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Thread: what is a TXV?

  1. #14
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    What Effect TXV on opperation efficiency

    This may be a silly question however, since I've seen so many folks chime in I have to ask?
    • So how does this effect the efficiency of my AC?
    • Will it give me a higher SEER rating?
    • Do most new high SEER rated system already have one installed?
    • If I don't have one on my current AC system should I get one?


    Inquiring minds want to know ....

    Thanks

  2. #15
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    Quote Originally Posted by AtticAce View Post
    Saturated vapor, liquid only if you leave off the TXV valve or restrictor.
    Sorry; but Contactor is correct. The evaporator has saturated liquid until near the end of the coil where it has become mostly saturated vapor, at that point it begins to superheat.
    If You don't have time to do it right, when will you have the time to do it over?

    "Perception is Reality" Look & Act like a Professional

  3. #16
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    Quote Originally Posted by timby View Post
    This may be a silly question however, since I've seen so many folks chime in I have to ask?
    • So how does this effect the efficiency of my AC?
    It assures the evaporator has the correct amount of metered refrigerant for a given heat load imposed onto the coil.

    It helps the coil run at a consistent saturated temperature, meaning in humid climates it will dehumidify the air better than a coil with a fixed restrictor metering device.

    It reduces the likelihood of liquid refrigerant reaching and damaging the compressor by closely controlling superheat. A fixed restrictor can't do that.
    • Will it give me a higher SEER rating?
    If your system does not already have one installed, you may see an uptick in SEER if you install one...closer to home you will see an improvement in performance, especially humidity removal.
      • Do most new high SEER rated system already have one installed?
    Many do, but not all. SEER is a numbers game. Tweak coil size, motor winding size...whatever it takes to make the numbers under the given test conditions. If the manufacturer can do it with a piston, he will. Otherwise, TXV.
    • If I don't have one on my current AC system should I get one?
  4. If you want one, get one. If you live in a hot, humid climate, definitely get one. If you live in Arizona or other arid region, you may do just fine with a piston, depending on how well the system is installed and charged.

  • #17
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    Actually the main reason for increase in efficiency comes in the off cycle. The non-bleed TEV keeps the liquid in the liquid line allowing the system to restart ready to go, rather than having to build the head and subcool and is quicker to feed a solid column of liquid to the TEV.

  • #18
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    True, but it also improves the "B" test results (82F ambient), providing better latent heat removal as shophound notes.

  • #19
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    Quote Originally Posted by Andy Schoen View Post
    True, but it also improves the "B" test results (82°F ambient), providing better latent heat removal as shophound notes.
    It would be more accurate to say "A hard shut-off TXV can and has been shown to improve dehumidification over a piston metering device in many cases".

    "Improves the B test results" isn't exactly what shophound noted. I'm aware that you are a TXV (excuse me "TEV") expert, but "B test results" could stand to be elaborated on, I think. Do these test results find, for instance, that the TXV is always going to improve dehumidification over a piston in the same system under the same indoor and outdoor conditions, regardless of other system features or settings, as implied by shophound?
    Last edited by hvacrmedic; 03-21-2008 at 03:57 AM.

  • #20
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    Quote Originally Posted by hvacrmedic View Post
    It would be more accurate to say "A hard shut-off TXV can and has been shown to improve dehumidification over a piston metering device in many cases".
    The fact that a non-bleed type TEV can hold a pressure differential during off-cycle has little to do with it providing improved dehumidification. The fact the TEV can control superheat despite varying head pressures allows for greater evaporator performance and improved dehumidification under these conditions.

    Quote Originally Posted by hvacrmedic View Post
    "Improves the B test results" isn't exactly what shophound noted.
    No, he stated you can get improved dehumidification with a TEV.

    Quote Originally Posted by hvacrmedic View Post
    but "B test results" could stand to be elaborated on, I think.
    Perhaps. But I've discussed this subject several times in the past. For those who are interested in reading the standard, it can be downloaded from here: http://www.ari.org/ARI/Content/Finda...Listing_PK=160

    Quote Originally Posted by hvacrmedic View Post
    Do these test results find, for instance, that the TXV is always going to improve dehumidification over a piston in the same system under the same indoor and outdoor conditions, regardless of other system features or settings, as implied by shophound?
    Now you're putting words in his mouth.

  • #21
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    Thanks for the Input

    ShopHound

    Thanks for the heads up.

    I'm currently looking at replacing my existing Goodman 4 ton HVAC with a more efficient one. When I had the last one installed I wasn't as informed a consumer as now (thanks to this site). The old , nearly 30 year old Carrier (SP?) died nearly 11 years ago in the middle of a heat wave in the summer in Texas. I didn't have the resources I have today and I had a company that had been doing my AC work come in and replace the dead unit with a new one. The system never performed as expected mainly because of the install. They increased the unit from a 3.5 ton to a 4 ton. They didn't replace the copper lines so they were undersized. They didn't cut the hole in the floor to the proper size thereby restricting the air flow to the unit. Also, I had some air duct problems that were not resolved. So to make a long story short, I've spent some time touching bases with the local AC guys to get someone that does a good install and have him recommend a new unit. So, it looks like I'll be getting a new 16 SEER (13 EER) rated unit in the next week or so.

    Thanks everyone for all the help.

  • #22
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    Quote Originally Posted by hvacrmedic View Post
    It would be more accurate to say "A hard shut-off TXV can and has been shown to improve dehumidification over a piston metering device in many cases".

    "Improves the B test results" isn't exactly what shophound noted. I'm aware that you are a TXV (excuse me "TEV") expert, but "B test results" could stand to be elaborated on, I think. Do these test results find, for instance, that the TXV is always going to improve dehumidification over a piston in the same system under the same indoor and outdoor conditions, regardless of other system features or settings, as implied by shophound?
    Medic, attached to this post is the B Test parameters from the ARI document Andy linked to.

