This system of piping is shown in Fig. 35. A single riser is carried directly to the expansion tank, from which branches are taken to supply the various drops to which the radiators are connected. An important advantage in connection with this system is that the air rises at once to the expansion tank and escapes through the vent, so that air valves are not required on the radiators.

Fig. 35.

## Pipe Connections

There are various methods of connecting the radiators with the mains and risers. Fig. 36 shows a radiator connected with the horizontal flow and return mains which are located below the floor. The manner of connecting with a vertical riser and return drop is shown in Fig. 37. As the water tends to flow to the highest point, the radiators on the lower floors should be favored by making the connection at the top of the riser and taking the pipe for the upper floors from the side as shown. Fig. 38 illustrates the manner of connecting with a radiator on an upper floor where the supply is connected at the top of the radiator.

The connections shown in Fig.. 39 and 40 are used with the overhead system shown in Fig. 35.

Where the connection is of the form shown at the left in Fig. 35, the cooler water from the radiators is discharged into the supply pipe again so that the water furnished to the radiators on the lower floors is at a lower temperature, and the amount of heating surface must be correspondingly increased to make up for this loss.

Fig. 36.

Fig. 37.

For example. - If in the case of Fig. 35 we assume the water to leave the heater at 180 degrees and return at 160 we shall have a drop in temperature of 10 degrees on each floor, that is, the water will enter the radiator on the second floor at 180 degrees and leave it at 170 and will enter the radiator on the first floor at 170 and leave it at 160. The average temperatures will be 175 and 165 respectively. The efficiency in the first case will be 175 - 70 = 105and 105 X 1.5 = 157. In the second case 165 - 70 = 95 and 95 X 1.5 = 142, so that the radiator on the first floor will have to be larger than that on the second floor in the ratio of 157 to 142, in order to do the same work.

Fig. 38.

Fig. 39.

Where the radiators discharge into a separate return as in the case of Fig. 32 or those at the right in Fig. 35, we may assume the temperature of the water to be the same on all floors and give the radiators an equal efficiency.

In a dwelling house of two stories no difference would be made in the sizes of radiators on the two floors, but in the case of a tall office building corrections would necessarily be made as described.

Where circulation coils are used they should be of a form which will tend to produce a flow of water through them. Fig.. 41, 42 and 43 show different ways of making up and connecting these coils. In Fig.. 41 and 43 the supply pipes may be either drops or risers, and in the latter case the return in Fig. 43 may be carried back if desired into the supply drop as shown by the dotted lines.

Fig. 40.

Fig. 41.