As the indirect heaters are placed in the basement, care must be taken that the bottom of the radiator does not come too near the water-line of the boiler, or the condensation will not flow back prop erly; this distance, under ordinary conditions, should not be less than 2 feet. If much less than this, the pipes should be made extra large, ap that there may be little or no drop in pressure between the boiler and the heater. A drop in pressure of 1 pound would raise the water-line at the heater .2.4 feet.
Direct-Indirect Radiators. A direct-indirect radiator is similar in form to a direct radiator, and is placed in a room in the same manner. Fig. 76 shows the general form of this type of radiator;
and Fig. 77 shows a section through the same. The shape of the sections is such, that when in place, small flues are formed between them. Air is admitted through an opening in the outside wall; and, in passing upward through these flues, becomes heated before enter ing the room. A switch-damper is placed in the duct at the base of the radiator, so that the air may be taken from the room itself instead f from out of doors, if so desired. This is shown more particularly in Fig. 76.
Fig. 78 shows the wall box provided with louvre slpts and netting, through which the air is drawn. A damper door is placed at either end of the radiator base; and, if desired, when the cold-air supply is shut off by means of the register in the air-duct, the radia tor can be converted into the ordinary type by opening both damper doors, thus taking the air from the room instead of from the outside. It is customary to increase the size of a direct indirect radiator 30 per cent above that called for in the case of direct heating.