I often come across heating system flue pipes which penetrate through the side of a brick chimney, usually in a basement. Many times the flue does not appear to be properly sealed at the penetration point or is the sealant is deteriorated. Of course I have no way of seeing how/if the flue is connected to a liner inside the chimney wall. My question is, what is the proper method or sealant to install a flue through a chimney wall. Thanks.
99.99% of masonry chimney flue entries I see are incorrectly done.
The best way is to have a 7-10"+ piece of flue pipe material (stainless steel is best for this little job due to its longevity) mechanically fastened to the supporting brick structure and extending into the flue. The outer end (male or female end out depending on the fuel) should extend outwards for 3-5" and be mortared/sealed to provide an airtight connection to the flue/chimney.
Then the flue pipe can be connected to this extending “breech pipe” and attached securely with 3 equally spaced sheet metal screws.