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Thread: Demand defrost on tranes

  1. #1
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    Demand defrost on tranes

    Guys, one of my guys is having issues with a Trane M#4TWA4048A. This unit has on demand defrost on it. I'm old school and have never worked on these but understand that their using the Delta T to bring on the defrost and temp termination. The issue is almost ever time these unit run through defrost and terminate they trip on low pressure, run through the soft restart over and over until a hard lockout. We can by-pass the low pressure switch and within 90 sec the unit will run fine till the next cycle. We are pushing the line set limits of 150 ft. The charge seems fine from what I'm being told. There are 5 units doing the same thing some don't expect a TXV, board or sensor issue issue. He has test all sensors and txv anyway.

    Any help would be great we don't work on many of these smaller units and this one is kicking his ass..

    Thoughts?

    Web

  2. #2
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    Quote Originally Posted by Web View Post
    Guys, one of my guys is having issues with a Trane M#4TWA4048A. The issue is almost ever time these unit run through defrost and terminate they trip on low pressure, run through the soft restart over and over until a hard lockout.

    Thoughts?
    I don't see any of these, but I think some of the defrost controls have a 1 minute LPCO bypass in/out of defrost, and others have a 3 minute.

    I don't know if that might be the issue - just a thought.

    What is your full model number of the unit and the control board number if you have it?
    Instead of learning the tricks of the trade, learn the trade.

  3. #3
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    Add a delay on make to the lps.

  4. #4
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    Quote Originally Posted by Web View Post
    Guys, one of my guys is having issues with a Trane M#4TWA4048A. This unit has on demand defrost on it. I'm old school and have never worked on these but understand that their using the Delta T to bring on the defrost and temp termination. The issue is almost ever time these unit run through defrost and terminate they trip on low pressure, run through the soft restart over and over until a hard lockout. We can by-pass the low pressure switch and within 90 sec the unit will run fine till the next cycle. We are pushing the line set limits of 150 ft. The charge seems fine from what I'm being told. There are 5 units doing the same thing some don't expect a TXV, board or sensor issue issue. He has test all sensors and txv anyway.

    Any help would be great we don't work on many of these smaller units and this one is kicking his ass..

    Thoughts?

    Web
    I’m pretty sure I know the board you are talking about. The LPS feeds directly into it on pink wires. It has a delay built into it so it doesn’t immediately respond to the LPS.

    Those boards create too many problems IMO. At one point Trane actually had us snip those pink wires and cap them so the board never sees an open LPS and then run the LPS in series with the contactor just like the HPS. From what I hear, the latest version of the firmware on those boards is better, but overall I think they’re garbage.
    "I think Quantum tunneling would work great... "

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  5. #5
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    I would verify the charge is correct first...could simply be low. Especially with the long line set....was the field charge accurately calculated?

    Funny when you can work on 100 ton rtus all day long, then a little 3 ton residential unit kicks your ass !! LOL!!

  6. Likes icy78 liked this post.
  7. #6
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    Quote Originally Posted by 71CHOPS View Post
    I would verify the charge is correct first...could simply be low. Especially with the long line set....was the field charge accurately calculated?

    Funny when you can work on 100 ton rtus all day long, then a little 3 ton residential unit kicks your ass !! LOL!!
    Very true! If the same person did the start up they may all be low for the same reason lol.
    "I think Quantum tunneling would work great... "

    "Call a technician for God's sake. Or we'll see you on the news or the Dark Side of the Moon."

  8. #7
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    So, this is a heat pump?

    Is it cold there?

    What is your definition of Delta T? Because that is the same thing as temp split (the difference between air in and out of a coil), and I don't think I've ever seen a heat pump go into defrost on a Delta T. Typically, there are options for 30, 60, and 90 minutes. After that interval, then, if the coil is cold enough, it goes into a defrost (if this is a heat pump). That same thermo disc on the outdoor coil is what terminates the defrost cycle.

    Did anyone change the low pressure control? Because it is very likely the manufacture has two different pressure ranges depending on if a heat pump or straight cool.

    Check your subcool before the outdoor coil metering device; too much, you got a restriction. Too little, you're low on charge.

    There's some starting points for you.
    I do a triple evac with nitro to remove non condensables.

  9. #8
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    Quote Originally Posted by 71CHOPS View Post
    I would verify the charge is correct first...could simply be low. Especially with the long line set....was the field charge accurately calculated?

    Funny when you can work on 100 ton rtus all day long, then a little 3 ton residential unit kicks your ass !! LOL!!
    Yes on this.^^^
    Also be aware of lineset rise and its affect on subcool at the txv. (If SC is being measured at the CU) (pressure differential between locations changes SC value)
    Probably long-lineset rules apply to those installs.


    Sent from my SM-G950U using Tapatalk

  10. #9
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    If this is the newer Trane defrost board that is rectangular instead of square, there was a service bulletin on the earlier versions. It had to do with the board not allowing enough "ignore" time on the low pressure switch during defrost. This was corrected in later versions or with the modification described by Jayman.

  11. Likes JayMan7 liked this post.

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