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  1. #1
    Join Date
    Jul 2012
    Location
    TX
    Posts
    33

    Determining correct charge on cool days

    Yep I'm a newby, but got a question for you more experienced techs. During this time of year a customer may be using AC one day and heat the next. If you need to check AC charge on a cool day, is there a different process to do this? Or, do you just slap gauges on and look at PT Chart? Also, should superheat and subcool targets be the same when you are so far away from design condition temps? Thanks for any help yall!

  2. #2
    Join Date
    Aug 2012
    Posts
    2,326
    There has to be some kind of load to determine sub and super heats.
    Trying not to be a Hack.

  3. #3
    Join Date
    Mar 2010
    Location
    Kent, WA.
    Posts
    183
    you can simulate a warmer day by raising head pressure by blocking some of the outdoor coil. as long as you have a decent load on the indoor you can get pretty close with your SH/SC.
    Experience
    Is what you get when you didn't get what you wanted.

    A positive attidude will not solve all your problems, but it will annoy enough people to make it worthwhile.

  4. #4
    Join Date
    Feb 2009
    Location
    Tallahassee, FL
    Posts
    5,821
    Metering device is important as txv will react different from fixed.

  5. #5
    Join Date
    Jul 2012
    Location
    TX
    Posts
    33
    Quote Originally Posted by SBKold View Post
    Metering device is important as txv will react different from fixed.
    Thanks to all the suggestions...

    So, really it sounds like its better to evaluate the system when there is a heat load I.D....then be sure to follow proper protocol per TXV or F.O.

  6. #6
    Join Date
    Apr 2010
    Posts
    18
    The only way to properly charge the system on a cool day is to raise the head pressure along with wet bulb, dry bulb, pressure and temperature checks. Of course this depends on TXV or Fixed Orifice. Once you have all your numbers you should charge according to the subcooling (if TXV) or superheat (if Fixed) that comparable to the numbers. This is important because the numbers change with the temp.

  7. #7
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,914
    If the indoor wet bulb temperature is below about 55F, you can pretty much forget about checking the refrigerant charge with any degree of accuracy.
    You really need to start with it higher than that, because you need the system to run long enough to stabilize, and especially with fixed metering, the coil to get wet.
    If more government is the answer, then it's a really stupid question.

  8. #8
    Wrap your OD coil with plastic and get your head pressure up around 250, and check like normal. Some Indoor load is ideal but not always possible. I believe in checking subcool and superheat regardless of txv or fixed. Subcool will let you know you have plenty of liquid and superheat with let you know your compressor protected .

  9. #9
    Join Date
    Jan 2007
    Location
    Mn the state where absolutey nothing is allowed
    Posts
    1,312
    you know blocking the outdoor coil changes the design temperature of the coil.

    check out Jim Bergmans videos hawking the fieldpiece charging blanket. does a good job of explaining the science of why blocking inlet airflow to raise head pressure is not the best idea.
    my boss thinks its possible to repeal the laws of physics

  10. #10
    Join Date
    Jan 2011
    Location
    Highland Park, CA
    Posts
    59
    I was going to ask if the flashing ad to the right of the screen about the field piece jacket has been tested in the field or not? Thanks ch4man.

  11. #11
    Join Date
    Jul 2012
    Location
    TX
    Posts
    33
    Quote Originally Posted by ch4man View Post
    you know blocking the outdoor coil changes the design temperature of the coil.

    check out Jim Bergmans videos hawking the fieldpiece charging blanket. does a good job of explaining the science of why blocking inlet airflow to raise head pressure is not the best idea.
    Thanks for the tips yall- ch4man- I went on Youtube and could only find one Jim Berman vid on fieldpiece charging blanket, but he seemed to be speaking pretty positively about how it worked....had it all mapped on his laptop showing HP and corresponding changes in SC...pretty interesting. Did you see a different one? Thanks

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