Sweetening the pan is just a generic term used for the first time you boil for the year or anytime you start fresh again. All your doing is boiling sap and as you boil it gets sweeter. If you batch boil you are sweetening the pans every time you put fresh sap in the pans. On a continous setup with a flue pan you are drawing off constantly and never empty the pan (unless a cleanup is in order, or your out of sap for an extended period of time) so the first boil you are sweetening the pans until you get your first draw.
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