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Thread: TXV Sensing Bulb

  1. #1
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    Lightbulb TXV Sensing Bulb

    Hey guys,
    I have recently installed a new TXV on Nordyne evaporator coil. Model number: B4VM-X30K-B
    Serial Number: 920773D. The issue I am having is when I have it in cool mode, my Suction pressure goes up to 125psi and drops down to 80, then it just keeps doing the same thing over and over. It takes about a minute for the pressure go up then back down. I have a feeling its my sensing bulb just not placed properly. Does anyone have any ideas as to what it could be? If it is the sensing bulb, some advice on placement and insulation to use over it. The system runs on R-410a. Any help would be really appreciated.
    Thanks

  2. #2
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    Quote Originally Posted by Matador429 View Post
    Hey guys,
    I have recently installed a new TXV on Nordyne evaporator coil. Model number: B4VM-X30K-B
    Serial Number: 920773D. The issue I am having is when I have it in cool mode, my Suction pressure goes up to 125psi and drops down to 80, then it just keeps doing the same thing over and over. It takes about a minute for the pressure go up then back down. I have a feeling its my sensing bulb just not placed properly. Does anyone have any ideas as to what it could be? If it is the sensing bulb, some advice on placement and insulation to use over it. The system runs on R-410a. Any help would be really appreciated.
    Thanks
    What are the SH & SC readings?

  3. #3
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    Thread Starter
    I only checked the SH, but it kept fluctuating going from 50 and dropping to 20. Im a new tech, still learning.

  4. #4
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    Please have your post moved from AOP to the tech to tech forum.
    AOP Rules: Rules For Equipment Owners.

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  5. #5
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    Sensing bulb. Depends how you installed everything too.

  6. #6
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    Whats it do when you remove it from the suction line?


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  7. #7
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    Quote Originally Posted by Matador429 View Post
    I only checked the SH, but it kept fluctuating going from 50 and dropping to 20. Im a new tech, still learning.
    Hello Matador and welcome...
    You say you are still learning, and that is good. All good technicians are still learning.

    What you are describing sounds as if the system is low on charge and the txv is not getting enough liquid... rather a slug of liquid every now and then. Txv systems need to be charged by sub-cooling.
    Keep asking questions it is how you learn. Good luck.

  8. #8
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    A quick check of subcooling will tell a lot. This number is essential to know when charging a system with a TEV.

  9. #9
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    Quote Originally Posted by frankhvac View Post
    A quick check of subcooling will tell a lot. This number is essential to know when charging a system with a TEV.
    Could be low on gas
    Could be incorrect bulb placement. (If in vertical position, cap tubes should be coming off the top of bulb, not the bottom)
    Possible you overheated it
    Could be a piece of brazing rod slag stuck in the equalizer line.


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  10. #10
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    Bulb should be place either between 2 and 5 o'clock position or the 7 and 10 o'clock position. NEVER on top or on bottom of pipe. It needs to be insulated. Once you do this verify subcooling. ~8-10° SC, depends on system. Report back what you did...
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  11. #11
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    Quote Originally Posted by Artietech View Post
    Bulb should be place either between 2 and 5 o'clock position or the 7 and 10 o'clock position. NEVER on top or on bottom of pipe. It needs to be insulated. Once you do this verify subcooling. ~8-10° SC, depends on system. Report back what you did...
    Which most mfrs DON'T do, insulate or position.
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  12. #12
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    Quote Originally Posted by pacnw View Post
    Which most mfrs DON'T do, insulate or position.
    That is correct, but as I pull the bulb off the line to braze, it does not bother me. I think Trane is the only one who ships it installed, which means some installers are overheating the bulb because they do not bother with pulling the panels off, let alone wrapping the bulb with a rag...
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  13. #13
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    Quote Originally Posted by Artietech View Post
    That is correct, but as I pull the bulb off the line to braze, it does not bother me. I think Trane is the only one who ships it installed, which means some installers are overheating the bulb because they do not bother with pulling the panels off, let alone wrapping the bulb with a rag...
    Lennox does as well. I can’t tell you how many horizontal installs I’ve seen where the bulb is still in the vertical position.

  14. #14
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    It’s a piece of copper pipe, it’s not gonna vary that much from 6 to 9 to 12, especially if insulated. When/if it frosts over (low/medium temp) its not like it just frosts in one place. Yes theoretically there could be a river of oil that insulated the bottom, and warmer vapors at the top. So mount it properly, but incorrect o’clock won’t cause 80 psi suction with 410A.

    Make sure it has good thermal contact along the length of the bulb and copper straps are tight. That bulb is measuring temperature just like we do when we check superheat. Kind of obvious but just saying. Do you really put your temp clamps at a certain o’clock to check superheat?

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  16. #15
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    Quote Originally Posted by knave View Post
    It’s a piece of copper pipe, it’s not gonna vary that much from 6 to 9 to 12, especially if insulated. When/if it frosts over (low/medium temp) its not like it just frosts in one place. Yes theoretically there could be a river of oil that insulated the bottom, and warmer vapors at the top. So mount it properly, but incorrect o’clock won’t cause 80 psi suction with 410A.

    Make sure it has good thermal contact along the length of the bulb and copper straps are tight. That bulb is measuring temperature just like we do when we check superheat. Kind of obvious but just saying. Do you really put your temp clamps at a certain o’clock to check superheat?
    Bulb placement does affect the valve operation. I have moved a bulb from the bottom to the manufacturer recommend position and the TXV stopped hunting....thermal contact with pipe and good insulation are key.

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  17. #16
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    Quote Originally Posted by Artietech View Post
    Bulb placement does affect the valve operation. I have moved a bulb from the bottom to the manufacturer recommend position and the TXV stopped hunting....thermal contact with pipe and good insulation are key.

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    I can see that it could help with hunting to get farther from the liquid. In refrigeration I see hunting quite abit, even with proper bulb location. I still don’t think it caused the OP situation of 80 psi.

  18. #17
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    Typically bulb position does not really affect operation until you get into larger pipe, just don't put it on the bottom where there can be a layer of oil acting as an insulation layer.

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  20. #18
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    Quote Originally Posted by Chuck View Post
    Typically bulb position does not really affect operation until you get into larger pipe, just don't put it on the bottom where there can be a layer of oil acting as an insulation layer.
    And make sure if mounting in the vertical position, the captubes on the end of the bulb are facing upwards


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