Carrier 38AKSO14 - Page 2
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  1. #14
    Join Date
    Oct 2002
    Location
    Lexington, NC
    Posts
    5,137
    Sorry about that statement. I was thinking about it wrong. The liquid line selonoid closes so the liquid pumps into the condensor, which should raise the head pressure and lower the suction. This is not the problem. I have all good numbers except the suction line temp. I agree that it is probably a bad TXV. This was my original thought, but I was thinking too hard about the pressures being correct. I assume that this TXV doesn't shut down all of the way, which would cause a pump down type senario with the pressures.

    I have not done much work with these larger TXV's that can be adjusted or the power heads changed out. I am wondering if someone can give me a briefing of how to adjust the TXV (I think that you take off the nut/cap on the bottom and turn the adjustment screw, but I don't know how much). I am also wondering if the power head can be changed out(if it is that type which should be evident by a nut at the bottom) without pumping down the unit. In other words, do the little rods that come through the valve body where the head screws off of the top leak freon by? Thanks guys!

  2. #15
    Join Date
    Jan 2008
    Location
    Dallas ,Texas
    Posts
    3,699
    Probably not a bad txv just running unloaded. There are several systems out there that use multiple txv`s on one evaporator.to reduce capacity there is a liquidline solenoid to stop flow through one txv which lowers the suction pressure to onload the compressor to match the load of the active txv. I believe that was what Jax was referring to. But this will also happen when there is no load in the conditioned space.

  3. #16
    Join Date
    Oct 2002
    Location
    Lexington, NC
    Posts
    5,137
    Ok, stick with me here and I will explain this system. They had 1 unit of the model @ that this post is named. That was not enough, so they added a second unit. So, in all reality, we have 4 stages of cooling on this one air handler when we take the unloader capabilities into account, which we should do by all rights. The machine that caries the first 2 stages is running right on the money. Now, there is a time lapse between when these machines were manufactured, and evidently Carrier made some big changes because the compressor is about 2/3 the size of the old machine, the unloader evidently is the type that works off of suction pressure, and there is no hot gas bypass line on the new model. I think that the hot gas bypass is for dehumidifying purposes, but that may be another post question.

    So, I have 1 saying there is a metering device problem and another saying that the unit is running unloaded. If it is running unloaded, then it is probably overcharged now. I have a 98 degree day tomorrow, so I will check the line temp to see what it tells me. Otherwise, I am wondering how to load up this machine with the suction type unloader, if there is a way to do so. Any other recomendations will be appreciated as well.

    So, what I am gathering here is that the TXV has not so much to do with the actual pressure on the low side, but just regulates the superheat. I thought that it had a little effect on both the pressure and the temp.

  4. #17
    Join Date
    Jan 2008
    Location
    Dallas ,Texas
    Posts
    3,699
    First things first you need to find out if that compressor is running loaded or unloaded. Pressure seem ok to me I think if there was a bad txv suction pressure would be lower and discharge higher. When your compressor loads up this will happen. Do they have the same refrigerant circuits or are they seperated? Also verify the compressor model numbers on the compressor .

  5. #18
    Join Date
    Oct 2002
    Location
    Lexington, NC
    Posts
    5,137
    Being that there is no selonoid to disengage in order to get this machine to unload, I do not know how to make it unload. Can you help me with this? As we have determined here, this works off of the suction pressure to unload. It should have been unloaded today if it ever will because it was 98 degrees here with high humidity.

  6. #19
    The piece you are describing on the compressor that is similar to a txv is your unloader, Carrier makes 2 types electric or manually adjusted.Yours is the manual type .Turning the large nut on top ccw will lower the pressure your compressor will load up[full capacity].

  7. #20
    Join Date
    Jul 2008
    Location
    Coastal North Carolina
    Posts
    10
    If this machine is running unloaded, The coil would be frozen at this point, and you would have noticed that for sure. With a suction pressure in the 60's, we can all take for granted that the compressor is, in fact, loaded. If it was unloaded, the suction pressure would be higher. Even more evidence is the superheat. If the compressor is unloaded, and the txv were working correctly, and the refrigerant charge was ok, then the superheat would be 10-15 degrees. All of the information I have read on this thread is pointing me to the txv.

    One thing I will ask though. With this being a multistage application, is the evaporator coil dedicated to this circuit, and this circuit only? In other words, the liquid line feeds either one or two txv's and the suction line for those circuits come back to only this unit? Answer that, and we can put this to bed.

    If the evaporator is, in fact, dedicated, and has only one txv, then what you will need to do is adjust it. Take the bottom cap off, and while measuring the pressure, converted over to saturated temperature, also measure the suction line coming out of the evaporator. If the superheat is too high, which it is in this case, turn the port counterclockwise one turn and recheck, allowing a good 5 minutes to allow it to adjust. Usually one full turn equates to 3 degrees, but that all depends on the type of valve and manufacturer. Adjusting txv's are like turning on water hoses, clockwise will cut off the refrigerant, counter clockwise will open it up.

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