My house used to use the basement for gas heat and the water heater that were using these, 3.5" and 5" ports in the chimney stack.
The whole chimney is no longer used but I want to keep the option of using it upstairs for a potbelly stove or such. In either case of not using it or maybe using it, how important is it to plug these holes and what should I use to plug them? E.g. some premade air duct plugs that fit or just patch it with brick pieces and mortar permanently? They will not be accessible soon as I plan to hang drywall at about where the camera was when the picture was taken.