pull. this will give an indication of the compressors health.

    I think i would be looking for a restriction, be it in the txv or piston, or a blocked drier or even a kinked pipe. I have seen a few txv's die once a system had been fixed for other issues, could be down to the pressure test or even that is has had to do some work now the evap is clean.

    Bad valves, comparisson af a good and bad comp on same site, note high side gauge is on good comp to show the difference.

    http://www.youtube.com/watch?v=FEtYI_OIHok

    Bad txv, not a very good vid, but shows how the third txv's inlet is cooler than the two good ones due to lower flow of refrigerant.

    http://www.youtube.com/watch?v=CY5ujrTXXp0

    Jon
    Mostly found in Oxfordshire, UK :)

  4. #4
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    Re: Split System. Low pressure but no leak

    Hey Guys,

    Thanks for your replies. Wouldn't a restriction in the orifice/piston cause HIGH head pressure and low suction pressure?? I have low pressure on both head and suction sides??

    Regards,

    JBW

  5. #5
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    Re: Split System. Low pressure but no leak

    It would give high head pressure if the condenser was full of refrigerant, such as if a tech had thought there was a leak and had topped up the system. But generally in a correctly charged system a restricted txv/piston or drier will give low pressure on both sides.
    Mostly found in Oxfordshire, UK :)

  6. #6
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    Re: Split System. Low pressure but no leak

    Originally, when the tech came onsite, he thought there was a leak, so he added about 2lbs of refrigerant. However, after no change in pressures, he stopped and recovered the refrigerant.

    Regards,

    JBW

  7. #7
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    Re: Split System. Low pressure but no leak

    OK. New development.
    We went back today. Recuped all refrigerant. Installed a new sight glass as well as liq line drier. pressure tested. vacuumed to 500 microns and held for 1 hour. charged factory amount taking into consideration liq line drier and line set length. started up. this time pressures were at 120 psi & 0 psi. Worked like that for 5 minutes then heard large hissing sound and the suction pressure jumped to 30 psi. worked for a few more minutes and compressor went off on internal overload. sight glass showed liquid with the odd bubble while it was working. Same conditions as before.. super duper high superheat with high subcooling. Discharge line from compressor to condenser blisteringly hot.

    ????

  8. #8
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    Re: Split System. Low pressure but no leak

    Definately sounds like a restriction to me.
    The compressor overheating is a sign of a lack of cooling gas (low mass flow), as is the low suction pressure.

    Cool condenser, liquid line and low head pressure tell me that all of the charge is staying in the condenser.

    My guess is the TXV. Does this show any signs of frosting? I guees not with 0psig. Seems like it is in permanant pump down situation.

    Change the TXV, or at least check the orifice for a blockage

  9. #9
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    Re: Split System. Low pressure but no leak

    i do hope that upon discovering a leak it was repaired before you dropped a few "pound" into the system what is it with yanks and sloppy workmanship have one at our firm and he himself told me " bang in the gas and leak check " rather then use ofn why not try changing your liquid line dryer and checking you reversing valve for possible restrictions

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