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Thread: Subcooling not increasing.

  1. #1
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    Subcooling not increasing.

    Hi experts. I can use some guidance.
    Charging a txv system with subcooling is simple.
    The problem I ran in today was really weird. The unit had low on gas and was freezing ice.
    After melting all ice I started adding refrigerant.
    The suction line gauge showed the pressure at 100 means temp was around 30deg or so so no wonder it was freezing. I added 410a as system needed the charge.
    My pressure increased. I charged till about 125psi on suction line and ~300psi on liquid line. My temperature on liquid line remained high. About 96deg (saturated temperature in the middle of condenser coil -reading on high side gage) and 94deg on liquid line temp. That’s just 2deg subcooling. My outside temp was around 81deg. Even adding refrigerant don’t help increasing the subcooling. This is odd. It should be simple but in this unit the liquid line temp increasing with the charge. What is wrong.
    It’s a 10yr old system Rheem 410a. I don’t see subcooling data anywhere on the unit.
    Please help.

    System type split (outside condenser and garage air handler)
    Refrigerant 410a
    Ice was built up on inside air handler and outside by the pilot valve (pic attached)
    No subcooling data
    16seer unit - Rheem 10yr old.
    Name:  IMG_4036.jpg
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  2. #2
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    Are you sure the evaporator was thawed? Beyond that, more data is needed. Type of system, refrigerant type, SH, TD, ODA, amperage, pressures for starters. Discharge line temp would be helpful too.

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  4. #3
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    Subcooling not increasing.

    Quote Originally Posted by vin lashon View Post
    Are you sure the ....line temp would be helpful too.
    Thank you. I updated the post with additional information I had and explained little better.

  5. #4
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    depending on other parameters, I have actually had a set of old yellow jacket gauges trick me on a txv system, it wasnt until I used the new set of electronic Testo gauges, which I bought just for the types of systems on walk in cooler and freezer, self contained units to measure sub cool/ superheat, they work well on HVAC stuff also, but what these yellow jacket gauges werent showing me was, the subcool would go sky high, then back down to not registering, then level back off and it would do that somewhat quickly then level back off, but the end result was, the evaporator would freeze, even after pulling the charge, vacuuming and adding back the data plate charge to insure I had the correct refrigerant amount in the system. Sometimes, there just isnt a quick fix to these things and we have to put a lot of time and effort into it, and we have to keep our tools and equipment in working order, like those gauges I had that were old, I have since replaced my gauges every couple of years, its just the cost of doing this kind of work.
    "Orange Man RAD!"

  6. #5
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    81*f AMBIENT? The head should be closer to 333(+-) psi. What was indoor air temp? If the evap was thawed out why is there frost/ice on the outdoor unit?

  7. #6
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    Line length, ll temp at condenser & evap, ssh, evap air temps, noncondensables test????

    Condenser approach low, liquid approach high, subcooling low = possibly low charge or extremely low air flow. Need ssh and evap temps info.

    Not enough info............

  8. #7
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    Sounds like you just don’t quite have it charged fully just yet. Txv systems can by odd to charge. What I mean by that is you’ll be adding gas and watching you sub cool and it won’t be going up too much, then all of a sudden you give it a squirt more and before you know it, it’s overcharged with 20 degrees. Take it slow and I like to add half a pound, let it stabilize then add some more if needed. If there’s no recommended subcool on the machine anywhere most people will charge to between 8-14 degrees. I didn’t see your superheat in the post or supply and return temps so with the info provided, I’d say you stopped charging too early and it’s still a little short.

  9. #8
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    Where did all the gas go ?

  10. #9
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    All this talk of refrigerant, what about airflow?

    If you don't have enough air going across the coil, there's less heat to absorb, meaning low sub cooling. How'd the coil look? TESP and blower speed? Suction pressure was low initially, but did you check superheat? Are you certain the coil is completely thawed? Are you certain the evaporator has a TXV and not a piston?

    Lots of missing info to assume it's low on refrigerant.
    Everything I’m going to say today are my conclusions and my opinions, my opinions are based on my education, my training, my experience. Different people have different experiences, so they have different opinions and I make no claim that my opinion has its origin in the mind greatness. - Paul Harrell

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  12. #10
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    One thing I have noticed when charging a unit with 410 is that if the inside conditions are not close to normal you can easily overcharge the system then have to go back and let some out. Don't know why, if it has to do with the charge in the TXV bulb or what but I have seen it happen more than once.

