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Thread: Bad TXV?

  1. #1
    Join Date
    May 2004
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    I know I probably wont give anyone enough info to give a good opinion , but what the heck.

    Worked on a 15 year old split a/c today. Carrier condenser with magic chef AH. Not sure about the tonnage. If i had to guess i would say 10 ton. data plates were gone. Anyways, my pressueres were 60 suction and 300 head. Airflow was good, coils were clean. Unit has about 100' line set, 25' of it is vertical. Suction line temperature at the compressor was only 36*. I'm thinking bad TXV. Return air was about 80*. Outside air at condenser was 85*. Anyone care to give your .02 ?

  2. #2
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    Oct 2004
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    Check your subcooling and suction superheat, then get back with us. Other than that there really is not enough info.
    Saddle Up!

  3. #3
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    Superheat stayed between 2-4 degrees

  4. #4
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    Your right, not enough info. Delta T's across the coils may show air flow problems and the superheat is dangerously low.

  5. #5
    need subcooling, if your more than 15 degrees then reduce charge to get 10 to 12 degrees then check superheat. high superheat would mean restriction or bad metering devise, low superheat would mean airflow across evap is low.

  6. #6
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
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    Although more info would be helpful, what info you gave can lead to some preliminary thoughts:

    Suction pressure = 60 psig/34°F SVT

    Head pressure = 300 psig/130°F SCT

    Suction line @ compressor = 36°F/2°F superheat

    What would be very helpful at this juncture is a subcooling reading. As it is your condensing temperature is 45 degrees over outdoor ambient.

    You stated that the condenser coils are clean. Are you sure? Double row coils can look clean from the outside and even the backside when you look inside the unit, but split them apart and...surprise! Years of crud and filth that no amount of surface washing will bust loose.

    As it stands from my vantage point, you may have a double case of inadequate airflow...double row condenser coil plugged where you can't see, and interior airflow problem, caused by belt slippage (10 ton unit is likely to be belt driven) or worn pulleys, restricted return, dirty filters, supply registers blocked off, etc.

    This would explain low superheat with high head pressure.

    Get a subcooling reading and check if condenser coils are double row. Let us know what you find.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  7. #7
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    May 2004
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    Metering devices almost never fail to the open position..............

    Dear Airworx,

    Regarding your previous post:

    “high superheat would mean restriction or bad metering devise, low superheat would mean airflow across evap is low.”

    High superheat “could” mean restriction, or bad metering device, but it could just as easily mean low refrigerant charge, condensing section reduced capacity, high internal heat load, higher than normal indoor airflow, or one of several other things, not just the two you posted.

    Low super heat “could” mean airflow across the evaporative coil is low, but it could also just as easily mean an overcharged system, low internal heat load on the evaporative section, or anyone of several other things, not just the one item you posted.

    Remember to say what you mean, and mean what you say.

    As for the comment as the metering device has malfunctioned, I post this question to all here at this thread:

    Based upon the inherent nature of the construction of the most common metering devices in the air conditioning marketplace today, namely the fixed orifice type, the capillary type, and the TXV type, don’t these valves “almost always” fail to the “closed position”, which of course would give us a symptom of too high of superheat?

    Please respond to my question……

    Respectfully Submitted,
    John J. Dalton


  8. #8
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    My two cents.............

    Dear Ace12,

    First and foremost, I agree with your assignment that additional information is required to begin properly diagnosing this refrigeration system. Given the fact that the system “could” be a 10 ton means the system “might” have a TXV for its metering device, given that fact, the system “could” be overcharged under the limited posted operating conditions as well.

    The following information is needed to start the diagnostic process:

    Discharge, suction, and liquid line pressures (posted at 300/ ? / 60)
    Suction and liquid line temperatures ( posted at 36/ ?)
    Outside air (posted at 85)
    Return and supply temperatures
    Compressor FLA or RLA
    Compressor actual operating amps

    As they say, the ball is in your court my friend…………….

    Respectfully Submitted,
    John J. Dalton

    PS: you were right in your original post in your assumption that additional information will probably be needed…………….

  9. #9
    Here's my .02 worth.

    What temp is the refrigerated space when you are taking these readings?

    Also ... when you are speaking of superheat ... are you referring to the evaporator superheat of the coil itself?

    Or are you speaking of the overall system superheat which you measured at the suction line, six inches from the compressor?

    Did you measure for pressure drop across any and all filter/driers?

    Have you been the only guy servicing this system? Do yo have acess to the records? Could someone have contaminated it with non-condensibles?

    Sounds to me like your on the right track here.
    Keep up the great work!

  10. #10
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    Re: Metering devices almost never fail to the open position..............

    Originally posted by john dalton
    As for the comment as the metering device has malfunctioned, I post this question to all here at this thread:

    Based upon the inherent nature of the construction of the most common metering devices in the air conditioning marketplace today, namely the fixed orifice type, the capillary type, and the TXV type, don’t these valves “almost always” fail to the “closed position”, which of course would give us a symptom of too high of superheat?

    Please respond to my question……
    John,

    Fixed metering devices fail closed when they get plugged. A capillary cannot fail open. But a piston type can 'fail open' if it gets some debris under it so it can't close properly. I've personally seen two do that.

    TXV's can fail in almost any position. I find them failed open about as often as I find them failed closed. They can also be "stuck" in some part open / part closed position where they would act like a fixed metering device.

  11. #11
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    Location
    Los Angeles, CA
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    Clarified Statement.................

    Dear Cxagent,

    Regarding your post:

    “Fixed metering devices fail closed when they get plugged. A capillary cannot fail open. But a piston type can 'fail open' if it gets some debris under it so it can't close properly. I've personally seen two do that.

    TXV's can fail in almost any position. I find them failed open about as often as I find them failed closed. They can also be "stuck" in some part open / part closed position where they would act like a fixed metering device.”

    Piston type metering devices can stick open with some debris, I give you that, but if 100 of these type metering devices “failed” the vast majority of these(over 95) would fail to the below open to full closed position. And I too have seen TXVs fail in the open position, but that’s been less than five times over a thirty-one year carrier, therefore I’ll clarify my position by the first statement, and add another statement about TXV failures in the field.

    When an A/C metering device fails, the vast majority of time, over 95% (more like + 98%) the device fails in the partially closed or full closed position.

    When a TXV metering device is condemned, over 90% (more like 95%) of the time, it has been misdiagnosed.

    How would you say to those statements Cxagent?

    Respectfully Submitted,
    John J. Dalton



  12. #12
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    Location
    Austin, Texas
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    Re: Clarified Statement.................

    Originally posted by john dalton

    <snip>

    When an A/C metering device fails, the vast majority of time, over 95% (more like + 98%) the device fails in the partially closed or full closed position.

    When a TXV metering device is condemned, over 90% (more like 95%) of the time, it has been misdiagnosed.

    How would you say to those statements Cxagent?

    That might be your experience, but it doesn't match what I've seen. Do you have a handle on the industry wide metering device failure data?

    I have to admit that I don't see that many 'metering device' failures. The ones I do see have probably had several tech's there before I'm called in.

  13. #13
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    May 2004
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    Thanks for the replies Gentlemen. i will Do my best to get all the requested info the next time im out there. As for right now, they say its running fine. but I know it is only a matter of time. With only 4* Total Superheat, I'm sure they will be calling back

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