MHU50-50,000 BTU, Big Maxx Natural Gas Unit Heater, 50,000 BTU Per Hour, Heats Up To 1250 SQFT, Spark Ignition Features Self-Diagnostic Control Module, Includes 2 Angle Brackets For Ceiling Mount, Ships As Natural Gas, Includes NG To LP Conversion Kit, 3" Flue Size For Vertical Vent & 4" For Horizontal, Category 1 Vertical: Category 3 Horizontal Venting.