Mr. Heater Corporation Vent Free Flame Propane Heater, 20k BTU, Blue 20,000 BTU per hour use with propane gas heater for spaces 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; Automatic low oxygen shut-off system (ODS) Blower fan kit (f299200 sold separately) - available for 20k units and up only