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  1. #1
    Join Date
    Aug 2009
    Posts
    10

    Superheat question

    I have had this happen to me two times in the past week and I am a little confused. When charging a R-22 system with a Fixed orifice or piston I aim for a 18-22 degree superheat. I was charging a system on friday that did something really weird and it happened again today. I was charging the system Really slow...had nothing else to go do after this job. I started with a 35 degree superheat and worked my way down (around an hour) to a perfect 20 degree superheat. Awesome!! I had a perfect 20 degree split between my supply and return also. Left my gauges on the system, went to the truck and wrote up the invoice, cleaned up my area, 20 minutes later....3-5 degree superheat!!! What happened? I went so slow and took my time, I cant believe I may have over-charged the system. Someone said its because of a oversized piston allowing too much liquid passed the piston. Even after a 3-5 degree superheat I still had a 19-20 degree split between S & R so I wasnt sure what to do. Can anyone help?? Thanks!

    about to start my 4th year apprecticeship..I know alot but definately not everything.

  2. #2
    Join Date
    Oct 2007
    Location
    Austin Texas
    Posts
    1,790
    I've had it happen to me a couple of times that high/low ambient conditions or low load conditions will mess with what I'm trying to get dailed in.
    Nemo me impune lacessit.

    How much blood do I have to bathe in to get clean?

    Don't look down on anyone unless you're helping them up.

  3. #3
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,915
    The evap coil may have been dry when you started, then as condensation formed on it and reduced the airflow, the superheat drops. If the weather was fairly mild, the load may have also dropped significantly.

    I'm curious, on what do you base your decision to charge to 18-22 superheat, or in this case, that 20 superheat was "perfect"?
    If more government is the answer, then it's a really stupid question.

  4. #4
    Join Date
    Oct 2005
    Location
    sacramento calif.
    Posts
    1,191
    when charging i try to minimize the time that the evaporator coil is below 0 deg. could you have iced up the coil?
    i was born under a wandrin star.

  5. #5
    Join Date
    Dec 2004
    Location
    Grand Rapids, MI
    Posts
    619
    did you happen to re-check the wetbulb as you were adding ?
    what was the subcooling before addition and at your good 20 SH ?

    luck, dan

  6. #6
    Join Date
    Aug 2009
    Posts
    10
    That's just what I have been taught. If I have no charging chart on the system, or any info that says "desired sub-cooling is ____" i aim for 18-20 superheat if the system is a piston. What do you recommend??

  7. #7
    Join Date
    May 2009
    Location
    South of Heaven
    Posts
    1,151
    Quote Originally Posted by danielworkerbee View Post
    did you happen to re-check the wetbulb as you were adding ?
    what was the subcooling before addition and at your good 20 SH ?

    luck, dan
    Required superheat always changes with available load. This ( fixed orface ) is not a " modulating " device such as an expansion valve. Go to a Carrier / BDP distributer and pick up a superheat calculator and you will understand.
    Last edited by tech45; 08-04-2009 at 09:44 PM. Reason: spelling

  8. #8
    Join Date
    Mar 2004
    Location
    Grottoes VA
    Posts
    5,856
    Quote Originally Posted by tech45 View Post
    Required superheat always changes with available load. This ( fixed oriface ) is not a " modulating " device such as an expansion valve. Go to a Carrier / BDP distributer and pick up a superheat calculator and you will understand.
    I agree you need a SH calculator.
    Karst means cave. So, I search for caves.

  9. #9
    Join Date
    Apr 2008
    Location
    Alberta Canada
    Posts
    2,246

    1 hour conditions do change

    Check intering wet bulb to evaporator and entering dry bulb to condensor. Then using slide ruler you will get SH reading and also you can check vapour pressure and leaving air on evaporator. Last but not least check SC. Good luck. SH reading will fluctuate alittle. R U charging with liquid or vapour.
    Do it right the first time.

  10. #10
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,915
    As others have said, you need a superheat calculator/slide rule/chart, and an instrument for getting the wet bulb temperature of the air entering the evaporator coil.

    Depending on the outdoor and indoor conditions, a correctly charged fixed metered system could have anywhere from 0 superheat to 30+ superheat.
    If more government is the answer, then it's a really stupid question.

  11. #11
    Join Date
    Sep 2002
    Location
    Warren, Michigan
    Posts
    84
    Dardin,
    I am very respectful of this site and the rules governing it. I tend to refrain from chiming in on many posts because of my regular member status.

    It's troublesome that you state: "When charging a R-22 system with a Fixed orifice or piston I aim for a 18-22 degree superheat". Based on that premise alone, it's obvious that you are in need of a little more training. All of the responders to your post have made valid statements although perhaps not answering the question entirely as to why some moments later you found the system to be operating at roughly 4 degrees superheat.

    I think you have two things going on here.

    First, a clear understanding as to how the load affects the charge requirements would be helpful. I suggest you forget all about superheat calculators, and charging charts, and work to understand the formula which generated them. Then, and only then will you see why monitoring the system variables will get you to your final destination regarding a proper charge. Study this forumula for fixed metering systems:
    (WBx3-80-Outdoor Ambient)/2=Required Superheat.

    Secondly, and speaking to the above: You took a good hour to charge a system, and some additional amount of time putting things away and doing a write up. Have you considered that you gave the system well more than an hour to operate? Ask yourself how much you have changed the load conditions indoors as well as what may have change outside as well. Throw in a good measure of the issues mentioned by other responders to this post and you'll have a much clearer picture of why your are having trouble.

    Hopefully, I am not guilty of posting greater than the rules allow. If so, It will never happen again!


    Respectfully, Toddfather
    Never argue with an idiot: They'll drag you down to their level, and beat you with experience!

  12. #12
    Join Date
    May 2009
    Location
    South of Heaven
    Posts
    1,151
    Well the reading of that wasn't exactly " light " now was it. kidding / lol.

    Viewing a superheat calculator under " real " conditions drove into my thick skull the relationship between latent heat / cooling effect / comfort. When everything is " fixed ' it seems to require your imagination to understand what is going on.

    After I got over that I just charge everything till it feels like a cold beer.

    Not a " frosty mug " mind you...just a cold beer. ROFL ( but true in a way )

  13. #13
    Join Date
    Jul 2008
    Location
    Metro Atlanta
    Posts
    70
    I also use the formula "toddfather" posted, it seems to work great if you can measure wb. Remember that the warmer the return air the higher the superheat. We usually charge systems when they are not working and it is hot inside! so there is almost always a lot of superheat, but think about when the return air is 70-74 because that is where it will be running. I actually charge systems with pistons fairly quick with liquid in the suction. No i dont open my gauge wide open. I too am an apprentice, been in school for 3 1/2 years and will be finished in march. what state are you in?

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