Mesquite in the grill is great for the flavor. While it burns very hot in the fireplace, if it is pleniful and used sparingly it can be used. Just don't get carried away and build a large fire. A small fire and fed frequently is will make a difference in the heating bill.  That said, mesquite is all that we have in a large quanity in this area. We pay large chunks of money to establish oak trees, no way are we going to cut one down.