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  1. #1
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    Nov 2019
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    Pressurized leak checking without first pumping out all coolant?

    I suspect I have a coolant leak somewhere in my AC system (410a), evidenced by the system now not cooling at all and quickly cycling due to the low pressure switch (after being away for some months). I had a service come out and check the pressure, and verified it was very low.

    We decided to then try a leak test by pressurizing with nitrogen to see if we might find an obvious and easy to fix leak source, and to judge the size and rate of the leak. The technician hooked up the nitrogen tank and pressurized the system up to ~300psi, but never pulled out the remaining coolant first. The test showed a rapid drop on the gauge, like a few psi per minute at first, and then still a continuous but increasingly slower drop for however long it sat. The conclusion was, "well, there's your leak." He never found any physical source though, neither by sound or with soap bubbles everywhere accessible. He didn't try a gas detector.

    But I was thinking later, if there would still be a couple pounds of coolant mixed with the nitrogen, what is the effect of that residual coolant on the overall saturation or temperature vs pressure sensitivity of the now mixed gas?

    Could some transient mixing or condensing or saturation be the cause of the pressure dropping on the gauge, rather than an actual, apparently massive leak? I don't know what the pressure-temperature curve would look like on some random mixture of 410a and nitrogen, or where we might have been on it. Any ideas or experience on how this mixed gas would affect the test readings, vs pure nitrogen?

    Just looking for a physical soap-bubbling leak this way is one thing, but with regard to tracking the pressure to quantify a leak, I wonder if the technician left in too many variables by not pumping out the coolant first and if we ended up with an incomplete test?

    The unit was left in that state for now, filled with nitrogen. We're not trying to run the system in this condition, of course.

  2. #2
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    If there was more than a few ounces of refrigerant the pressure would not have dropped. He should have got the sniffer out and found where it was leaking.

  3. #3
    Join Date
    Jan 2014
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    The reason he left the refrigerant in the system with the nitrogen is that a leak detector does not pick up nitrogen. So what’s the reason he left you hanging? Soap suds can be used to leak test also.

    At that rate of pressure dropping, it should not be hard to find the leak. Have them use the electronic to locate, then use soap to pinpoint, have him show you exactly where the leak is with soap, or have him take a photo(s) then show you. Don’t accept just the electronic, as they can make it read a false positive.

  4. #4
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    Nov 2019
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    Thread Starter
    Quote Originally Posted by Bazooka Joey View Post
    The reason he left the refrigerant in the system with the nitrogen is that a leak detector does not pick up nitrogen. So what’s the reason he left you hanging?
    He didn't want to use a leak detector, saying that it will go off too easily anywhere in the area of the unit if there is an apparently big leak and not really pinpoint where the leak is - for which he preferred soap bubbles to search. But even with soap he found no leak spot. Therefore, he assumed there must be a leak in an inaccessible area, like the back side of the evaporator coils.

    But is this a failsafe assumption? What I was wondering is if using a mixed nitrogen+410a gas while tracking high pressure will actually give a stable pressure reading, or if it would naturally fluctuate? Would the new high pressure nitrogen also quickly compress the couple pounds of remaining 410a, increasing its pressure and temperature momentarily (how hot? I don't know), and as the pressurized system sits there, would that 410a cool off somewhat from ambient air convection around the lines, thereby lowering the corresponding total pressure reading, even without there being a leak? I'm curious how the mixed gas would behave, I really don't know how much effect, if any, that residual coolant would have.

    Can this situation provide a false pressure drop reading?

  5. #5
    Join Date
    Jun 2014
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    Refrigerant reacts quickly and very noticeably with changing temperature.
    Trace refrigerant is used with a high pressure nitrogen test to give the leak detector something to detect.
    Assuming your leak is somewhere without proof could mean your spending money to change parts and still wind up with a leak.
    I havent failed. Ive just found 10,000 ways that wont work. - Thomas Edison

    Its not whether you get knocked down, its whether you get up. - Vince Lombardi

    "In this house we obey the laws of thermodynamics" - Homer Simpson

    Local 486 Instructor & Service Technician

  6. #6
    Join Date
    Nov 2019
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    Florida
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    Thread Starter
    That was my concern, using assumptions without full proof.

    There was much more than trace refrigerant, probably 2-3 lbs (out of 6lbs total) were still in the system.

  7. #7
    Join Date
    Oct 2016
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    It shouldn't be stably dropping even if it has refrigerant in it.

    If anything, it would go down, then up, then down, etc.

  8. #8
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    Sep 2008
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    VA
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    I don't know how other techs do it, but I pull out the electronic leak detector first.

    If it's not showing up quickly, then I step up the pressure with nitrogen, and search again.

    Once I find the area, I can pinpoint with bubbles if possible. Bubbles are a great tool to use, but I don't typically start with that unless I see signs of oil deposits.

    If the tech can find the leak, it doesn't really matter what methods they use if the job gets done properly.

    If I can't find the leak, then it's UV dye if the customer wishes to go further.
    "The only true wisdom is in knowing you know nothing" Socrates

  9. #9
    Join Date
    Dec 2005
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    Cincinnati, Oh
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    Based on the information you have a leak.

    A leak of that size could be heard with just the ear. Have him find the leak.

  10. #10
    Join Date
    Aug 2002
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    Adding N2 to the remaining charge for leak checking is Illegal. (Good Luck with that. Inform your local DEC / EPA if you want)

    Pressurizing the system with 5PSIG of R-22 and following with N@ is the proper way of leak checking. Start slow 100 - 150#'s check with the electronic detector, increase pressure slowly until reaching the limit of the equipment all the time checking for a signal!

    Find a different tech!

  11. #11
    Join Date
    Dec 2005
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    Quote Originally Posted by pecmsg View Post
    Adding N2 to the remaining charge for leak checking is Illegal. (Good Luck with that. Inform your local DEC / EPA if you want)

    Pressurizing the system with 5PSIG of R-22 and following with N@ is the proper way of leak checking. Start slow 100 - 150#'s check with the electronic detector, increase pressure slowly until reaching the limit of the equipment all the time checking for a signal!

    Find a different tech!
    Ive never heard of adding nitrogen to an existing charge for leak checking to be illegal.

  12. #12
    Join Date
    Aug 2002
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    Southold, NY
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    Quote Originally Posted by hvacvegas View Post
    I’ve never heard of adding nitrogen to an existing charge for leak checking to be illegal.
    That's to prevent someone from claiming the remaining charge is now a Leak Test too get around recovering it!

  13. #13
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    Dec 2005
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    Cincinnati, Oh
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    Can you provide a link for it?
    Im extremely interested because I do it all the time.

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