Abstract: This invention is comprised of a thermostatically controlled safety circuit and device used in conventional hot air oil burning furnaces which automatically turns the oil burner off when the temperature of the room heated by the furnace becomes abnormally hot. By so doing an overheated furnace and possible fire is avoided. The invention is comprised of a line voltage or safety thermostat positioned at or near the location of the conventional furnace thermostat. The line voltage thermostat is connected between the limit switch and the control circuit of the furnace control circuit. A variation of the invention is comprised of the line voltage thermostat or safety thermostat connected or ganged with the conventional furnace thermostat so that the line voltage thermostat will be activated at a higher temperature than the conventional thermostat.