Q&A installation tips

QandA Posted By QandA, Oct 5, 2001 at 2:20 PM

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  1. QandA

    New Member 2.
    Staff Member

    Nov 27, 2012

    Just stumbled upon your web site. Very impressed lots of real good information. I wonder if you can help me with a simple problem. I am constructing a new concrete masonry chimney to vent an oil fired furnace. I plan to use 16 x 16 square masonry units with a 6 inch square terra-cotta liner. The furnace will be located in the basement. The furnace vent will have to pass through the 8 inch concrete block foundation about 12 inches below the wood floor joists.
    1.My question is what is the correct way to tie the single wall metal flue from the furnace to the square 6 inch terra-cotta liner of the chimney ?
    2.What is the best way to make the transition through the concrete block foundation ?
    3.Have you ever heard of filling the air space between the terra-cotta liner and the block chimney with Pearlite or Vermiculite to add insulating value to the chimney. I thought this detail may allow the chimney to retain heat and draw better.


    1. You use a "crock" which is a round piece of terracotta to make the connection through the wall. There is an article on passing through walls at https://www.hearth.com/what/specific.html. You can purchase a flue liner for the chimney with a round hole already cut into the side of it.
    2. As above. You may want to use a piece of insulated class A chimney for the wall pass-through since you are only 12" below the joists instead of the required 18".
    3. Yes, this will definitely help. You must center the liner in the block. This can be done with several methods. Some use a nail embedded into the mortar between the blocks to hold the flue tile in place. Another method is to mix some cement and sand with the Vermiculite insulation and pour a ring of it (about 2" think) every few feet as you go up the chimney. Fill the area between these rings with loose poured insulation.

    More info at:


    Link: Article on passing through a wall
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