Our house was built in the 1950's. We have a big stone fireplace in the living room. It has two vents above the mantel, I assume for heating the room when a fire is going. We would like to be able to put a TV on the fireplace. Any advice on how to go about closing off the vents. Can we just have them sealed up and put stone over the holes? These are the only vents, there are not any below the hearth.