I have a 12 year old house with a rather poor quality fireplace insert. The flu has problems staying closed when not in use and the is a great deal of cold air coming thru the lower louvers. I have to wedge a piece of wood to hold it closed. When is is windy outside you can feel this across the room. Here is the real question, there is some kind of vent control on the bottom left of the insert. It moves only about half an inch in and out but it seems to move more when is is moved to the right. Regardless of the position, there is a lot of cold air coming thru the lower louvers. What is this and what is it supposed to do? Naturally there is no brand name nor model number anywhere. I really don't feel like replacing it because of the expense.