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  1. #1
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    Mar 2013
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    Chicago
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    409

    Bestway to determine refrigerant levels accurately in residential A/C systems

    Hello Guys,

    Im new on here and this is my first post. Im looking forward to learning much from you guys.
    Im new in the HVAC field, just about finished with school.

    My question :

    What is the most reliable way to find out how much refrigerant is in a residential system ?
    I would think recovering the refrigerant and weighing it will obviously be the most accurate. Then you know how short or overcharged the system is, right ?
    My instructor said the only way it should be done is by taking superheat readings
    Can you please advise ?
    Thanks,
    Bob

  2. #2
    Join Date
    Sep 2008
    Location
    VA
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    1,845
    Quote Originally Posted by Bobe5531 View Post
    Hello Guys,

    Im new on here and this is my first post. Im looking forward to learning much from you guys.
    Im new in the HVAC field, just about finished with school.

    My question :

    What is the most reliable way to find out how much refrigerant is in a residential system ?
    I would think recovering the refrigerant and weighing it will obviously be the most accurate. Then you know how short or overcharged the system is, right ?
    My instructor said the only way it should be done is by taking superheat readings
    Can you please advise ?
    Thanks,
    Bob
    You are correct, weighing in is usually most accurate.
    Superheat alone will not tell you what amount of refrigerant is in the unit.
    Superheat should be used on fixed metering device systems along with other measurements, to ensure the accuracy of the system charge.
    Superheat alone is not going to ensure proper charge.

    Are you sure he didn't say that superheat should always be checked to ensure a proper charge?
    That I would agree with.
    "The only true wisdom is in knowing you know nothing" Socrates

  3. #3
    Join Date
    Dec 2011
    Location
    North Texas
    Posts
    668
    X2.

  4. #4
    Join Date
    Mar 2013
    Location
    Chicago
    Posts
    409
    Thanks for the reply. But, doesnt weighing it in ensure a proper charge also ?
    Im just assuming that if you go to a service call and you suspect an incorrect charge, cant you recover all the freon and then weigh it- and then add / subtract freon to get to the design amount ?
    This may not be the most efficient way to do it but it seems like the most accurate way to charge a system with the correct amount of refrigerant.
    I see guys on youtube adding freon into a system and just feeling the suction line ( with their hand )to determine if they added enough refrigerant. And thats something the school instructors warned us about..... that is the absolute wrong way to do it.

  5. #5
    Join Date
    Feb 2009
    Location
    Tallahassee, FL
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    6,051
    Yes but the data plate charge is not always the correct charge in a split heat pump.

  6. #6
    Join Date
    Sep 2008
    Location
    VA
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    Quote Originally Posted by Bobe5531 View Post
    Thanks for the reply. But, doesnt weighing it in ensure a proper charge also ?
    Im just assuming that if you go to a service call and you suspect an incorrect charge, cant you recover all the freon and then weigh it- and then add / subtract freon to get to the design amount ?
    That is probably a last option. Removing refrigerant and recharging is going to take time. Your time is money. Yes, do the best job you can, but someone needs to pay for this time, either your boss or the customer.
    There are other ways to check for proper operation without going to this extreme.
    "The only true wisdom is in knowing you know nothing" Socrates

  7. #7
    Join Date
    Mar 2013
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    Chicago
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    Ok Thanks for the replies. I do appreciate them. Like I said Im a novice so, whats the most efficient accurate way to check / adjust the charge ? How do you guys do it ? Just referring to residential split systems.
    Thanks,
    Bob

  8. #8
    Join Date
    Sep 2008
    Location
    Western PA
    Posts
    25,957
    There are two questions being asked here with three possible answers.

    #1. Are we trying to determine exactly how much refrigerant is contained in the system? This could really only be done accurately by recovering and weighing in the charge.

    #2. Or are we trying to determine if the system is properly charged? The answer here is far more complex...

    Before we can determine if a system is properly charged, we've got to establish a few things.
    Coils clean?
    Filter clean?
    Airflow across evaporator correct?
    Airflow across condenser correct?
    What type of metering device does the system have?
    Is it a matched system?

    Now, we'll measure superheat and subcooling, compare them to either the manufacturer's required numbers or to a field computed value absent manufacturer's data and adjust the charge as required.

  9. #9
    Join Date
    Mar 2013
    Location
    Joplin,Missouri
    Posts
    310
    Even when weighing in a charge there are factors to consider lets say you are installing new dry charged R22 13 seer unit on an existing 10 seer evaporator coil. with a short lineset if the unit says field charge 9 pounds and you add 9 pounds you will have probably overcharged the system because of the short lineset and smaller 10. seer indoor coil and the proper pressure may be a little off from the chart on the unit like everything else it takes practice and experience

  10. #10
    Join Date
    Sep 2009
    Location
    Kennesaw, GA
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    619
    I have something to add to this question. Superheat and subcooling are a reflection if there is enough refrigeration in a system. Actually more of a overcharge or undercharge. But if let say you need 10 degree superheat and your getting 7 or 8 degrees... Does it ever occur that the pressures may not reflect that? Like its at 68 psig, the little refrigerants you put to bump it up to now to 10 superheat still reflects 68 psig. Like i am asking is there a blurry line sometimes,especially if someone is not knowledgeable or well experianced in superheat and subcooling. Also I am not talking about something that is totally undercharged as of low pressures and same as overcharged with high pressures. The above example I gave is for superheat with fixed metering device and r-22. I hope I am clear here.. Loll . I am also asking this because unlike me, you guys have probably hooked up gauges thousands and thousands of times.

  11. #11
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    Jul 2010
    Location
    nebraska
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    Quote Originally Posted by mofotech View Post
    I have something to add to this question. Superheat and subcooling are a reflection if there is enough refrigeration in a system. Actually more of a overcharge or undercharge. But if let say you need 10 degree superheat and your getting 7 or 8 degrees... Does it ever occur that the pressures may not reflect that? Like its at 68 psig, the little refrigerants you put to bump it up to now to 10 superheat still reflects 68 psig. Like i am asking is there a blurry line sometimes,especially if someone is not knowledgeable or well experianced in superheat and subcooling. Also I am not talking about something that is totally undercharged as of low pressures and same as overcharged with high pressures. The above example I gave is for superheat with fixed metering device and r-22. I hope I am clear here.. Loll . I am also asking this because unlike me, you guys have probably hooked up gauges thousands and thousands of times.
    In your example,assuming it's a fixed orifice, adding refrigerant would be going the wrong way. The answer to the question is yes. There will be times when adding or removing a few oz of refrigerant does not change the pressures but your SC and SH will.

  12. #12
    Join Date
    Sep 2009
    Location
    Kennesaw, GA
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    619
    Quote Originally Posted by martyinlincoln View Post
    In your example,assuming it's a fixed orifice, adding refrigerant would be going the wrong way. The answer to the question is yes. There will be times when adding or removing a few oz of refrigerant does not change the pressures but your SC and SH will.
    im sorry, loll I goofed that one. You add when superheat is high. So better example is superheat is 20 and you need like 16. I guess my assumptions were right. Anyways its always critical one knows that when he walks away from the unit; its properly charged!

  13. #13
    Join Date
    Feb 2012
    Location
    Durham, NC
    Posts
    73
    The most accurate way is weighing it in.

    The second most accurate is manufacture charging chart. But keep in mind that manufacture charging chart assumes the airflow is correct, coils are clean, filters are clean, lineset and ductwork are sized properly. Here's one that trane uses for charging a heat pump in heat mode.

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