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txv issue

Hello to all

Having an issue on a residential split system I maintained last year for the first time and had no issues.  This year they called and said it was not working.  I could not get the suction pressure above 30 degrees Fahrenheit. (I always think in temp. so Im not sure of the pressure, 105 psi?, r410a) I was for sure overcharging it because head pressure went up, subcooling went up high but suction pressure did not budge.  liquid line was cool and suction line not cold.  I took the txv bulb off the suction line and let it hang outside the unit and suction pressure went up to about 42 degrees. liquid line got cold but still seemed to appear undercharged based on subcooling and head pressure.  Is this a txv problem and why?

thanks a lot, Michael.


  • Eugene Silberstein_2
    Eugene Silberstein_2 Member Posts: 349
    What is the Superheat

    The TXV has a "one-line" job description: To maintain constant evaporator superheat.

    If the TXV is indeed maintaining constant superheat, the valve is operating. Let's take a look at an example:

    You mentioned that the evaporator saturation temperature was 30 degrees (98 psig). Assuming that the superheat setting on the valve is 10 degrees, the temperature of the thermal bulb will be about 40 degrees, giving us about 118 psig inside the thermal bulb.

    Now, you mentioned that you removed the bulb and let it hang and the evaporator saturation temperature rose to 42 degrees. If the valve is operating properly, the superheat will still be about 10 degrees, the temperature of the thermal bulb will be about 52 degrees, meaning that the pressure in the bulb will be about 148 psig.

    Having not calculated the superheat, I would be inclined to say that the TXV is indeed operating since you, in essence, warmed the thermal bulb and the valve opened. With the bulb sensing an increase in suction line temperature (you removing the bulb from the line), the valve opened to allow more saturated refrigerant into the evaporator coil.

    Also, I believe you mentioned that the high subcooling indicated an undercharge. High subcooling is an indication of an overcharge, not an undercharge.

    Since you are experiencing an excess of refrigerant on the high side and what seems to be a deficiency of refrigerant on the low side, I would examine other liquid line components such as filter driers to see if there is a liquid line restriction, which is what I suspect.

    Please keep us posted and let us know what you find.
  • meplumber
    meplumber Member Posts: 678
    Liquid Line restriction

    Charlie, to me this screams liquid line restriction.  As the Professor stated, the TXV is doing what it is supposed to do.  Check the filter drier or look for a new kink in the liquid line.

    Good Luck.
  • charliechicago
    charliechicago Member Posts: 130
    Thank you

    Dr Silberstein and meplumber for the quick response. I will check the filter drier. What is an acceptable temp drop accross drier. 2 or 3 degrees? Also check for kink. What about strainer at txv? What I don't understand is shouldn't the txv be opening up like it does with bulb off, to compensate for the high superheat? I will also check superheat. Thanks, mike.
  • charliechicago
    charliechicago Member Posts: 130
    Went back today

    It ran until today ok with bulb off. I replaced filter drier. When I shook it it didn't have the normal rattle and seemed a little heavy but it wasn't heavily clogged I could blow through it. When I came back it was running at about 34 degrees. After drier change 35. Pressure slowly drops from 42 to 35. 23 superheat at ah. Could not find any kinks. Liquid line is hot at start up but then cools. 15 sub cooling. Any suggestions. Strainer clogged. Thanks. Mike.
  • Techman
    Techman Member Posts: 2,144
    edited June 2014
    txv issue

    This is where an "electronic sight glass" comes in handy. Hook it up at inlet to TXV ,  got bubbling then look up stream , no bubbling then look to the TXV.
  • charliechicago
    charliechicago Member Posts: 130
    Electronic sight glass

    What's that?
  • Techman
    Techman Member Posts: 2,144
    edited June 2014
    E-Sight Glass

    I have a TIF4000A Electronic Sight Glass. Put two clamps on the LL just before the TXV and the "Ultrasonics" lets you know if there is activity in the LL or not. My meter is 30 years old, I purchased it to check an overhead oil system for cavitation ,which was there. I do not know of a "newer " ESG.

    The system was run w/ the TXV thermal bulb not connected to the suctionline? That's pretty "iffie" as far as protecting the compressor. If the LLFD was replaced ,no kinks in the LL ,then its the TXV turn to be checked.But 1st I would install a SightGlass/MoistureIndicator @the AHU to "see" if there is any activity in the LL. 
  • McGee
    McGee Member Posts: 14
    Electronic glass

    I have the same electronic glass since around 1980. Question is wouldn't there be a full column of liquid upstream to txv? If there were bubbles at that point and with high sc wouldn't that indicate a restriction? Sorry if I misunderstood but I have used this expensive tool very little and you have given me new hope for it. If you have other uses I would really appreciate you sharing. Thanks
  • Techman
    Techman Member Posts: 2,144
    edited June 2014
    TXV issue

    The LL going into the TXV is supposed to be full as you said ,no bubbling/activity is allowed. That's what the ESG is for. A full SG/MI at the cond does not mean there is a full LL at the TXV.Charliechicago,  the TXV thermal bulb that is hanging off of the SuctionLine, was the thermal bulb in the attic air or the air inside the AHU ? What was attic air temp? I also think in terms of temperature as compared to pressure, its easier.

    McGee, the ESG is for "seeing" full or not LL's, for TXV, cap tube, orifice, and also for "flood back "in the Suction Lines. And also fuel oil lines.
    EBEBRATT-Ed Member Posts: 14,572
    edited June 2014
    Air flow

    it may not be a refrigeration problem. You need to check the air flow across the evaporator-400-450 cfm/ton. should be a 20 deg TD depending on the load and humidity.

    sounds like the txv is working  but closing down due to low air flow. A cold liquid line=high sub cooing restricted or overcharged. low air flow, dirty filters, plugged coil, slipping belt dampers in ducts closed down undersized ducts????