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Thread: TXV surprise

  1. #1
    Join Date
    Sep 2005
    Location
    Columbia, TN
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    58

    Question TXV surprise

    Is there anyway you can tell from the nameplate or other furnace characteristics that will let you know if the metering device is a txv or fixed metering device, without actually seeing the metering device.

    I am finding my self trying to charge by superheat and right when I think i'm on target, the txv adjust and there goes my superheat and my time. Is this common or am I missing a clue.

    Thanks

    Mike

  2. #2
    It will usally state what it is on the Air Handeler. If you can't figure out what you have then open it up and find out. The big diffrence between a TXV and a fixed bore device is that the txv uses a receiver to control superheat during various load conditions to adjust with the load.

  3. #3
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    It takes less time to look if its a txv, then it does to undo what you just did by guessing.

    Not all installers mark if they installled a txv.
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  4. #4
    Join Date
    Apr 2005
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    new jersey
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    I thought that subcooling is the only way with TXV installed

  5. #5
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    Aug 2004
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    North Richland Hills, Texas
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    Quote Originally Posted by rick00721 View Post
    The big diffrence between a TXV and a fixed bore device is that the txv uses a receiver to control superheat during various load conditions to adjust with the load.
    What? A receiver? Please explain how you think a TXV works...
    If more government is the answer, then it's a really stupid question.

  6. #6
    Join Date
    Aug 2003
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    Fort Worth, TX
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    Quote Originally Posted by rick00721 View Post
    The big diffrence between a TXV and a fixed bore device is that the txv uses a receiver to control superheat during various load conditions to adjust with the load.
    I've been around some old residential split system equipment in my time, have yet to see a receiver on any condenser I've come across.

    Perhaps you meant to say that a TXV uses a thermal bulb attached to the suction outlet of the evaporator to control superheat?

    As for how to tell if a system has a TXV or a piston on it without opening up the evap casing or air handler, your gauges, superheat, and subcooling readings can give you a clue. Piston superheat tends to wander, whereas the TXV equipped system will show a steady superheat, even if you adjust charge up or down a bit. On a piston system it doesn't take much refrigerant either way to affect a superheat change. Vary the charge a little with a TXV and you will likely see the superheat remain unchanged.

    Piston superheat is affected both by evap heat load and head pressure. You can partially block the condenser coil on a piston system and watch your superheat change (and your suction pressure rise). Do the same thing with a TXV and the superheat, if it changes at all, will change little. Suction pressure won't change much, either.

    An older school way to tell if a system had a TXV on it, in the days when recip hermetic compressors reigned, was the presence of a start capacitor and potential relay for the compressor. With the advent of scrolls that method isn't as reliable.
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    • Refrigeration makes the HVAC psychrometric process happen.
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  7. #7
    Join Date
    Jul 2004
    Location
    Massachusetts
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    I find receivers on heat pumps at the outdoor unit but am clueless about a receiver for a TXV. Maybe you're talking about heat pumps with TXV's and that's the only place you're seen a TXV. But straight cooling systems also use TXV's. They used to be a rarity but with the advent of high efficiency equipment, the old TXV is coming back into vogue. It's a policy at our company that installers mark the coil if there's a TXV installed. Some field installs of TXV's leave no doubt because the valve is outside the coil jacket. But OEM installed TXV's are always inside the jacket. If it's not labelled and you can't tell by the equipment number, just be like the people in Missouri. Look.

  8. #8
    Join Date
    May 2007
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    18
    Quote Originally Posted by rick00721 View Post
    It will usally state what it is on the Air Handeler. If you can't figure out what you have then open it up and find out. The big diffrence between a TXV and a fixed bore device is that the txv uses a receiver to control superheat during various load conditions to adjust with the load.
    I think he is calling the thermal bulb a receiver in a sense that it receives the information needed to make the correct SH load adjustments. That what it sounds like to me anyway.

  9. #9
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    Quote Originally Posted by skippedover View Post
    I find receivers on heat pumps at the outdoor unit but am clueless about a receiver for a TXV.
    I believe on a heat pump, being on the true suction side, it is called an accumulator. Receivers are downstream of the condenser.
    • 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.

  10. #10
    Haha yes your right sorry about that No you won't see a receiver on a residential. I was thinking more on the lines of refrigeration not A/C my mistake. So no don't go looking for a reciever because you will never find one out there lol

  11. #11
    Commercial systems may have a receiver but im not sure.

  12. #12
    Quote Originally Posted by rick00721 View Post
    Commercial systems may have a receiver but im not sure.
    Do not dig yourself any deeper.
    Commercial RTU's with TXV's do not have receivers either, they also are critically charged systems.

    Refrigeration systems with TXV's have receivers for storage of unused refrigerant during different loads.

  13. #13
    Quote Originally Posted by smilies View Post
    Do not dig yourself any deeper.
    Commercial RTU's with TXV's do not have receivers either, they also are critically charged systems.

    Refrigeration systems with TXV's have receivers for storage of unused refrigerant during different loads.
    Ahh so they don't. See im used of working with commercial refrigeration dont get out much if you know what I mean. Learn somthing new everyday thanks smilies

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