This invention relates to respiratory humidifiers and heated breathing conduits used to couple a patient to the humidifier. A conduit overheating detection system for a conduit having heating wire or element is disclosed. The overheating detection system may be utilized with a in a single limb conduit system or dual limb system. In each of these systems the conduit overheating detection system monitors the current in the heating element(s) and alters the power to the heating element(s) to prevent the occurrence the heating element(s) and/or conduit from overheating.