A Direct-Indirect Radiator is a standard direct radiator that sits on a device called a box base. The box base brings in air from outside via a louvered opening in the outside of the wall known as a wall box. The wall box connects outside air to the radiator box base thru a short duct arrangement known as an extension sleeve. The point of an arrangement like this is to introduce fresh air into a room (like a school room) and have that fresh air heated so as to remain comfortable also. The radiator is called a direct-indirect radiator because some of the radiator is exposed to the room and directly heats existing room air, but the majority of sections of the radiator are encased in the sheetmetal walls that extend up from the box base and these sections indirectly heat the room. Thus direct-indirect radiator. Many of these box bases were equipped to close off the outside air intake and recirculate the existing air thru the units. This was only done in severe weather.