I just bought a 45 year old house and am starting to winterize the house. The house has an original wood fireplace which has been converted to gas, there is a chimney with a working flue. In the back wall of the fireplace there is a small sliding metal door. It is about 2 inches tall and 10 inches wide. It is located about 8 inches from the floor. What is this door for? Is is a vent of some kind or it is to deposit ashes from the original wood fireplace? I'm asking because quite a bit of cold air is coming in through the little door and I'm hoping I can seal it up.