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