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  1. #1
    Back when i went thru Trade School and beyond...youd see ALOT of sight glasses used on TXV split systems ; made charging the system not only a breeze, but, accurate also. Used to see nearly every TXV system with a field installed Sight Glass -- same on automobiles. I understand the approach today is via the subcooling chart (hopefully) with the A/C unit, but, i sure miss watching the bubbles disappear thru the Sight Glass. They're only $8.00 each.


  2. #2
    Senior Tech Guest
    God invented gross margins...

  3. #3
    Join Date
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    Originally posted by senior tech
    God invented gross margins...
    Amen!


    Asside from that, charging by the sight glass never was even close to the best way to charge a system, but it was so simple a monkey could do it, so thats what was taught.

    As efficiency became more of an issue, the accuracy of the refrigerant charge became ever more critical. Even though a TXV system is by nature more forgiving of being slightly under or over charged, on many residential split systems, being off by even as little as 4 ounces can have a measurable impact on the efficiency and capacity of the system.

    Charging by sight glass will almost never get you the correct charge. Under high load conditions, a clear sight glass likely means the system is grossly overcharged. Under light load conditions, the sight glass can be clear, but still be significantly undercharged.

    Always charge TXV systems by subcooling, and check the superheat to verify correct operation. Some manufacturers have charging instructions and/or charts for thier TXV systems, but all of them are basicly varriations of charging by subcooling.

    The only manufacturers instructions I can remember reading on an older TXV equipped system that even mentioned a clear sight glass as part of the procedure was on some old Carrier systems.
    Even then, the instructions required a certain range of return air temperatures, and that you block off the condensor coil with a piece of cardboard to maintain a specific head pressure.
    I have seen those instructions in both R-500 and R-22 units were 25+ years old.
    If more government is the answer, then it's a really stupid question.

  4. #4
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    Charging by sight glass can only tell you that there is liquid in the line. At best it may get you close to how much liquid is needed in the line but does not tell you if there is too much liquid in the system.

    I like sight glasses for visual inspection of refrigerant and checking for moisture, that's it.
    Government is a disease...
    ...masquerading as its own cure…
    Ecclesiastes 10:2 NIV


  5. #5
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    Yup. The moisture indicator is about all they are good for. Many systems were overcharged in the past because technicians charged by sightglass.

    I recall some jobs in the past where there were still some bubbles in the glass after I charged by superheat and subcooling. Then the resident maint guy tries to tell me that I did not completely charge the system and wants me to return and get it right because he sees some bubbles.

    Or, he tells me he had to add "a little" after the last time I was out.

    We are better off without sightglasses when there are ignorant people around.



    [Edited by NormChris on 06-22-2005 at 12:28 AM]

  6. #6
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    16,122
    I agree with all about the sight glasses but what do you tell a customer when they call up and say my unit needs antifreeze
    I can see bubbles in my sight glass, how do you tell them nicely that that don't mean a thing go so back in and finish reading the newspaper.
    __________________________________________________ _______________________
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  7. #7
    Join Date
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    Originally posted by mrbillpro
    I agree with all about the sight glasses but what do you tell a customer when they call up and say my unit needs antifreeze
    I can see bubbles in my sight glass, how do you tell them nicely that that don't mean a thing go so back in and finish reading the newspaper.
    What do you mean go back to reading the newspaper? He read in an air conditioning textbook that you charge until the glass is clear! Even the books are misleading.

  8. #8
    Join Date
    May 2002
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    Yea went to house Sunday morning not cooling sight glass at condenser no bubbles.
    Checked every thing out TXV SC way off removed 4# R-22 set it at 10* SC working much better. The previous company came out on last Turkey day removed old suction drier left spun copper drier in unit and installed new suction drier. No doubt they charged by sight glass. When I left there were lots of little bubbles in the glass.

    Janitrol 4-ton with 35-year-old TXV coil and GMP100-4 furnace.

    Ya gotta just love the competition I am going to be putting new Amana RSG 5-ton AMV8 with 3-zone system and strip seal and wrap the old snap lok ducts.

  9. #9
    Join Date
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    I'll bet that new two stage, variable speed blower furnace won't be a 100,000 Btu either, will it?

    It's a wonder that condenser lasted that long with a TXV. Someone must have installed hard start components somewhere along the line.
    Government is a disease...
    ...masquerading as its own cure…
    Ecclesiastes 10:2 NIV


  10. #10
    Join Date
    Jan 2004
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    Originally posted by NormChris
    Originally posted by mrbillpro
    I agree with all about the sight glasses but what do you tell a customer when they call up and say my unit needs antifreeze
    I can see bubbles in my sight glass, how do you tell them nicely that that don't mean a thing go so back in and finish reading the newspaper.
    What do you mean go back to reading the newspaper? He read in an air conditioning textbook that you charge until the glass is clear! Even the books are misleading.
    He said

    How do you politely tell the customer to go back to reading their newspaper.
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  11. #11
    Originally posted by mark beiser
    Originally posted by senior tech
    God invented gross margins...
    Amen!


    Asside from that, charging by the sight glass never was even close to the best way to charge a system, but it was so simple a monkey could do it, so thats what was taught.

    As efficiency became more of an issue, the accuracy of the refrigerant charge became ever more critical. Even though a TXV system is by nature more forgiving of being slightly under or over charged, on many residential split systems, being off by even as little as 4 ounces can have a measurable impact on the efficiency and capacity of the system.

    Charging by sight glass will almost never get you the correct charge. Under high load conditions, a clear sight glass likely means the system is grossly overcharged. Under light load conditions, the sight glass can be clear, but still be significantly undercharged.

    Always charge TXV systems by subcooling, and check the superheat to verify correct operation. Some manufacturers have charging instructions and/or charts for thier TXV systems, but all of them are basicly varriations of charging by subcooling.

    The only manufacturers instructions I can remember reading on an older TXV equipped system that even mentioned a clear sight glass as part of the procedure was on some old Carrier systems.
    Even then, the instructions required a certain range of return air temperatures, and that you block off the condensor coil with a piece of cardboard to maintain a specific head pressure.
    I have seen those instructions in both R-500 and R-22 units were 25+ years old.

    This is a dead-on-balls-accurate post, and I thank you!

  12. #12
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    At least put them at the coil if they think they need sight glasses, you loose subcooling between the od and id units.
    But those guys don't want to walk inside to check, do they?
    Col 3:23


    questions asked, answers received, ignorance abated

  13. #13
    Thank you everyone for your replies ; I personally, would settle for the Sight Glass located right at the Evaporator so i know i have a full head of liquid...and use subcooling measurements in addition . Sadly, i came across an almost new 10 ton split system that didnt have a Sight Glass nor even a Filter Drier on the system -- TXV's on both 5 ton AHU's.

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