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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.

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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.

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    I would use fire brick mortar to seal them up , you don't want any chance of back drafts caused by bathroom or kitchen fans making the house negative pressure and pulling the deadly CO , carbon monoxide into the living area. – Ed Beal Feb 13 '16 at 1:17
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    If you don't plug them, cold air, bugs, and other nasties will find the their way in. – Tester101 Feb 13 '16 at 1:18
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Yes. In addition to the comments listed already, as heat rises in your house it can cause more cold air and moisture to enter through the holes.

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