While renovating my kitchen, I came across an old exhaust duct in the wall. It goes through the chimney bricks and into the interior of the chimney. The boiler uses a duct within the chimney that is not connected to this old piece of duct. So, I don't think fumes can escape from it, but I would like to plug it well anyways. I also want it more or less flush with the chimney so I can cover it with drywall. I was thinking of trimming the edge that sticks out, plugging it with a plug-type galvanized end cap, and then sealing around it with gutter/flashing sealant. Does that sound sufficient, or should I do something else? Thanks.