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  1. #1
    Join Date
    Sep 2000
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    Greenville, SC
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    922
    I have a question about TEV (TXV) performance. As we know, TEVís are sized by tons, selected based on application, they come external or internal equalized, attempts to maintain superheat, and matched bulb with known refrigerant. My question is what happens with an external equalizer TEVís bulb begins to since suction line temperatures in the range of a refrigerator rather than a high temperature A/C coil? Does the TEV lose control and begin to hunt? Does the TEV shut down, restrict completely, and then fails to pull back out because coil eventually freezes up?

    I am beginning to face more and more TEVís 13 SEER systems whose has poor airflow designs, bad duct layout, and restricted airflows. Therefore, the poor airflow is leading to low superheat and TEVís seemly acting squirrelly. Have anyone faced these problems. Are you finding your TEVís losing control or not?
    A switch is a "switch" is a switch".

  2. #2

    Unhappy Hi Backspin

    Sorry, I never faced that problem before.

    However, a TEV, being a control valve with seat, WILL hunt if the opening is too small, and there is liquid forcing through from upstream.

    If it is closing due to low superheat at the outlet of the evaporator, then it is working fine.

    Does the compressor shut off when this happens? If it does, then the solenoid valve and/or the low pressure switch is working fine.

    Have the system been tuned for proper superheat? Is the coil dirty?

    If everything is ok, then resizing the ducts, and reducing bends (if at all possible) within the duct might solve the problem.

    [Edited by mjk_na on 04-26-2006 at 07:03 PM]

  3. #3
    Join Date
    Sep 2000
    Location
    Greenville, SC
    Posts
    922

    TXV settings

    The TXV is a factory install with no adjust been made to it.

    Yes it appears that the TEV is shutting down due to low superheat, but then the coil begins to freeze of slowly and slowly and slowly. Finally a block of ice is back to the suction line and valve is completly covered too. Compressor doesn't shutt off, but keeps running. No low pressure control.

    Superheat obvoiusly goes up, but the valve bulb is so cold, it doesn't exert enough pressure to open. This is my suggestion of what is hanppend. Does it seem plausbile?

    Just courious.
    A switch is a "switch" is a switch".

  4. #4
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,376
    Originally posted by Backspin
    I have a question about TEV (TXV) performance. As we know, TEVís are sized by tons, selected based on application, they come external or internal equalized, attempts to maintain superheat, and matched bulb with known refrigerant. My question is what happens with an external equalizer TEVís bulb begins to since suction line temperatures in the range of a refrigerator rather than a high temperature A/C coil? Does the TEV lose control and begin to hunt? Does the TEV shut down, restrict completely, and then fails to pull back out because coil eventually freezes up?

    I am beginning to face more and more TEVís 13 SEER systems whose has poor airflow designs, bad duct layout, and restricted airflows. Therefore, the poor airflow is leading to low superheat and TEVís seemly acting squirrelly. Have anyone faced these problems. Are you finding your TEVís losing control or not?
    I'll give my take on TXV's and low load conditions momentarily but what strikes me foremost is your second paragraph. You're facing more high SEER systems with TXV's tied to poor air distribution. And getting a customer to spend money on correcting crappy ductwork is like looking to buy a Hummer that gets 40 MPG. In the city with a/c on.
    So...what to do? If the customer won't go for improving airflow, why even bother with these higher SEER boxes? If contractors won't design and install good air distribution...same question. Might as well rip it all out and put window shakers in every room. At least then the customer would have zoning!

    TXV's work off of three pressures. Evaporator, spring, and bulb. Two of these three are valve closing pressures - evap and spring- only one is an opening pressure - bulb. As load drops off, so does evap and bulb pressure. Bulb pressure drops because superheat diminishes under low load conditions. Spring is still same pressure...it does not change with the other two variable pressures. Spring tension (if possible) is what is adjusted to change a superheat setpoint.
    As the load falls off, so does superheat, thereby dropping bulb pressure on the powerhead. The valve begins to throttle closed. At the same time, superheat will begin to rise as the increasingly clamped down condition of the valve tends to starve the evap. The valve reacts and begins to open due to increasing bulb pressure on the powerhead, which will in turn result in lower superheat as more refrigerant is admitted to the coil. This will again force the valve closed.

    To answer some of your questions more directly, you could say the TXV has lost control when an evap is iced over. More accurately, the TXV is doing all it can to maintain a minimum flow of refrigerant through the evaporator in an attempt to maitain superheat setpoint. In a frozen evap condition, caused by poor airflow, the TXV is being asked to operate below its minimum design limits. It can't maintain superheat if there isn't any there to be maintained. The bulb pressure will not be there to provide any normal sort of modulation for the valve.

    The compressor will shut off with a frozen up coil is if it has low pressure cutoff protection, a freeze stat, the compressor itself fails, or someone pulls the plug after freaking out over all the ice and water on everything.

    When a TXV fails closed, you'll know it. You'll have very low suction pressure and not a lot of head, either, even if you dump a ton of refer into the system. If the coil is frozen, de-ice everything and restart the system. If you get frosting immediately at the TXV, it's either restricted or failed closed. I had this happen on a Lennox split system heat pump TXV not too long ago. Changed the valve and everything was cool again (pardon the pun).

