I need some help, we installed a Englander 28-3500 add -on and are not getting very good burn times or heat. the flue goes into a old teracota chimney that I 13x9. I am going to purchase a pre insulated 6 inch flue liner to see if that helps. question is will the flue liner fit in my old flue number one ,number two, my chimney liner will be 30" long will that be to long of a run for a 6 inch flue to get the right draft? thanks for any help!