Hydronic heating systems for apartments are gravity systems whose purpose is to heat one or several apartments situated on the same floor. The main difference between an apartment heating system and those described earlier is that in this system the centre line of the boiler is often situated higher than the centre line of the radiators; i.e. in the circulational head, the gravitational part (h) is absent or negative. Putting a negative number or zero in the formula in Figure 1,we get negative or zero circulational head. Such a heating system is called «clamped». The motion of the warmth carrier in the system is provided only by decreasing the density and increasing the volume of the heated water. The tops of the radiators are hot and the bottom parts are cold.
How to force the warmth carrier to circulate in such systems, to «unclamp» the system? First, as has already been said, it is necessary to try to install the boiler as low as possible, for example to cut a depression in the flooring and to position the boiler in a way such that the return pipe to the boiler is on the same level as the return pipes of the system. Second, to use the gravitational force of the water to overcome the internal resistances of the pipes and radiators, the slopes of supply and return pipelines should be not less than 0.005 (at least for the supply pipes; for the return pipes, probably, quite often, this is not possible, but you should try). Third, to use the difference in densities of hot and cooled water to full extent (the second part of the formula in Figure 1), the vertical pipeline with hot water must be heat insulated up to the branch pipelines and these must be situated under the ceiling of the apartment. In the previously described systems, it was possible to place heat insulated branch pipelines in the attic and in the basement, but in an apartment, this pipeline can be put only under the ceiling, accessible and not insulated. Thus we can consider that the thick supply pipe is itself a heating device and together with the radiators, the actual center of the heating devices is higher than the centre of the boiler, and circulational head results. In other words, although it would seem from the scheme of such a system that the centre of the boiler is above that of the radiators, and so the water would not circulate in the system, in fact, if we take into consideration the supply and return pipes, then we see that the actual centre of the heating devices is situated above the centre of the boiler and the water will circulate. But really, the circulation head in an apartment heating system is quite low and therefore, to provide normal circulation it is recommended to use all available means: the force of gravitation for the water (using large diameter pipes and steep pipe slopes); difference in densities of hot and cold warmth carrier (heat insulation of the vertical pipeline); gravitational component (difference of heights between centre of heating devices and centre of the boiler); and also to decrease hydraulic resistances in pipes and radiators as much as possible (choosing pipes and devices with the lowest resistance).
The system of hydronic apartment heating consists of: the boiler, which is placed in the kitchen; the main vertical pipeline, which is insulated to improve circulation and decrease overheating of the kitchen; hot supply pipeline, which goes under the ceiling; the hot vertical pipelines; heating devices; return vertical pipelines; and the return pipeline which goes under the heating devices (see Figure 11). The expansion tank is installed in a heated room and is connected to the main vertical pipeline. There is a pipe going from the expansion tank to the kitchen sink which serves simultaneously as overflow, air purger, and signal pipe. To purge air and get rid of water from the system, the supply and return pipelines are installed to slope downwards in the direction of the water flow in the pipeline. The water from the system can be eliminated through a drain pipe with a valve which is placed at the lowest point of the pipeline. Filling the system with water is done from the local water supply. It is necessary to put in this unit both a valve and a check valve and also to include a dirt filter in it.