The 2 photos below are of a water heater flue pipe that is oversized. As a result, the flue gases aren't drafting properly up and out to the atmosphere. They are cooling off before they make it to the top, churning, and causing the B-vent to corrode. This could also lead to backdrafting of carbon monoxide and other flue gases into the home under the right circumstances. On a side note: notice the B-vent touching the drywall in the 1st photo and touching the wood shake in the 2nd. It needs a minimum 1" clearance to combustibles.
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