I have a slight issue I can't quite figure out. I only burn on weekends. It all started a few weeks ago I burned all day Sunday shut it down and when I came home from work my co detector was going off . I called a HVAC guy and had the furnace and water heater checked . All was good. Well last night it did it again. I had a fire Friday but not Saturday but there was still some coals in there and when I opened the door you could feel the cold air pushing down. So it had a back draft. How can I stop. this . My set up is a avalon petition insert in the basement of my trilevel home. It has around 25 feet of out side wall chimney. With a insulated 6" flex liner. I have noticed the draft changes with the weather sometimes but once you warm the flue it has a strong draft.