Problems related to residential installations.
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From your description "... HSI does not shut off after the burners have ignited and the relay which controls the HSI starts smouldering." it seems the relay is fusing closed due to the current draw. You can try replacing the relay itself from the control board or get a new board. Check the HSI to be sure there isn't an obvious visible defect that might cause excessive current draw (short) that was the cause of the relay fusing closed.