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Split air handlers fuses blown

worldlyone | Posted in General Questions on

Hello All! I successfully installed a split air system in my home about 5 years ago with help from this forum, and have been enjoying it every year as it gets warmer and warmed in the Pacific NW. I have three indoor air handlers: a 18k BTU ceiling cassette type, and two wall mounted units, one with 12k and the other with 9k. Last used the system in late September last year, had some warm weather a few weeks ago so turned on the ceiling unit, it started up and worked perfectly. (As did the outdoor unit). I then went to fire up the two others, both dead a a door nail. Checked incoming power on each, present on both. Then extracted the control boards (no easy task) to discover the single fuse on both boards were blown. Replaced the fuses, checked for continuity, and reassembled, still completely dead, tho the new fuses did not blow. I have ordered two new control boards but am leary of just installing them without getting some feed back from folks here. We’re not prone to power surges in my area and I find it really odd that this happened to two of the three units and while the units were turned off sometime during the winter. Could some other component within the air handler, other than the circuit board, have caused this and might fry the new boards I install? Sorry for the long read, and I would sincerely appreciate any and all thoughts.

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  1. walta100 | | #1

    Do you have a wiring diagram for your unit?
    Did I miss seeing the make and model numbers for your units?
    Will you please post a photo of the board you think is bad? We may see a fault you do not see.


  2. worldlyone | | #2

    Thanks W.A. It's a Thermocore T118D system, and the two air handlers are their "T1" models. I will upload two pics of one of the boards, and of the wiring diagram for the air handlers. I will be receiving the new boards, today, so can send a cleaner pic of those when I get it, if that would help. REALLY appreciate your thoughts on this.

  3. Expert Member
    BILL WICHERS | | #3

    My guess is something failed within the high side of the small switching power supply on that control board. This may be due to a surge or spike on the power coming in, or it might just be a General “it died” issue with no real cause. It’s hard to say with any certainty what caused it. I’ve seen this before on other units with power supplies like this. Some designs just seem prone to die early, I suspect due to a lack of enough integrated protection components.

    The new control board will probably solve the problem. I’d recommend getting a small TVSS device for your electric panel as some extra insurance. There is a company in Florida that makes nice ones for around $50 or so. I’ve posted info about these before, but I can look up the model I recommend and post it again if you're interested.


  4. worldlyone | | #4

    Bill, Thank you SO much for your time and thoughts on this! I will try installing and testing just one board to start with, and go from there. I will also look into your recommended TVSS. Anything to help insure we have A/C when we need it.

    1. Expert Member
      BILL WICHERS | | #5

      I recommend the Ditek DTK120/240CM+ TVSS device for residential use. I use one myself as a secondary protector (I use an expensive polyphaser device as a primary protector ahead of my ATS, but I have frequent lightning issues to deal with).

      You will also want to check if you have any “sneak paths” that surges can get in. The easiest thing to check is to be sure that ALL cables coming into your home enter at, AND ARE GROUNDED AT, your electrical service entrance. This means power, phone, cableTV, any satellite cables, etc, should all come in at the same location and have all their grounds connected together. This is current code, but often overlooked — especially with older homes.


  5. Expert Member
    Akos | | #6

    Definitely blown power supply on your board. You can see the poped FET (8 pin chip with one corner missing).

    Most likely as Bill said, either bad design or power surges. If you don't use your mini split for heating, the best is to turn the breaker off after the heating season. This also reduces your standby power, some units can consume 40W to 60W for the compressor heater even if you don't use the unit.

    1. Expert Member
      BILL WICHERS | | #7

      Wow, good eye! Now we know exactly why it’s dead.

      One thing that MIGHT MAYBE help a little to add some reliability is to get a small ferrite toroid of mix #31, and wrap 4-5 turns of BOTH supply power leads around it. You wind these by looping the wire through the “hole in the donut”, each pass through is one turn. This forms a choke which can help limit some fast rise transients. No guarantee it helps with whatever specific issue caused the failure you saw though.


  6. worldlyone | | #8

    You guys ROCK! Thanks so much for all the insight and advise. I will certainly install the recommended to help keep this from happening, again. And I didn't even think about pulling the breaker during the off-season (we don't use it for heat). Thanks for that, also. The delivery of my new boards got pushed back to today, so waiting patiently, grateful that we have cool weather in the immediate forecast.

  7. walta100 | | #9

    I agree the power supply chip is blown apart and bad.
    The new board will likely get you going.

    Good eye Akos.


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