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  1. #27
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    Oct 2009
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    good alternative to the fan delay mounted to the coil to bring the fan on.i assume you still use a sensor going to X to take defrost out, once the coil has melted the frost.so no fan on thermostat, only a defrost termination stat?Does 5 minutes seem to be a proper time for the coil to frost up and bring the fans back on?

  2. #28
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    Jul 2019
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    VTP99: Leaving the fan so it starts as soon as defrost ends just blows foggy air around. That leaves moisture in the air to condense on literally everything. Flooding? Nah. I have seen units that have had the 3 wires simply spliced together and the "X" wire disconnected at the timer. Some have been that way for YEARS! Just an annoyance in most cases, but the frost covering everything is often the giveaway.

  3. #29
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    Jan 2010
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    Quote Originally Posted by MilfordsMrFixIt View Post
    VTP99: Leaving the fan so it starts as soon as defrost ends just blows foggy air around. That leaves moisture in the air to condense on literally everything. Flooding? Nah. I have seen units that have had the 3 wires simply spliced together and the "X" wire disconnected at the timer. Some have been that way for YEARS! Just an annoyance in most cases, but the frost covering everything is often the giveaway.
    In my case the box was empty so it didn't matter. But as for the flooding I'd still be concerned. Superheat on most freezers is low to begin with like 4-6 so it wouldn't take long for liquid to pass right through that coil. Especially with the condensing unit right above it.

  4. #30
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    Jul 2019
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    Gleng: Right, you still need to terminate defrost. The fans can be connected to the compressor output and a delay timer to delay the fans. 5 minutes is the non-adjustable setting on the Grassling timer, but you might need more or less. In a -20 Warehouse, there's 8, 10 hp units, that we control with an Arduino, that we delay the fans by 6:30 after a 35 minute defrost every 8 hours.. Just one unit defrosts at the top of every hour.

  5. #31
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    Jul 2019
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    VTP99: Understandable. You need to determine the right amount of time. I usually use a thermometer embedded in the coil close to the suction to determine how long it takes to prechill

  6. #32
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    Quote Originally Posted by MilfordsMrFixIt View Post
    VTP99: Understandable. You need to determine the right amount of time. I usually use a thermometer embedded in the coil close to the suction to determine how long it takes to prechill
    It was a considerable amount of time but then it was from a 80° box starting point.
    Typically I'm servicing a pulled down box so that makes a huge difference.
    This was a Heatcraft coil with the switches mounted on the end sheet. I believe the return bend type would respond much quicker. I've also seen coils with a tee at the u that can have a mounting spot there.

  7. #33
    Join Date
    Mar 2013
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    Quote Originally Posted by VTP99 View Post
    Thanks for the clarification.
    Just this past week I had to start up a walk-in freezer.
    The high limit had failed open and was not permitting defrost.
    I picked up a new one as well as the DTFD switch.
    After starting it up a good half hour had lapsed with the coil well frosted over but the switch not triggering the fans. I had to get going to my next call so I installed the new DTFD to play it safe. It still would not switch over so I moved it over to the power terminal to get them running knowing I'd be back the next day.
    Next day box was down to temperature and I repositioned the fan wire where it belonged. All was good so I came to the conclusion that that box just had to pull down before the surface at the switch was down to temp.
    I wonder how much flooding would have happened had I just let it rock as was ?
    I have had to bang on brand new DTFDs with the handle-end of a screwdriver for it to start the fans before. I knew for a fact they were seeing the correct temperature.

    I never had a problem with it after that so I'm thinking that just by sitting in storage for a while maybe caused it to get lazy.



    Sent from my Moto Z (2) using Tapatalk

  8. #34
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    Jul 2019
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    I think it was an old Larkin, from when they were Larkin, not a part of Heatcraft, that had a part soldered on to one of the 'u-turns' that was copper and about the same size as the flat mounting flange of the common DTFD. Those worked great!

  9. #35
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    Apr 2003
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    Quote Originally Posted by shellkamp View Post
    I have had to bang on brand new DTFDs with the handle-end of a screwdriver for it to start the fans before. I knew for a fact they were seeing the correct temperature.

    I never had a problem with it after that so I'm thinking that just by sitting in storage for a while maybe caused it to get lazy.



    Sent from my Moto Z (2) using Tapatalk
    Sometimes new walk in freezers do take forever before the fans turn on, even when the DFTD starts to get surrounded by frost. If I get tired of waiting I will sometimes grab an upside down CO2 cylinder and blast it with dry ice particles. That usually does the trick and gets it to start working.
    If at First You Don't Succeed, Skydiving Is Not for You.

  10. #36
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    Quote Originally Posted by ammoniadog View Post
    Sometimes new walk in freezers do take forever before the fans turn on, even when the DFTD starts to get surrounded by frost. If I get tired of waiting I will sometimes grab an upside down CO2 cylinder and blast it with dry ice particles. That usually does the trick and gets it to start working.
    We have a tech that does that on self contained.... “fans not coming on had to blast fan delay with CO2”
    And when we run the Callback we find an undercharge or evap starved for refrigerant.

  11. #37
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    May 2014
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    Bay Area California
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    On a hot pull down, it's not uncommon to see the fans cycle on and off a few times until the box cools down some.


    Quote Originally Posted by VTP99 View Post
    It was a considerable amount of time but then it was from a 80° box starting point.
    Typically I'm servicing a pulled down box so that makes a huge difference.
    This was a Heatcraft coil with the switches mounted on the end sheet. I believe the return bend type would respond much quicker. I've also seen coils with a tee at the u that can have a mounting spot there.
    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.

    I use 56% silver on everything except steel.

    Did you really need the " If you were a real tech " ??

  12. #38
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    Jan 2010
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    Quote Originally Posted by VTP99 View Post
    . I've also seen coils with a tee at the u that can have a mounting spot there.
    Exhibit A
    Attached Images Attached Images  

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