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A coil freezing up....fast

So I get this job handed to me. 4.5 ton coil, 5.5 ton condenser, maybe 900 sq' of space with vaulted ceiling and 5 registers max. I forget furnace size but way oversized. R-22. Coil freezes into a solid block of ice that grows out of the ducting.

My local rep says just put a TXV on it and a 3-ton condenser. Should be good to go. I do this and still freezing up the coil, within a few hours even. I adjust the TXV all the way in but still freezing. Charge is correct, TXV was installed exactly where the instructions said to put it.

Thoughts? Heading up there tomorrow. This is happening with outdoor temps in the mid to high 90's. Sorry I don't have more numbers for 'yall. Last trip up it was 76 in the room and stat was set for 69 and the coil was solid ice, all the way across.

On startup, it pulls a ton of heat out of the rooms and the AC feels nice and super cold. A few hours later with the coil frozen, not so good.

Thanks,    Tim
Just a guy running some pipes.


  • Frozen Coil

    If the coil is frozen solid, you can usually conclude that there is an airflow problem through the evaporator coil.

    There are other things to consider as well. The purpose of the condenser is to reject heat and the evaporator is intended to absorb heat into the system. With such a large condenser coil, the system is rejecting large amounts of heat, while the smaller evaporator is not absorbing much heat into the system. This will result in much lower operating temperature, bring your evaporator saturation temperature well below the freezing point.

    By closing the TXV you are further reducing the saturation temperature in the evaporator.

    Now... One question... What happened when you closed the txv? did the entire coil still freeze, or did only the first few passes freeze?
  • Timco
    Timco Member Posts: 3,040
    Thanks for the reply!

    When I closed the TXV I cracked it back open a 1/4 turn and the entire coil froze again. solid. I would love to take some action while I am up there tomorrow but am just getting my feet wet in AC. Easy to get a new system sized right up & working, but issues like this will require advise and field schooling.

    Thanks,  Tim
    Just a guy running some pipes.
  • meplumber
    meplumber Member Posts: 678
    I agree with the professor.

    Gotta start with airflow. You have no heat exchange going on. For the coil to ice that quickly, you are not changing state in the evap coil.

    Remember 800 cfm per ton is a good number to start with.
  • Definitely Airflow

    Your problem is definitely airflow related.

    Not to throw monkey wrenches, but 400 cfm per ton should be your starting point.
  • JStar
    JStar Member Posts: 2,752

    If you post some numbers, we can head you in the right direction...

    High side, low side pressures. Superheat, subcooling. Air temperature difference. Outdoor air temperature. And if possible, an indoor wet-bulb temperature, and the total static pressure of the blower.

    Those are the minimal amount of readings you'll need to get an idea of what's going on. Anything less will just leave you guessing.

    Right off the bat, it sounds like the system is oversized by nearly 100%. There isn't enough air in the whole house to put the right load on the evaporator coil. I'm sure the ductwork is questionable as well. My house is 900 sq. ft. and I'm running with a comfortable 2 ton system.
  • meplumber
    meplumber Member Posts: 678
    Sorry Eugene.

    400 cfm. My fingers and my brain were not on the same page.

    Thank you.
  • Timco
    Timco Member Posts: 3,040
    Today's results

    So I did not have any AC equipment with me today but we went up there. What I found was that the vast majority (90+%) of the return was coming from the bedroom closest to the furnace and almost none was being pulled from the great room or other bedroom. By blocking 3/4 of the closest return, it pulled a lot more from the great room. Also, when I cleaned the inside coil, lots and lots of blue build-up came off the coil as I rinsed it. Now, lots more air goes through. Last, I sealed all the places where air got around the coil, and now 100% of the air gets sent through the coil. With the system on for 30 minutes and the TXV closed or in all the way, the vents blew frosty cold, the return was even and favored the great room, and the condenser was putting out very hot air. When it finally got the room down to 73 from 77 there were no signs of frost on the suction line.

    I can take actual readings Monday.

    Thanks,  Tim
    Just a guy running some pipes.
  • Techman
    Techman Member Posts: 2,144
    Frozen coil

    Tim, listen to JStar. The answer usually lies in the "readings".So get a couple of digital thermometers that have the decimal point. Like 38.8* , 38.7*  ,38.6* .  SuperHeat/ SubCooling. Pressurer readings help alot in troubleshooting. And humidity levels help also. Indoor/outdoor temps. Temp drop/rise of coils are also good to know.Help your self help you,help us help you!
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