The relatively small amount of electricity used to operate the fan is converted to additional heat, so that efficiency remains at 100%.
Most installed fuel fan heaters in the first world use a heat exchanger and external ventilation, avoiding that risk by venting the combustion gases to the outdoors.
The picture immediately to the right (the top on the mobile site) shows most of the component parts of a typical plug-in electric fan heater.
The thermal fuse (right) is a fail-safe backup device that will blow and disconnect the heating element permanently should the bimetal cutout fail to operate (e.g. due to its contacts welding together) and in so doing prevent extreme overheating which could result in a fire.
Industrial fan heaters can be used in warehouses, shipping containers, clean rooms, shops and other general purpose heating applications.