Invisible radiators are just that, invisible to the occupants of a room. The heating elements are cleverly placed inside a wall cavity opening which is then plastered over. There is a slot or grill at the bottom of the wall which lets the cold air in to be heated, and there is a grill at the top on the wall which lets the heated air out into the room. Pretty clever huh? There were many makers of these back in the day, among them was Modine, and Trane. You will notice on the Trane link that the same element gives off more btu's if the higher grill outlet is used. The taller the invisible radiator was the better the chimney effect was in getting the air to move. The Invisible radiator was generally used in dining rooms and living rooms, but could in fact be put just about anywhere.