How to seal up the hole in chimney where the wood burning stove thing used to vent through...??? The house we are renovating has 2 chimneys. One is completely dead.. has nothing hooked up to it for the past half century. The 2nd.. currently has furnace (in the basement) hooked into it and vents upward and outward. The home inspector told me we needed to concrete up those holes (now covered with round, about the size/shape of a dinner plate, cover) to make sure no fumes or carbon monoxide could escape into the room. I guess they used to have wood burning stoves hooked up at them. I have the concrete.. just have NO idea how to mix it and definitely no idea how to concrete these holes up! I'm not too worried about the one in my dining room as it is not hooked up to anything and if I push concrete too far into it and it falls down the chimney base there is nothing it can really ruin. However, the one in our family room has the furnace hooked into it and I want to be sure no material falls down into that one. How do I seal these things up? A friend of mine suggested putting some bricks in the holes and then concreting.. I should mention that once you move the cover plate.. it is like a metal or aluminum tube or whatever in the wall (about 8 inches in diameter) and then the drop off that goes down to the basement (through the chimney) or up to the roof. I hope someone understands my ramblin'!