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you need a new board, not a repaired one. It is likely a timer or signal in the board that causes the relay issues. If under warranty request a new board.
the problem is that there is nothing in the unit that draws heavy enough power to cause electrical issues. The fact they sent the board away to be tested tells me there is/was a manufacturing issue with the board.
When a board is bad under warranty it is replaced....unless...boards are at a premium because of low stock, high failure rate.
I had one company that reworked their board 6 times before it was able to be below "recall" rate.
I would ask the contractor :
To get a new board
Was there revisions made to the board
Does warranty start over or just is for remainder of initial warranty
Does the factory rep have any input since this unit has had multiple issues.