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  1. #1
    Join Date
    Jun 2005
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    eastern iowa
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    Lightbulb

    Recently came across a 15 year old Amana split system on a PM check. The following parameters are what I found during the PM:

    1. Temp Diff.= 12 deg
    2. Suct Press= 48 lbs
    3. Head Press= 165 lbs
    4. SH= 35 deg
    5. SC= 13 deg
    6. Outdoor Ambient= 78 deg
    7. Return= 67 deg

    With the evaporator outlet by indoor coil and suction line warm to touch and filter clean I decided that if unit wasn't low on a charge then system probably had Txv and that could have bad powerhead or internals damaged or maybe even poor bulb location or lack of insulating.

    After adding charge to system Suction stayed right @ 48lbs and head continually went up and as expected suct line temp remained warm. After looking found that unit did have Txv and that bulb mounting was @ 10 and 2 and insulated well. I did not warm up bulb to see if I could open valve, looked pretty obvious, bad powerhead or valve.

    Told homeowner of situation I came into and because powerhead and valve body could not be seperated a new Txv should be in the works. He agreed and I came back 2 days later with bossman to put new one on.

    To boss said initially looked like a great call till after completing Txv changeout he looked and found that A-coil was partially plugged and that this was the culprit. He cleaned before we tested out unit with new Txv. Unit worked great after everything was done.

    My question is: who was right? I performed the initial start up suct press went right down to 48 lbs and stayed and suction line was always warm to the touch. SUCTION LINE AT BULB LOCATION ALWAYS WARM. Txv never hunted. He said poor airflow on Txv system usually looks like low charge or refrigerant restriction. Is he right? Do I need to go back to school?

    Appreciate feedback, caught hell over this job from owners and management.



  2. #2
    Join Date
    Jan 2004
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    Lancaster PA
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    He's right, to an extent.

    You skipped an important part of the call.

    VERIFY AIR FLOW!!!!!!

    If you had, and cleaned the coil, and then had the same readings, you would have a happy boss, and customer.

    Theres a good chance that the txv was bad, but now the customer thinks they're paying for some thing they didn't need.

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  3. #3
    Join Date
    Aug 2003
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    Fort Worth, TX
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    Was the original TXV adjustable? If so, it could've been "adjusted" by a previous tinkerer.

    Otherwise, I'd say a power head failure would act similar to what you stated in that the valve would tend to starve the evap if there was no opening force being provided by bulb pressure acting on the head's diaphragm. The readings you gave show a high (I'm assuming) evaporator superheat - you did measure superheat at the evap outlet and not the compressor since you were concerned with checking a TXV's performance, right?

    If the airflow is diminished on a TXV system with normal charge for whatever reason, the TXV will react by throttling closed, because low airflow translates to less superheat for a number of reasons. The TXV will attempt to maintain a consistent superheat temperature up to a point. When system charge is low, superheat in a TXV system will be high. When the charge is too high, the valve will react against a tendency toward reduced superheat by throttling closed.

    If you added refrigerant and you did not see a corresponding drop in superheat, I would also be led to suspect the valve as not reacting properly.
    As you added refrigerant, did you observe an increase in subcooling? Subcooling is how you'll know if you're over or undercharged on a TXV system. Overcharged, liquid refrigerant tends to "stack" in the liquid line. Undercharged, the superheated vapor off the compressor must travel through most of the condenser before it becomes a liquid, leaving less time for it to cool in the subcooling loops of the condenser coils, hence less subcooling occurs.

    EDIT: I do agree with beenthere, however. Verify airflow...should be a normal part of any PM work.
    • 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.

  4. #4
    Join Date
    Jun 2005
    Location
    eastern iowa
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    9
    Thanks for your reply BEENTHERE. Blower motor was operating fine and am guessing it was in high speed with plenty of draft coming from vent in living room I involuntarily felt after making t-stat adustment. Failed to check static press, secondary, or blower wheel

    If he is right can you or someone else help me out with why. The way I understand Txv's is that when the bulb reacts to the temperature of suction line at the evaporator outlet. In this situation this particular area was always room temperature to the touch. At this temperature shouldn't the Txv throttle open temporarily until that area becomes cooler causing the valve to reclose? Having hard time understanding the logic.


