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Old 06-21-2012, 07:18 PM   #1
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Bad main board, reversed polarity? Suddenly a problem?


I work in the commercial building automation field, so I'm fairly good with controls, but I'm not familiar with the diagnosis a friend had after her home AC quit working.

This is her quote from the tech. They want to replace the main board. "He found a bad main board due to reverse polarity on furnace. Posible further problems."

I don't know what further problems there could be, other than some burned out contacts, but how could this happen at this point in it's life? The system was installed in 1985, and I don't think there's been much work on it since. She's been in the place for 2-3 years and hasn't had any problems or any other work done on her house, so I don't understand how reversed polarity could have suddenly happened.
Any help would be appreciated!
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Old 06-21-2012, 07:29 PM   #2
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Bad main board, reversed polarity? Suddenly a problem?


What was the initial problem?

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Old 06-21-2012, 07:33 PM   #3
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Bad main board, reversed polarity? Suddenly a problem?


something fishy going on.....
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Old 06-21-2012, 07:35 PM   #4
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Bad main board, reversed polarity? Suddenly a problem?


most furnaces are 115V with a transformer within 115V/24V then it lands on that board R and C...that R/C is the only polarity that might cook the board..the board mounted relay handles the 115V feed to the supply fan motor so that could take an mperage hit starting over the years it should have ratings on it just like a hand held relay... http://www.bestbuyheatingandaircondi...tegory_Code=CB
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Old 06-21-2012, 07:41 PM   #5
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Bad main board, reversed polarity? Suddenly a problem?


Some boards are sensitive to reversed polarity. And there are some boards that will give you that error when the board itself goes out. Even though the polarity isn't reversed.

Was any electrical work done in the last few months?
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Old 06-21-2012, 07:56 PM   #6
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Bad main board, reversed polarity? Suddenly a problem?


The initial problem was no AC. I don't think the main fan is running either.
As far as I know, there hasn't been any other electrical work done since she moved in.
That might be it Beenthere. I didn't realize that code can show up for a board failure, good info again!
I guess I'll have to stop by with my meter.
Thanks guys

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