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Thread: TXV

  1. #1
    Join Date
    Jan 2005
    Location
    Bakersfield, CA
    Posts
    209
    I've read a few posts on the issue of TXVs. I am building a new home in Bakersfield, CA. The climate is hot in summer (often >100), relatively low humidity. Winters rarely are below freezing.

    One contractor has offered me the option of having TXVs for an additional $350 per unit. He isn't making a hard sell, he's just offering the option and tells me that opinions vary as to the usefulness of TXVs.

    There seems to be a fair amount if disagreement between the professional members of this site as to the value of TXVs.

    I would welcome any opinions or hard facts that the professional members have on this issue.

    Thank you.

  2. #2
    Join Date
    Nov 2001
    Posts
    1,874
    This maybe more stupid then helpfull but Here goes.

    Every day can be alittle different. The sun beats down 1 day, and it's cloudy for 2. The cooling load on those cooler days is different from when the sun beats down.
    A TXV will fluctuate the flow of refrigerant as needed to handle the load on the evaporator. A fixed metering device will allow the same flow no matter what the load is.

    Lets say you have the kitchen sink on about 1/3 of the way. You've been in side and lazy all day. Your not very thursty but 1/3 flow fills you cup and your good to go.
    Now the next day your in the yard it's 90* and your working very hard, At your kitchen sink that 1/3 flow just can't provide enough water to conqour your thurst. So you have to increase the flow to satisfy your needs. Or stand at the sink all day waiting for enough water.
    What if the sink could only provide 1/3 flow, you'd be hurting on those hot days huh ?

    Like I said kind of stupid but there it is.
    you'll increase your eff and today thats important.
    If you try to fail, and succeed.
    Which have you done ?



  3. #3
    Join Date
    Apr 2004
    Posts
    7,680
    Good attemp at an analogy Tool.

    However, the difference is, that on a hot day, a fixed metering device will flood the evaporator which is a good thing within reason. On a mild day the fixed metering device will starve the evap. With that said, a mild day will flow less refrigerant through the fixed metering device and thusly the suction pressure is lower still as is the saturation temperature which means good dehumidification. This is why fixed metering devices work pretty good overall.

    Now you stand to gain some efficiency when installing a TXV and there is something to be said for inherent protection against slugging when a TXV is used.

    For a guy throwing in a 10 SEER unit in a dry climate, he might get 11 (no promises). Is that worth the added cost? That of course is the question. If he is installing a 12 or 13 SEER unit then the TXV should be required. It should be noted that there are people who sell units on SEER but fail to properly match the indoor unit accordingly and frankly that in and of itself should be frowned upon. Check with the mfg spec sheets to find out what difference it will have in efficiency to justify the added cost. Post the equipment you selected so the match can be verified.

  4. #4
    Join Date
    Jan 2005
    Location
    Bakersfield, CA
    Posts
    209

    TXV

    Thanks very much for the information. I will get the model information (Trane 14 SEER I know but will have to ask for model number).

    I'm afraid I do not know what you mean by the inside unit.

  5. #5
    Join Date
    Apr 2004
    Posts
    7,680
    Dont worry about it, I dont know about Trane but if its a 14 SEER unit it better have a TXV.

    Doc <----keeps his mouth shut

  6. #6
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,371
    Doc, I can see how a piston can tend to flood an evap under higher OAT and tend to starve it under lower OAT. However, my experience with piston systems is that under lighter load conditions, the superheat tends to drop, indicating more refrigerant in the evap. Under higher loads the superheat will rise, indicating less refrigerant in the evap.

    The main advantage of a TXV, as you well know, is that it maintains a constant superheat under a wide variety of operating conditions. This is especially advantageous under lower sensible/higher latent conditions when dehumidification is more in demand than sensible heat reduction.
    The superheat of a piston is all over the place, guaranteeing that the compressor will be at risk of slugging under lower load conditions, and may not be sufficiently cooled by the returning vapor under high load conditions.

    Piston systems operated under low ambient conditions with no head pressure control are set up to slug the compressor. TXV's are no less exempt, but probably do allow a lower ambient threshold tolerance before the system would require head pressure control.
    • 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.

  7. #7
    Join Date
    Apr 2004
    Posts
    7,680
    Originally posted by shophound
    my experience with piston systems is that under lighter load conditions, the superheat tends to drop, indicating more refrigerant in the evap. Under higher loads the superheat will rise, indicating less refrigerant in the evap.

    Well I suppose if you were speaking strictly from an indoor load this could would be true (such as affected by airflow issues). Generally, in a proper system the load variable is increased or decreased based on outdoor temperatures.

    However, outdoor temperature directly effects the head pressure and the head pressure determines how much refigerant will flow through a fixed oriface. The greater the pressure, the more will feed through, and lower the superheat. The lower the pressure, the less feed through a fixed oriface and thusly the higher the superheat.

    Look at a superheat charging chart like a carrier slide chart and using different outdoor tems and similar indoor WB temps you will see what I mean.

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