I have a combination Wood and Coal burning fireplace insert, there are 2 sets of grates for the bottom of the firebox. One set (2 pieces)is a has multiple "vent holes" and sit in a larger opening in the floor over the clean out. The second set ( 2 Pieces) are just a plate the covers the openings for the other set. I assume one is designed for wood and the other is for coal. Can anyone provide guidance? Thanks