Split air handlers fuses blown

Discussion in 'HVAC Heating & Cooling' started by scottmcll, May 22, 2020 at 12:21 PM.

  1. scottmcll

    scottmcll New Member

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    Issaquah, Washington
    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.
     
  2. WorthFlorida

    WorthFlorida The wife is still training me.

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    On the circuit board there might be a label or printed number next to the fuse. It could give an idea what it might be for. Generally, if the fuse is a ceramic cover (no glass), it's on the line voltage side, perhaps to a transformer. A glass fuse will be low voltage. The original fuse may have a voltage rating stamped on it. Possible causes could have been a power surge since two separate units blew its fuse and components failed
     
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  4. scottmcll

    scottmcll New Member

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    Thank you WorthFlorida. They are glass fuses, so interesting to know they are low voltage. The printed number next to the fuse is T10A/250VAC, if that tells you anything., and that's what I replaced them with to no affect. Clearly there is additional damage to the boards other than the fuses.
     
  5. fitter30

    fitter30 Active Member

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    There should be some kind of flow charts how to check out the circuitry before powering up the boards.
     
  6. scottmcll

    scottmcll New Member

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    Thanks fitter30. That may be a bit beyond me. I'll look into it.
     
  7. fitter30

    fitter30 Active Member

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    You didn't mention the brand of mini split but if one unit was down maybe but two theres a problem.
     
  8. WorthFlorida

    WorthFlorida The wife is still training me.

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    That is a 10 amp slow blow at 240v AC, it's may not be low voltage. To tell for sure I have to look at the circuit board. It may only be for a 120 v fan motor or a step down transformer.
     
  9. scottmcll

    scottmcll New Member

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    Fitter 30, the brand is Thermocore. And WorthFlorida, I will upload a couple of pics of the board I took when I had it out. I am supposed to receive MVIMG_20200512_144506.jpg MVIMG_20200512_145519.jpg the new boards, today, so can upload a cleaner pic of one of those after I get them.
     
  10. scottmcll

    scottmcll New Member

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    And here is the wiring Diagram. Wiring Diagram.jpg
     
  11. WorthFlorida

    WorthFlorida The wife is still training me.

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    I see that the fuse is soldered to the board and the print on the board is for another use. The fuse symbol is etched but not used. That is definitely the high voltage side and it is wired for 240v (BLK & RED north America power). Not sure what the yellow is. Probably the neutral (White wire from the breaker panel) to run 120v fan motor and the 24v ac for relays and contactors.
     
    Last edited: May 23, 2020 at 5:34 AM
  12. Bannerman

    Bannerman Well-Known Member

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    The transformer appears to have been overheated. Look also at the IC below the transformer, above the large capacitor as the IC appears to have blown as there seems to be a large chunk missing.
     
  13. WorthFlorida

    WorthFlorida The wife is still training me.

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    I think it was a surge since two out of three failed. If the transformer overheated you would see burn marks on the board. It's possible the cap failed but it would usually be swollen.

    board.jpg
     
  14. Bannerman

    Bannerman Well-Known Member

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    Look also at the discoloration around the diodes directly above the circled IC. I suspect the tranformer too has been affected. Without a schematic to confirm, I anticipate the affected components are associated with the low voltage DC power supply for the circuit board which has taken the brunt of a power spike (lightning?).

    The On/Off control for the unit is probably low voltage so unless AC power is disconnected during the off season, the circuit board will continue to be powered.

    As the circuit board appears to be susceptible to power damage, suggest installing a surge suppressor to the AC power line feeding each unit. Alternatively, surge protection could be installed at the main electrical panel so as to protect all circuits within the home.
     
  15. scottmcll

    scottmcll New Member

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    Location:
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    Thanks so much guys! From your observations, the board certainly had a a series of failures that, clearly, just replacing the one fuse was never going to resolve. I will certainly follow your advise for measures to help prevent this in the future. Guess I'm lucky that the outdoor unit and the ceiling cassette handler weren't damaged, as well.
     
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