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  1. #1

    problem with an older AAON unit form 1992

    The original service call was for no heat... after checking out the unit the transformer was tripping out when the unit was calling for heat. I narrowed the problem down the a shorted 24v coil on the 2nd contactor for the strip heaters. I pulled the 24volt side of the coil and the unit started up when the power was brought back on with only the 1st and 3rd heaters energized.

    Ok so i thought that was the problem... went to the supply house bought a new contactor and came back and installed it. Started the unit and all three contactors for the electric heat pulled in ( delay on the third) and the unit was running fine. Checked my amp draws for the heaters and called it a day.

    Two days later the customer called me saying that the unit wasn't working again... so went back to check out the unit and the same problem. The coil had shorted again causes the transformer to trip. Im not sure how long the unit ran before failing again could of been 10 minutes or a day....

    So let me give you some of the details of the unit... the transformer has been replaced a few years ago ( i checked to make sure that the VA was the same as the original, checked voltage on the secondary side of the transformer had 24.6 volts but when checking voltage at the coil i only had 23.5 volts... this should be fine right?) The wiring for the heat goes from w1 through 2 limit switchs (in series) then to a to a time delay relay (enables w2 and the third contactor for the electric heat) and the 24 volt coils for the first and second contactors (wired in parallel) with the way they are wired it doesn't make sense to me that only one 24 volt coil would fail??
    There has been a problem in the past with contactors failing according to the customer so im not a 100 percent that the problem has ever been fixed. Im looking for things to check for that could be causing the problem... any help would be appreciated thanks

  2. #2
    Join Date
    May 2002
    Location
    evansville indiana
    Posts
    943
    Does this unit have one trans ,,or as I've seen in a lot of aaons two transformers paralleled
    ?
    You sure someone didn't pull two and went back with one ?

    Or do you have two ? be absolutely sure of your secondary VA requirements.

    You said 14 volts at trans.
    Doesn't make sense? i assume you had 24 @ secondary and made a typo.

    Is something being energized later causing a big voltage drop?or a faulty component causing one?
    Sounds like a voltage drop or undersized transformer .although an undersized trans would probably blow the fuse or burn out before a coil did.

    Pull all components "in" and measure your secondary trans amp draw .
    If a 75va trans at 24volt that equals an allowable 3.125 max amps to be pulled . VA divided by secondary voltage equals allowed max amps... although I would allow some wiggle room.

    .

    I suspect a voltage drop thru components ,probably one feeding that particular contactor or undersized trans. If it's all rough how much would the factory limits,contactors,tdr etc cost vs repeat calls.

    I also suspect that you might be "leaving " some voltage on that coil when it's not energized . Maybe pulling current thru it but not enough to pull it in.

    Also is your primary tapped correctly? is it a multitap trans ,if so check that as well ,although usually if tapped wrong your secondary should be off as well
    240 volt supply = trans tapped for 240 not 208 or vise versa,,,,
    Last edited by mikeacman; 11-27-2012 at 09:38 PM.
    mikeacman

  3. #3
    Join Date
    May 2002
    Location
    evansville indiana
    Posts
    943
    http://www.franklin-electric.com/fra...teristics.aspx

    I hope I explained myself well ,,,I type all this on a crappy windows phone that doesn't like this site....

    http://www.franklin-electric.com/fra...teristics.aspx
    mikeacman

  4. #4
    Yes it was 24.6 volts sorry i edited the post...

    I am going to check the voltage to the coil of the contactor when i get back with the replacement contactor. I just dont understand why the 1st contactor would be fine and then second would short out with them wired in parallel?? that doesnt make sense right?

    So after replacing the contactor im going to make all electric heaters call with the fan and check my amp draw at the secondary side of the transformer... this will tell me if the transformer is the problem?? not to exceed 3.125 amps

    Then if i find any voltage from across any components replace them... what is the least amount of voltage to the coil that would work?

  5. #5
    Join Date
    May 2002
    Location
    evansville indiana
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    943
    It depends on the contactor and it may say on it ...

    No that doesn't make since if truly wired parallel. Voltage the same on both coils?

    Check for proper VA again

    make sure primary side of trans is tapped correctly.


    Voltage drop??? Maybe get a chair and sit for awhile.

    Maybe something miswired?

    Look for current draw on the problem contactor ,get a amp meter and make sure no current when it's de-energized by the tstat .

    I still suspect a voltage drop,undersized trans ,miswirw or current being pulled when supposed to be off...

    having to small of a trans should burn the trans or blow the trans fuse if fuse is properly sized but since your there its a pretty quick check.

    wish I could see it ,I'm better with my hands and mind while there than when trying to see it blindly from miles away.
    mikeacman

  6. #6
    Join Date
    May 2002
    Location
    evansville indiana
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    943
    Remember ...VA divided by secondary voltage equals allowed maximum amp draw....
    mikeacman

  7. #7
    What would cause it to draw amperage when it is supposed to be off?

  8. #8
    Join Date
    Mar 2009
    Location
    Mid-Mo
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    3,596
    This is going to sound odd but how far away is the thermostat and how many conductors do you have going there?

  9. #9
    Join Date
    May 2002
    Location
    evansville indiana
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    943
    Quote Originally Posted by newyorkjets View Post
    What would cause it to draw amperage when it is supposed to be off?
    something miswired or "bleeding thru" taking about the coil not heaters
    mikeacman

  10. #10
    Quote Originally Posted by ryan1088 View Post
    This is going to sound odd but how far away is the thermostat and how many conductors do you have going there?
    the thermostat is about 40 feet away and there is 7 conductors

  11. #11
    Quote Originally Posted by mikeacman View Post
    something miswired or "bleeding thru" taking about the coil not heaters
    If something was miswired it would have to be in that heating control curcuit?

    What do you mean by breeding through?

  12. #12
    Join Date
    Dec 2003
    Location
    Northern VA
    Posts
    622
    Some of the AAON machines size their VA's very closely. Look for damper actuators,added relays (like for fire alarms) etc. Not much additional capacity is available with the OEM transformer.
    "Iron sharpeneth iron..."

  13. #13
    Join Date
    Apr 2012
    Location
    Louisiana
    Posts
    690
    Check the limit wiring I've found the same problem on some aaon units and rub through on the limit wiring was the culprit.

    Also some aaons are setup to run electric heat for dehumification with compressors running.

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