I had an inspector come through and said that I should fill this to prevent carbon monoxide. It's my main chimney for my oil tank and is in constant use when it's cold. I'm in southern Vermont and I wanted to know what I can use to fill it. It's pretty big and I'm not aure how to tackle it.enter image description here

  • Not sure of their exact name, but they make though the wall chimney fixtures. Large square pieces of thin steel with hole/pipe in the centre for chimney pipe to enter. They keep the proper air gap/space between the hot pipe and the wood wall.
    – crip659
    Commented Sep 8, 2023 at 14:43
  • @crip659 I think that is for living room fire places going through a stud wall. I think here the pipe is entering a chimney flue, hopefully no wood.
    – jay613
    Commented Sep 8, 2023 at 16:54
  • 1
    @jay613 They are, but should also work to seal that hole and also to make easy remover of the pipes in case work/cleaning needs to be done. There are probably other ways of doing it also, just that big square hole seems like it would do.
    – crip659
    Commented Sep 8, 2023 at 17:00
  • If that chimney was built using lime mortar and case-hardened bricks, lime mortar. If it was build using more recent materials, I'm guessing hydraulic cement would be the easiest patching material but don't know.
    – keshlam
    Commented Sep 8, 2023 at 17:00
  • @crip659 ya that's probably easier and more maintainable than my suggestion! Make it an answer, just don't mention "the wood wall" :)
    – jay613
    Commented Sep 8, 2023 at 17:01

2 Answers 2


They have large square thin steel fixtures to fit chimney pipes though a wall.

Not sure if they are called wall pass thoughs or thimbles or something else.

They have a hole/section of pipe in the centre for the chimney pipe to fit/attach to.

You should only need the measurements of the hole, get bigger to cover the hole, and the diameter of the pipe of fitting.

Some screws and high temperature caulking on the outside of the square should seal it up tight.

  • That soulds like what I know as a "register plate".
    – Simon B
    Commented Sep 8, 2023 at 20:17
  • Maybe but I would use that term for the duct openings in the walls/floors.
    – crip659
    Commented Sep 8, 2023 at 20:23

Use mortar and bricks to replace the missing row beneath the pipe and where practical replace small pieces elsewhere around the perimeter. Repoint other damaged gaps with mortar. Finish around the pipe with high temperature caulk.

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