I have a 1955 farm home which has a chimney currently being used to exhaust an oil furnace. The furnace is in the basement, and on the main floor there was a thimble extending out on each side of the chimney. When I had a chimney expert out to inspect the chimneys, he suggested I brick/mortar one side up and place a thimble cover plate on the other side. In case the next owners move away from the oil furnace and want to put in a wood burning stove. So I followed his advice and blocked in one side and mortared it up. The side that still has an accessible thimble is in the kitchen and it currently has an original thimble cover plate (tension rods on back that slip into thimble) on it that was hand painted. I also have carbon monoxide detectors which have never gone off.
Problem: home inspector said I have to remove the cover plate and seal the thimble to prevent carbon monoxide leaks.
I reached out to the chimney guy that I originally worked with and he said that he knows of no building code regulations that say how to seal the thimble so the thimble plate w silicone caulk should be fine or to just brick/mortar the hole (which I don't want to do).
Question: can I use a high temp silicone caulk around the thimble cover plate so it's not permanent like brick/mortar but still air tight. Is there other methods of sealing a thimble so carbon monoxide doesn't escape but aren't permanent?