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  1. #1
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    May 2018
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    3 wire DTFD sensors

    I am fairly new to refrigeration, but at work we now have 70+ walk in freezers and coolers.

    They are all basic pump down style systems and all have 3 wire defrost termination/ fan delay in the evaporators.

    My question is.... when I am turning the systems on from room temperature the defrost termination side is closed and the fan side open.... how long does it typically take for the switch to flop and start the evap fans?

    We have been told by a refrigeration guy that it will “eventually get cold enough on its own and switch over”, but no matter how much time I give it (even 45 minutes) it doesn’t seem to switch.

    I have resorted to just jumping the evap fans out, coming back once the box is down to temp and wiring it back to normal. Everything runs fine then with fan delay closed.

    Am I not waiting long enough? After 30 or 40 minutes with no fans the coil starts lookin pretty frosty. I thought I was seeing bad DTFD’s at first, but they all seem to behave the same.

  2. #2
    Join Date
    Aug 2018
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    Quote Originally Posted by 3.14 View Post
    I am fairly new to refrigeration, but at work we now have 70+ walk in freezers and coolers.

    They are all basic pump down style systems and all have 3 wire defrost termination/ fan delay in the evaporators.

    My question is.... when I am turning the systems on from room temperature the defrost termination side is closed and the fan side open.... how long does it typically take for the switch to flop and start the evap fans?

    We have been told by a refrigeration guy that it will “eventually get cold enough on its own and switch over”, but no matter how much time I give it (even 45 minutes) it doesn’t seem to switch.

    I have resorted to just jumping the evap fans out, coming back once the box is down to temp and wiring it back to normal. Everything runs fine then with fan delay closed.

    Am I not waiting long enough? After 30 or 40 minutes with no fans the coil starts lookin pretty frosty. I thought I was seeing bad DTFD’s at first, but they all seem to behave the same.
    The DTFD is based on temperature. It should have a temp stamped on it or you can look up the part number to see what temp it has to reach in Order to close for the evap fans. I’ve seen freezers range from 20-30 degrees so mounting location is important. If it’s just hanging in the airstream which I’ve seen before it will take a lifetime! They need to be mounted to the evap coil itself to work correctly. So I would start there.


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  3. #3
    Join Date
    May 2014
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    Also, from a hot box pull down, it's not uncommon for the fans to cycle off a time or two.
    If you were a real tech, you'd solder a relay on that board and call it good to go.

    I do a triple evac with nitro to remove non condensables.

  4. #4
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    May 2018
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    Thread Starter
    Yes, they are all mounted in the electrical compartments clipped to suction side tubing. They are all in the same spot and appear to be factory installed.

    Definitely understand that they are temp operated.... I honestly haven’t looked at what the temp is as I’m waiting.... I just hate walking away without seeing the fans on.

    I realize there could be a lot of variables and a ballpark time may not be realistic.... but, lol.... does 45 minutes seem excessive..... or have you seen them take an hour or two?

  5. #5
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    Quote Originally Posted by 3.14 View Post
    Yes, they are all mounted in the electrical compartments clipped to suction side tubing. They are all in the same spot and appear to be factory installed.

    Definitely understand that they are temp operated.... I honestly haven’t looked at what the temp is as I’m waiting.... I just hate walking away without seeing the fans on.

    I realize there could be a lot of variables and a ballpark time may not be realistic.... but, lol.... does 45 minutes seem excessive..... or have you seen them take an hour or two?
    A lot of variables come into play to put a time on it. If your worried I would put a temp probe next to the switch on the coil to monitor the coil temp at the switch. Then find out what temp it is manufactured for and see if it correlates. May be a bad switch. I’ve seen 45 mins before fans come on but as BBeerme said. Once they come on they will pull heat over the coil which raises suction line temp and they may cycle back off. They can do this several times before the coil stays below rated temp of switch.


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  6. #6
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    Sep 2013
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    I’ve seen well over 45min before, but also will jump the fans out sometimes depending on circumstances.

  7. #7
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    May 2018
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    Thread Starter
    Will do next time.

    My worry has been coming back to find an ice block with no fans, but I guess even if it has significant icing, the heat from the box starting up would still take care of it.

  8. #8
    Join Date
    May 2014
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    Does sound a bit excessive.

    The real test is how long does it take when the box has reached temp after a defrost.

    Some things you just can't do until the box has reached temp.
    If you were a real tech, you'd solder a relay on that board and call it good to go.

    I do a triple evac with nitro to remove non condensables.

  9. #9
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    May 2018
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    Thread Starter
    Well, I haven’t sat with one to watch on a normal defrost cycle. But I have also not come across one with refrigerant flowing and no fans.

    I believe they are all running full clock duration and never flopping into defrost termination.

    All the freezers seem to be set to 30 minutes, four times a day.

    I have run across them many times with the heaters still screaming and not a drop of ice on them.... I wonder if their positioning is less than ideal?

  10. #10
    Join Date
    Apr 2004
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    Waterloo N.Y.
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    Had the same issue. Some evaps like Larkin, mount the switch on a panel instead of the coil tube itself. It takes 45 min at least on a hot pull down. It sucks when you have to wait for the fans while there is another call an hr. away waiting for you.
    There's TREACHERY AFOOT!!!

  11. #11
    Join Date
    Aug 2002
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    Southold, NY
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    Just be careful bypassing the control....that can lead to overloading the pump!

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