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  14. #11
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    Subcooling not increasing.

    Some more data.
    Yes TXV system. I checked by removing cover.

    Before I got to the job site the costumer had the AC turned off for atleast 24hours so all ice was fully melted. Coil was clean

    When I got to the site and charged the system (yes slowly), it started cooling with the gauge readings I provided (125 and ~300). The house cooled to 75deg or so inside temp.

    I didn’t check the superheat but will measure today.

    The unit has been cooling for 24hours now with no freezing issues and the house is cooling (75deg inside).
    Inside blower airflow is good (running full speed) and airflow out of ducts are good also.

    The difference in ambient temp and the air blowing out of outside condenser was about 22 degrees so it is removing heat.
    Both indoor and outdoor coils were clean. I did change the inside filter just for kicks.

    Nonrestriction on liquid line. Temp outside before dryer is same as after dryer and also same by air handler.

    With all those the subcooling won’t come up. With more charge the head pressure keeps increasing but also the liquid line temp keeping the subcooling low. I don’t want to overcharge.

    Yes my gauge is good.
    Please help.

  15. #12
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    No ice? I spy w/ my little eye something that begins w/ the letter "i"? (In that photo). Charge them for a SGMI at the evap and labor to put it in. Oh yea, and 2 SAE flare nuts.

  16. #13
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    You need to measure more than just “blowing good” out of the vents. Duct work could be undersized, duct insulation could have fallen, blower speed set incorrectly, etc. You need to measure TESP and check airflow against the blower performance chart, otherwise your pressures and line temperatures are going to be off.

    More than likely you’re right about it being undercharged, but unless you want a concrete answer to your question, we still need airflow to be verified.

  17. #14
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    For the record, being off for 24 hours does not mean there is no ice unless one looks. A couple years ago my sisters unit iced up. I told her to shut it off and turn the blower on. She shut it off but did not turn on the blower. I got there 24 hours later and there was still a huge slab of ice on the coil so I had to melt ice before I could diagnose she had a bad TXV. Luckily it is a heat pump so that sped up the process.

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  19. #15
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    Subcooling not increasing.

    Quote Originally Posted by TechmanTerry View Post
    No ice? I spy w/ my little eye something that begins w/ the letter "i"? (In that photo). Charge them for a SGMI at the evap and labor to put it in. Oh yea, and 2 SAE flare nuts.
    This pic was taken by the costumer when it iced. The obvious thing for any HVAC tech


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  20. #16
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    Quote Originally Posted by CircusEnvy View Post
    You need to measure more than just “blowing good” out of the vents. Duct work could be undersized, duct insulation could have fallen, blower speed set incorrectly, etc. You need to measure TESP and check airflow against the blower performance chart, otherwise your pressures and line temperatures are going to be off.

    More than likely you’re right about it being undercharged, but unless you want a concrete answer to your question, we still need airflow to be verified.
    Very good point. I endup checking all that. No issues there at all. All duct work intact and blower working great per its chart.


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  21. #17
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    Quote Originally Posted by BNME8EZ View Post
    For the record, being off for 24 hours does not mean there is no ice unless one looks. A couple years ago my sisters unit iced up. I told her to shut it off and turn the blower on. She shut it off but did not turn on the blower. I got there 24 hours later and there was still a huge slab of ice on the coil so I had to melt ice before I could diagnose she had a bad TXV. Luckily it is a heat pump so that sped up the process.
    I see your point but being an HVAC tech I did my due diligence and checked all that in detail and throughly. No ice at all plus I didn’t expect to see any with 90deg weather in Florida


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  22. #18
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    Quote Originally Posted by Laeeq421 View Post
    I see your point but being an HVAC tech I did my due diligence and checked all that in detail and throughly. No ice at all plus I didn’t expect to see any with 90deg weather in Florida


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    Ssh ? Eat ? Lat ? Updated readings ?

  23. #19
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    Subcooling not increasing.

    Quote Originally Posted by ehsx View Post
    Ssh ? Eat ? Lat ? Updated readings ?
    I didn’t save them unfortunately but I remember having a conversation with my supervisor of them being normal.

    That’s the off part about it that we have never seen anything like that before on any unit. Everything is within range and it’s just that the liquid line temp won’t drop or even it it does after discharging the head pressure also drops. Or the other way around which keeps the subcooling around same.
    SMH

  24. #20
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    have you checked discharge temperature? High discharge would mean it is not moving the refrigerant it should which would lean towards still being low on refrigerant.

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