Because of the larger proportion of hot water than cold water, cold water flows downward and hot water goes upward. The water will be heated continuously between water tank and vacuum tube. In other wards, hot water always floats on cold water. The hot-water flows to the water tank from the vacuum tubes. Because of the proportion, cold water flows to vacuum tubes from the water tank.
The cold water is heated in vacuum tubes. After it becomes hot water, it flows to water tank again. The water circulates constantly between water tank and vacuum tubes; the cold water in water tank will be hotter and hotter. Finally, all cold water will be heated to be hot water. This is the heating course of cold water.