The pipeline for hot centralized water supply cannot be made according to the scheme of cold water supply. Cold water supply pipelines are dead-end, i. e. they end at the last point of the water system. If you make the hot water supply in an apartment building using the same scheme, then water at night when it is not used too much will be cooled in the pipeline. Besides there can be a situation, for example, when residents of a five-storey building which are located on one vertical pipeline go to work in the day, the water in the pipeline gets cooled and suddenly one of the residents on the fifth floor needs hot water. After turning the tap on, he must drain all the cold water from the vertical pipeline, wait until the water becomes warm and then hot — that is excessively wasteful. Therefore the pipelines of the hot water supply are made in a loop: the water gets heated in the boiler room, thermal unit, or boiler and is supplied via the supply pipeline to consumers and then comes back to the boiler room on the other pipeline which, in this case, is called circulational.
In the centralized hot water supply system, the installation of pipelines in the building is made with two-piped and one-piped vertical pipelines (see Figure 111).
A two-piped hot water supply system consists of two vertical pipelines, one of which supplies water and the other takes it away. The heated towel rails are placed on the return circulational vertical pipeline. The water is heated anyway and is sent to consumers but whether they use it or not and at what time is unknown, so why waste it? Let this water heat up towel racks and the air in the bathrooms which is, by definition, damp. Besides, towel racks serve as a П-shaped compensator for thermal elongation of the pipes.
One-piped hot water supply system differs from the two-piped version in that all the circulation vertical pipelines in it (within one section of the building) are combined in one and this vertical pipeline is called «unmarried» (has no consumers). For better water distribution to individual points of water consumption and in order to have pipes of the same diameter along the entire height of the building in one-piped hot water supply systems, the vertical pipelines are looped. While using a ring scheme for buildings of height up to five floors, the diameters of the vertical pipelines are 25 mm and for buildings with six and more floors, the diameter is 32 mm. Heated towel racks in one-piped settings are put on the vertical supply pipeline and this means that during weak water heating in the boiler rooms, it is cool by the time it reaches further consumers. Hot water will be taken not only by close consumers but also it will be cooled in their heated towel racks. In order to prevent the water from become cool and reach the far consumers still hot, a bypass is installed in heated towel racks.
Two- and one-piped hot water supply systems can be made also without heated towel racks, but then these racks must be attached to the heating system. In this case, in summer time, the heated towel racks will not work and in winter time the total expense for hot water supply and heating will increase.
In order to purge air from the system, the pipes are installed with a slope of not less than 0,002 regarding the input of the pipeline. In lower-setting systems, air is removed through the upper water tap. In upper settings, the air is removed by automatic air purgers installed in the upper points of the systems.