Mr. Heater Corporation Vent Free Radiant Natural Gas Heater, 20k BTU 20,000 BTU per hour; For use with natural Gas; Heats up to 500 sq. ft. Radiant heat burner Quiet integrated blower for gentle circulation of heated air Thermostat for automatic temperature control; Automatic low oxygen shut-off system (ODS) Blower fan kit (f299200 sold separately) - available for 20k units and up only