My home was built in 1802. There was a massive six ft wide fireplace built in what originally was the kitchen. The entire cavity entrance was bricked up many years ago. We have an oil fired boiler in the basement which heats water for radiators throughout the house. Apparently when the previous owners installed the boiler they ran the vent (exhaust) into the bottom of this old fire place and did not install a liner. It is using the old chimney (structurally sound) as part of the vent. We never noticed a problem until this last winter when the boiler needed a service and we noticed fumes in the room due to the boiler not burning properly. This told me that we are probably getting CO into the room and not noticeing it when the boiler is operating properly. A stainless steel liner would be an easy fix and well within my capability but here is the question. The exit of the oil boiler into the firepit (one floor above the basement) is through a comparatively new construction of a square brick stack that starts from the floor of the basement with a clean out door at the bottom below where the metal fluepipe (8", in good condition) enters the square brick stack from the boiler. I now have access into the old fireplace and can see the continuation of this square brick stack and it is level with the floor of the firepit. I would like to cement an adapter in place to connect an 8" stainless steel liner.

Is there a concern about what material this adapter is made from? I would like to use stainless steel but most of the parts that are available from suppliers are aluminized steel or galvanized steel. There is then a lot of text about one manufacturer's parts not working with the parts from another. Is this something that I should have custom made by a local HVAC firm? The furnace is shut down until repairs are completed.