I was "gifted" an old but unused ember hearth wood burning fireplace insert my current fireplace is funcioning properly nut isn' efficient in heating I would like to install this insert but my first question is I have no liner what should be in there it is just open brick to the top where it has a flu that has a cable that pulls down and closes at the very top of the chimney also the top or exit point of the wood burner is rectangular 4.5" by 12.5 I beleive do I need something else I'm new to this all please help