This scheme differs from the high setting version in that the supply pipeline goes below, near the return pipeline (see Figure 7) and water through the vertical supply pipes goes from the bottom to the top. Passing through the heating devices, the water through the return pipes and the vertical return pipes comes back to the return pipeline, and from there to the boiler. Getting rid of air from the system is done through the air purgers (Mayevsky valves) which are installed on every heating device. Also water can be deleted from the system using automatic air purgers, which are installed on the vertical pipelines or special air lines. Heating systems with both high and low settings can be designed with one or several contours with either «dead end» or «continuous flow» motion of water in supply and return (see Figure 8).
Gravity systems with low setting are used very rarely because they have a lot of end radiators which require air purgers. Because such systems have expansion tanks open to the atmosphere and they get air into the circulational loop, then the procedure of purging air from the radiators has to be done almost every week. To overcome this disadvantage, the hot water supply pipelines are made in a loop with so-called air pipelines which collect air and put it into the expansion tank above its water level (see Figure 9). Such systems are used even less often because they are similar to the high setting systems and require almost the same quantity of piping. In general, the disadvantages outweigh the advantages: vertical pipelines go in the rooms from floor to ceiling; the advantage of lower setting systems was that there were no vertical pipelines in the rooms, at least in the highest floor.
