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 tank is merely a part of the heater, forming an additional reservoir for the return water. It is possible to do away with the tank entirely, connecting the returns direct into the water chamber of the heater, but as the water space of the heater is generally comparatively limited, the water level in such cases is subject to more or less extreme fluctuations, due to the fact that the return water does not always come back with a uniform flow. This is especially the case with large office buildings, when the building is being heated in the morning and a number of cold radiators are apt to be turned on at nearly the same time. In the same way it is possible also to do away with the receiving tank represented in Figure 7, but this is subject to the same objection as in the other case, only to a more extreme degree, as the small governor provides scarcely any reservoir volume for the return water and the pump is subject to sudden changes in speed. In the arrangement shown in Figure 7 the main return pipe is generally connected at the point, F, and not directly to the tank.

The writer has installed a number of large plants with open heaters and no receiving tanks whatever which have given perfect satisfaction; and recently installed a plant having over 16,000 square feet of radiating surface with practically the same arrangement as indicated in Figure 7, but without any receiving tank. This system requires rather careful attention, especially in the early morning, but it was impracticable on account of local conditions to put in a receiving tank, and the system has given thorough satisfaction.

It should be noted here that the system represented in Figure 8 is practically a gravity system, the heater and tank taking the place of the boiler represented in Figure 1 to 4, and acting both as steam-producing chamber and reservoir for return water, both being at this point under precisely the same pressure. The water level in the return pipes and return risers will stand at a higher level than in the heater or boiler in the case of Figures 1 to 4, by a distance representing the difference in pressure required to force the steam through the system, just as in the ordinary gravity system. As a matter of fact, also, it is found that in the system shown in Figure 7 the pump operates much more smoothly and uniformly if the system is made a gravity system by connecting a small equalizing steam pipe between the main steam supply of the heating system and the top of the receiving tank and governor.