When thinking about a gas furnace and venting it into your chimney you should really consider relining your chimney with a stainless steel chimney liner. Why? Today’s furnaces know how to break down masonry chimneys. It’s what they do. They find ways to make block crumble and create issues for your home. Some will say to reline the chimney with an aluminum chimney liner. You can, but plan on replacing it within 10-15 years. Aluminum chimney liners simply don’t last. If they don’t last they can break down and even create blockages in your chimney. If this happens, the toxic flue gases that are supposed to go outside your home can end up inside. People have died from this, it’s nothing to play games with. If you want the best and safest chimney, reline your chimney with a Flex King Stainless Steel Chimney Liner.