    [Edited by shophound on 04-26-2006 at 01:39 PM]
    • 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.

  5. #5

    Lightbulb Oh my!

    Originally posted by Backspin
    ...the coil begins to freeze of slowly and slowly and slowly. Finally a block of ice is back to the suction line...
    I think we have a winner here. Your statement of,

    1. "...ice building slowly and slowly"...

    2. and, "...ice is back to the suction line..."

    Suggests that you have insufficient refrigerant inside the air conditioning unit.

    Please check for refrigerant charging. The symptoms that you have just posted seems like a classic INADEQUATE charging case.

    Charge using superheat method, as you're using TXV. I think that will solve it.

  6. #6
    Join Date
    Apr 2002
    Location
    Dallas, TX
    Posts
    2,987
    Originally posted by Backspin
    My question is what happens with an external equalizer TEVís bulb begins to since suction line temperatures in the range of a refrigerator rather than a high temperature A/C coil?
    The TEVís thermostatic (bulb) charge designed for a/c is typically more than capable of handling evaporator temperatures in the refrigeration range. You will find heat pump units having TEVs on both the outdoor and indoor coil have the same thermostatic charge, and the TEV on the outdoor coil works in the refrigeration range most of the time anyway.

    Originally posted by Backspin
    Does the TEV lose control and begin to hunt?
    The hunting is usually the result of low refrigerant flow and capacity that results from the unit operating at low suction pressures. The TEV is essentially oversized for these conditions.

    Originally posted by Backspin
    Does the TEV shut down, restrict completely, and then fails to pull back out because coil eventually freezes up?
    Nope. The TEV will still control superheat. The coil freezeup is simply caused by the evaporator operating at too low a suction pressure.

  7. #7
    Join Date
    Apr 2002
    Location
    Dallas, TX
    Posts
    2,987

    Re: Oh my!

    Originally posted by mjk_na

    Charge using superheat method, as you're using TXV. I think that will solve it.
    Don't do that! unless you are checking subcooling also.

    Restrictor systems: charge by superheat

    TEV systems: charge by subcooling

    Weighing in the charge, priceless

  8. #8

    Unhappy

    What a stupid mistake from myself. Yes, it should be through subcooling.

    Sorry for the clumsiness.

  9. #9
    Join Date
    Sep 2000
    Location
    Greenville, SC
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    Shophound, Any, and Mjk_na thanks for your replies.

    My comment about poor airflow and duct design stems from choices you have to make when you are either changing out pre-existing units, servicing units already installed by antoher company, or as you mention homeowners who refuse to upgrade. With the January passing the 13 SEER law, there are fewer and fewer 10 and 12 SEER equipment to purchase.
    ---------------
    The comment about the ice slowly building is not due to refrigerant low charge. Operating pressures and temperature start off fine. But as the TXV loses it, then the coil temperature drops and then the problems begin to occur.

    I had a zone system with a bypass damper dumping too cool of air back to my return and I really caused me problems.
    -------------


    Of course things are made worser with low ambient running conditions. Although the TXV is supposed to handle this condition, poor airflow (heat load) seems to be my biggest problem.

    Shophound, you said you replace the TXV on a lennox due to a quick freezing up at the metering device. I'll look at this closer.

    Thanks for all comments.

    A switch is a "switch" is a switch".

  10. #10
    Join Date
    Apr 2004
    Posts
    7,680
    To add to whats already been said you have to understand that the TEV controls superheat ONLY, not suction pressure or suction line temperature. In a low load condition a TEV will slow the flow into the evaporator which in turn will lower the suction pressure and thusly the saturation temperature. Since it controls superheat it will prevent floodback and yes it will hunt when the system is not loaded. That said in a low load its more likely a system with a TEV will freeze sooner that one with a piston. Of course with the piston you will be killing a compressor.

    Bottom line is before you do anything you need to fix the low airflow (or low load) problem, because with either metering device you will have problems and in addition your efficiency, capacity and reliability will suffer.

  11. #11
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
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    14,915
    I have seen different type/brand TXVs respond in different ways to the same kind of conditions.

    The ballance port type TXVs seem to respond somewhat differently in some situations than valves designed for a particular capacity too.

    I have seen many systems where low airflow/load caused the coil to start icing up, and the fixed capacity TXV kept pinching the refrigerant flow off to the point of there being very little refrigerant flow, so no ice formed past the coil.

    The Ballance port type TXVs that will work for a range of size systems seem to only pinch off the flow so far, then loose control of the superheat.

    Just last week my boss thought a system had a bad TXV, but when I went back out to change it, I found the problem was actually very poor airflow and a compressor not pumping efficiently. The system had low superheat and low sub cooling, , but the pressures were close to normal for the indoor and outdoor conditions, head pressure just a tad low.
    Low airflow/load was causing the TXV to pinch off the refrigerant flow to control superheat, but the inefficient compressor was artificially inflating the suction pressure.

    I'm just glad I looked more closely before swapping out the TXV my boss condemned. I figured I best check it out when I noticed the air handler sounded like a vacuume cleaner sucking through a straw, due to not having enough return.
    If more government is the answer, then it's a really stupid question.

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