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If running heater with damper open, non-combustible material above fireplace opening is not needed. Go to Installing Damper Clamp Accessory for Vented Operation, page 9. Minimum Non Combustible Material Clearances If Not Using Mantel Note: If using a mantel, go to column 3. If not using a mantel, follow the information on this page. You must have non combustible material(s) above the fireplace opening. Non combustible materials (such as slate, marble, tile, etc.) must be at least 1/2 inch th