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Thread: Double TXV?

  1. #1
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    Double TXV?

    What is the function of this double TXV on an old Carrier, 50DD054600XC, S#8895569?
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  2. #2
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    Is there a solenoid valve on one? Staging!

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  4. #3
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    thats probably a hot gas bypass

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  6. #4
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    Agreed. The fact that the inlet on the valve in the foreground is smaller than the background provides a good chance it is for hot gas. Follow the line back to the source, likely it will be on the discharge line and have a solenoid valve

  7. #5
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    And it is piped the best way too for HGBP .
    At the outlet of the TXV.
    "Iron sharpeneth iron..."

  8. #6
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    Your reasoning is close. Rather, look at the inlet and outlet of that 'second' valve. Note how they are the same size. That's the only clue we have that this is a hot gas valve. Well, besides the way it is plumbed in.

    If you were on site, the next clue would be that that bulb is just hanging in the return air, and not strapped to the suction line. Then, of course, we could trace out the source of the inlet to that second valve.


    Quote Originally Posted by mrautomation View Post
    Agreed. The fact that the inlet on the valve in the foreground is smaller than the background provides a good chance it is for hot gas. Follow the line back to the source, likely it will be on the discharge line and have a solenoid valve
    A skilled Tech would solder a relay on that board and call it good to go.

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  10. #7
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    Hot gas bypass valve. Temp controlled hot gas bypass valve. Sporlan DR style probably. Model will be stamped in the brass body and of course it's piped from the discharge too,

  11. #8
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    Is it for defrost or heat?

  12. #9
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    Most likely capacity control, like under a low load condition. Keeps the evap pressure above freezing.


    Quote Originally Posted by joejax View Post
    Is it for defrost or heat?
    A skilled Tech would solder a relay on that board and call it good to go.

  13. #10
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    On the 50DD the hot gas bypass was only used on the VAV version of these units. And those good ol step controllers and pressure controlled unloaders. Workhorse units they were...

  14. #11
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    How does the valve work. Is it a reverse acting txv? Wouldn't think you would want it to dump even more hot gas into the evap when the SH rises. Like a normal TXV would.

  15. #12
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    It's not an TXV. It dumps hot gas into the evap. HG can be press or temp activated, installed at the TXV or suction lines with bull head liq inj.

    VAV units air flows can change quickly and drastically especially with vane dampers. Suction pressure will also drop quickly and freeze up, so they inject hot gas into the evap coil when coil temp starts to freeze, then the TXV will open due to the false load, and it'll run basically idling until cooling will start again when the airflow increases, then the HG will close and resume normal cooling. Much better than shutting off on LP and restarting every few mins.

    HG not used often on standard small AC systems but are real common on VAV, environmental Chambers, refrigeration, Lieberts, etc.

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  17. #13
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    It's used to maintain suction pressure during low and variable load periods of operation. It helps keep the system from shutting down on low pressure control and maintains temperature by means of the cap tube/powerhead. Hot gas by pass should be used for comfort cooling when retun air is at 72 degrees F/ 50% rh or less.

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