    The improvements I have in mind in dehumidification performance given by a TXV/TEV vs. a piston are practical in basis. When a house in a humid climate needs latent capacity, a TXV, to my observation, will deliver more consistent latent capacity over a wider variety of real world operating conditions than a piston can. We don't live in a world of "B Tests", but B and other tests are necessary for establishing a benchmark for rating system capacities for a cross section of manufacturers.

    What I believe Andy is saying (don't want to put words in his mouth ) is that B Test results show the TXV under same conditions as a system with a piston will yield greater moisture extraction from the air passing over the coil. To my understanding, this is because the TXV can accurately keep more of the coil's length under a saturated vs. superheated condition. Again, more heat transfer occurs from the air to the coil when the section of coil under consideration is experiencing a phase change of refrigerant within the coil tubing. Superheating refrigerant does not produce the same refrigerating effect as refrigerant in a phase change. Sections of the coil with superheated refrigerant will be warmer than sections of the coil where the refrigerant is in a phase change condition. The warmer coil sections will extract less moisture from the air, for it has a higher ADP.
    Attached Files Attached Files

  • #23
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    Quote Originally Posted by timby View Post
    ShopHound

    Thanks for the heads up.

    I'm currently looking at replacing my existing Goodman 4 ton HVAC with a more efficient one. When I had the last one installed I wasn't as informed a consumer as now (thanks to this site). The old , nearly 30 year old Carrier (SP?) died nearly 11 years ago in the middle of a heat wave in the summer in Texas. I didn't have the resources I have today and I had a company that had been doing my AC work come in and replace the dead unit with a new one. The system never performed as expected mainly because of the install. They increased the unit from a 3.5 ton to a 4 ton. They didn't replace the copper lines so they were undersized. They didn't cut the hole in the floor to the proper size thereby restricting the air flow to the unit. Also, I had some air duct problems that were not resolved. So to make a long story short, I've spent some time touching bases with the local AC guys to get someone that does a good install and have him recommend a new unit. So, it looks like I'll be getting a new 16 SEER (13 EER) rated unit in the next week or so.

    Thanks everyone for all the help.
    Timby, I'm glad this discussion has been helpful for you. Sometimes us tech nuts get a little wild and wooly throwing around refrigeration theory and the like, but if you walk away from all that with a benefit, that's super.

    In your case, you likely were just another sad victim of a contractor throwing in a larger unit, thinking such would cover his tail end (you would not call him up on a 100+ degree day, complaining his system isn't keeping you cool). His error is that he insured himself against a day that occurs only a few hours a year.

    I also live in Texas, I know we get miserably hot days, but we also get many days under 100 degrees with higher humidity levels that require our a/c systems to run and remove humidity as well as keep indoor temperatures comfortable. An oversized system is exactly incorrectly equipped to handle these scenarios.

    This time around, insist your contractor conduct a thorough heat load calculation for your house. If he wants to replace like for like because that's what you already have, send him packing. You deserve a well designed and installed system this go around, having suffered from one that was sized only to benefit the installing contractor, IMO, vs. you, the person who has to live and pay for this system for years to come.

  • #24
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    Quote Originally Posted by shophound View Post
    The improvements I have in mind in dehumidification performance given by a TXV/TEV vs. a piston are practical in basis. When a house in a humid climate needs latent capacity, a TXV, to my observation, will deliver more consistent latent capacity over a wider variety of real world operating conditions than a piston can.
    It would be difficult to argue in favor of a restrictor over that of a TEV when it comes to latent heat removal given real world operating conditions.

    Quote Originally Posted by shophound View Post
    We don't live in a world of "B Tests", but B and other tests are necessary for establishing a benchmark for rating system capacities for a cross section of manufacturers.
    Quite true.

    For those who are RSES members, the Journal will publish a nice article on programmable thermostats within the next few months. This could form the basis for an interesting discussion on how these thermostats typically control equipment relative to the current SEER cyclic test (D test), i.e., 6 minutes on and 24 minutes. Hint: the D test may not be terribly relevant nowadays.

  • #25
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    Quote Originally Posted by shophound View Post
    Timby, I'm glad this discussion has been helpful for you. Sometimes us tech nuts get a little wild and wooly throwing around refrigeration theory and the like, but if you walk away from all that with a benefit, that's super.

    In your case, you likely were just another sad victim of a contractor throwing in a larger unit, thinking such would cover his tail end (you would not call him up on a 100+ degree day, complaining his system isn't keeping you cool). His error is that he insured himself against a day that occurs only a few hours a year.

    I also live in Texas, I know we get miserably hot days, but we also get many days under 100 degrees with higher humidity levels that require our a/c systems to run and remove humidity as well as keep indoor temperatures comfortable. An oversized system is exactly incorrectly equipped to handle these scenarios.

    This time around, insist your contractor conduct a thorough heat load calculation for your house. If he wants to replace like for like because that's what you already have, send him packing. You deserve a well designed and installed system this go around, having suffered from one that was sized only to benefit the installing contractor, IMO, vs. you, the person who has to live and pay for this system for years to come.
    Timby, fishindad here. I like 410A systems. Get info on that type of refrigerant system, and I strongly agree with shophound about getting a load calc on your home. 16 seer machines might only be 410A, but dont take my word 4 that. and your lineset (copper lines between the unit) those will have to be changed out if different (freon)/refrigerant is used and either a 7/8 or 1 1/8 vapor line will be installed for those higher effiency units. if the last co. put a bigger system in they need to also size your ducting and return air accordingly, TIP if you don't know already keep those filters clean, I love a clean filter

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