I want to install a multi fuel stove in my living room. Is it safe to connect a 6" stove flue pipe to my 9" clay chimney liner? If I use an adapter, what if any problems might I face? The clay liner is in excelent condition as the house is only two years old.

I personally connected a 6 inch wood/coal stove to a approx 8X10 inch square clay chimney liner approx 15 years ago. Still works fine. Clean your chimney yearly, brushes are cheap. also when the draft gets bad. usually caused by burning garbage or green wood.Also recommend not having multiple inlets into chimney, IE:more than 1 stove. In case of chimney fire you can choke it out easy by closing the draft, multiple stoves make that alot harder, more dangerous.

I just moved to a three year old house with a large fireplace. A cast iron wood burning/multi fuel stove would fit perfectly in the large opening. My problem is that the stoves I’ve looked at have six inch flue pipe diameters, while the clay liner coming from my chimney has a nine inch diameter. Will I need to have something fabricated or does anyone know a company that has an off the shelf solution that doesn’t cost a fortune?

You should be able to find one off the shelf at a hardware store and just to clarify what others are saying it should be at least 26 gage or better yet 24 gage for wood/coal burning stoves.30 gage is used for ductwork and is too thin.