Registers are made of cast iron and bronze, in a great variety of sizes and patterns. The universal finish for cast iron is black "Japan"; they are also finished in colors and electroplated with copper and nickel. Fig. 17 shows a section through a floor register in which "A" represents the valves, which may be turned in a vertical or horizontal position, thus opening or closing the register; "B" is the iron border, "C" the register box of tin or galvanized iron and "D" the warm-air pipe. Floor registers are usually set in cast iron borders, one of which is shown in Fig. 18, while wall registers may be screwed directly to wooden borders or frames to correspond with the finish of the room. Wall registers should be provided with pull cords for opening and closing from the floor; these are shown in Fig. 19. The plain lattice pattern shown in Fig. 20 is the best for schoolhouse work as it has a comparatively free opening for air flow and is pleasing and simple in design. More elaborate patterns are used for fine dwelling-house work. Registers with shut-off valves are used for air inlets while the plain register faces without the valves are placed in the vent openings. The vent flues are usually gathered together in the attic and a single damper may be used to shut off the whole number at once. Flat or round wire gratings of open pattern are often used in place of register faces. The grill or solid part of a register face usually takes up about 1/3 of the area, hence in computing the size we must allow for this by multiplying the required "net area" by 1.5 to obtain the "total" or "over all" area.

Fig. 17.

Fig. 18.

For example, suppose we have a flue 10 inches in width and wish to use a register having a free area of 200 square inches, what will be the required height of the register? 200 X 1.6 = 300 square inches which is the total area required, then 300/ 10 = 30, which is the required height and we should use a 10" X 30" register. When a register is spoken of as a 10" X 30" or 10" X 20", etc. the dimensions of the latticed opening is meant, and not the outside dimensions of the whole register. The free opening should have the same area as the flue with which it connects. In designing new work one should provide himself with a trade catalogue, and use only standard sizes as special patterns and sizes are costly. Fig. 21 shows the method of placing gossamer check valves back of the vent register faces to prevent down drafts, the same as described for fresh-air inlets. Pipe Connections. The two-pipe system with dry or sealed returns is used in indirect heating. The conditions to be met are practically the same as in direct heating, the only difference being that the radiators are at the basement ceiling instead of on the floors above. The exact method of making the pipe connections will depend somewhat upon existing conditions, but the general method shown in Fig. 22 may be used as a guide with modifications to suit any special case. The ends of all supply mains should be dripped, and the horizontal returns should be sealed if possible.

Fig. 19.

Fig. 20.

## Pipe Sizes

The tables already given for the proportioning of pipe sizes can be used for indirect systems. The following table has been computed for an efficiency of 640 B. T. U. per square foot of surface per hour, which corresponds to a condensation of 2/3 of a pound of steam. This is twice that allowed for direct radiation in table XIII. of Part I., so that we can consider 1 square foot of indirect surface as equal to 2 of direct in computing pipe sizes.

Fig. 21.

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 properly; this distance should not be less than 2 feet under ordinary conditions. If much less than this, the pipes should be made extra large so 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.