  5. #5
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    Jun 2005
    Location
    eastern iowa
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    Thanks for reply SHOPHOUND. Valve was non-adustable.I failed to include at first that i overcharged the system to where the back pressure was close to 200lbs and SC hiked to above 30. I told the bossman about it, didn't change anything. Airflow is usually first and foremost to me on PM's as it was on this one, but after seeing what i saw on my guages and from the line temperature i took airflow out of the equation

    [Edited by greenhorngreg on 06-07-2005 at 07:25 PM]

  6. #6
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
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    Originally posted by greenhorngreg
    Thanks for reply SHOPHOUND. I failed to include at first that i overcharged the system to where the back pressure was close to 200lbs and SC hiked to above 30. I told the bossman about it, didn't change anything. Airflow is usually first and foremost to me on PM's as it was on this one, but after seeing what i saw on my guages and from the line temperature i took airflow out of the equation

    [Edited by greenhorngreg on 06-07-2005 at 07:01 PM]
    Chalk it up to experience, Greg. It's easy to get thrown off track by something that smacks you in the face as appearing way out of whack, which it was. Your boss seems like a straightforward guy in that he didn't hide from the customer that the coil was dirty, but that info revealed made both his life and yours a wee bit uncomfy, yes? You know the plumber's rule...effluent descends from higher elevation to lower elevation (sh@t flows downhill).

    Question: when your subcooling was up over 30, what was your superheat?
    • 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
    Jun 2005
    Location
    eastern iowa
    Posts
    9
    35 degrees

  8. #8
    Join Date
    Jan 2005
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    340
    Originally posted by greenhorngreg
    To boss said initially looked like a great call till after completing Txv changeout he looked and found that A-coil was partially plugged and that this was the culprit.
    Did you "no charge" the TXV changeout, or did you stick the homeowner with the cost of the TXV plus cleaning the evaporator?

  9. #9
    Join Date
    Jun 2005
    Location
    eastern iowa
    Posts
    9
    hey dude i didn't stick nobody, if anybody did it was the bossman. I do everything by the book. as far as i know he charged for the valve and installation

  10. #10
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    Jan 2004
    Location
    Lancaster PA
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    By the superheat you posted, I think your right about the txv.


    You missed the air flow, so now you look bad for awhile with your company, it will pass.

    The customer, will never forget.

    Thats the bad thing.

    We've all made simulair mistakes some where along the line.

    Its called the school of hard knocks.

    An important thing, is that you have learned from the mistake.

    You felt good air from 1 supply and assumed air flow was good.

    You'll know better next time.
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  11. #11
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    Jun 2005
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    eastern iowa
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    you're right there BEENTHERE, i guess you live and learn. Thanks for the replies

  12. #12
    Join Date
    Apr 2004
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    7,680
    You said a high side pressure of 165, that has a saturation temperature of about 89 degrees. If you have a subcooling as you say of 13 degrees, thats a liquid line temperature of 76 degrees which is colder than ambient.

    Now frankly I am betting you are not reading accurate temps, if how ever we assume you are, then you have a restriction in the high side prior to your point of reading.

    If you have a restriction in the high side then you are not likely feeding a solid column of liquid to the TXV and that means it will not work properly. If the coil was dirty it needs cleaned, no doubt about that.

  13. #13
    Join Date
    Sep 2003
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    1,311
    Originally posted by greenhorngreg
    1. Temp Diff.= 12 deg
    2. Suct Press= 48 lbs
    3. Head Press= 165 lbs
    4. SH= 35 deg
    5. SC= 13 deg
    6. Outdoor Ambient= 78 deg
    7. Return= 67 deg
    Here's a tid-bit of info that will help out: Whenever suction pressure gets below a saturation temperature of 32 then there is a risk of the coil icing up. 48 PSIG has a saturation temperature well below 32 which, among other things, means the coil is probably icing up.

    In addition to what doc said about the liquid line, if the external equalizer tube is plugged then the TXV won't operate correctly, either. Some of the most frustrating systems had to do with self-piercing saddle valves which fed the external equlizer... and the saddle valves didn't pierce or didn't completely pierce.

    The TXV shouldn't have had 35 SH even with reduced airflow. 12 TD with 35 SH would mean a really starved evaporator and possibly iced (thus the 12 TD). In an orifice system the low airflow would lead to a flooded evaporator, not a starved evaporator. It sounds like the TXV wasn't working right. I would go with doc in trying to determine if there was straight liquid getting to the TXV and if that proved correct then try determining if something else might have been causing the TXV to provide an unreasonable superheat.

    [Edited by sadlier on 06-09-2005 at 01:22 AM]

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