Question: We are looking at installing a B-vent L P gas insert in a masonry fire place. Our masonry fire place has a combustion air vent from the attic. Would it be best to leave this vent open when installing the insert or close it off and use air from inside the room for combustion air. We are look at purchasing the Earth Stove BayFire XL. Answer: The problem is that you might not have a method to easily seal off this opening when it is not in use. If this is the case, you might have cold air coming into the home when the fireplace is not being used. If your house is not extra tight, then the Earth Stove may have enough combustion air without this inlet.