The invention relates to heat engineering, primarily to industrial thermal technology, namely to the field of fluid heating, e.g. water, using electricity. The invention can be used to simplify and cheapen manufacturing of any multi-phase appliances intended to heat fluids, primarily in industrial plants. For example, the invention can be used in the systems of circulating water heating, in self-regulating fluid heaters for autonomous heat and hot water supply, mobile heating and hot water systems, as a universal device for different electric heaters, to produce large amounts of hot water especially at industrial, agricultural and other facilities. The invention can be applied also in domestic water heaters if multi-phase power supply is available. The invention may be used to generate hot water in movable facilities, i.e. vessels, air-planes, vehicles. The invention has the following objectives: to increase the ease of manufacturing, assembly, operation and cleaning of multi-phase electrode assemblies and electrode heating boilers in whole, to increase reliability, design simplicity, mounting and operation safety. Another objective is to improve safety of devices and their components, in particular, electrode assemblies due to inaccuracies during assembly. Another objective is to increase the device durability, service life and maintainability. This is done by improving the appliance safety from changes in due course of the electrode shape and orientation in assemblies that is especially important at a large amount of electrodes in multi-phase industrial boilers. The invention solves the problem of expanding functional capabilities, versatility and flexibility of the device, expanding the possible product range and enhancing the adaptability to solve various particular problems. In addition, the invention enables to intentionally change and improve convection properties of water heating boilers. Also, the invention aims to increase the appliance protection against breakup between electrodes, to decrease uneven loading of phases by current, to protect electrodes against non-uniform deformation during operation in dynamic modes. The objective also includes simplification and expansion of variability ranges for the design, typical sizes and power output of multi-phase boilers that is especially important for industrial plants. According to the variant 1, the electrodes (1) of the device are directed inside the cylindrical casing (2) downward in near- vertical directions. The electrodes (1) are directed vertically downwards with a slight deviation from the longitudinal axis of symmetry of the casing (2) and from the longitudinal axes of all electrodes (1) of each electrode assembly (3). The electrodes (1) can be installed on various unequal distances from each other. The longitudinal axes of the electrodes (1) of all assemblies (3) may also form non-zero angles to each other of a part or all electrodes (2), which is simultaneously congruent with unequal distances between the electrodes (1). The electrodes (1) are grouped on a spatial basis - in three groups with three electrodes in each group, forming the assembly (3), which consists in this variant of three electrodes (1). Each electrode (1) in the assembly (3) can be a phase electrode, and in such case has a separate current lead (5). Also, the electrodes in the assembly (3) can be electrically connected together within one assembly (3). In this case, the electrode assemblies (3) are the phases and each assembly (3) has a separate current lead (5). The casing (2) is covered with the lid (6) and the electrode assemblies (3) with the electrodes (1) are fastened on the lid (6) outside the casing, so that the electrodes (1) pass through it into the casing (2). In this case the lid can be made either of a heat-resistant dielectric material or of a metal; then, the electrodes require an electrical insulation (7) from the lid (6). The lid (6) can be secured with bolts (8) and can be removable or it can be welded to the casing (2); and the electrodes (3) can be removable. The electrode assemblies (3) can be made removable as well. The multi-phase water boiler can be used independently; either its casing (2) is inserted into the open or into the circulating water heating system in any desired location. The water heating system is filled with water, treated in a usual manner, lapping its electrical resistivity, and the electrodes (1) of the boiler are connected, using leads (5) disposed outside its casing (2), to an external electrical polyphase circuit, e.g. a three-phase. Connection is performed depending on the particular variant of the device - either each electrode (1) of each assembly (3) is connected to its phase or the assembly (3) is completely connected to its phase. The cooled water from heating radiators passes in the casing (2) of the boiler through the inlet (4), where it is heated by current passing through the electrodes (1). The heated water comes from the casing (2) to consumers, e.g. heating radiators. The convection processes originated in the casing (2) of the boiler, when the water is heated between the electrodes (1), can be intentionally arranged by means of mutual orientation and location of the electrodes (1) and electrode assemblies (3) in such a way that the boiler can serve as a circulation pump without any forced water pumping in a closed system. The offered option of the mutual orientation and possibility of asymmetric arrangement of electrodes (1) against each other both in the casing and in the electrode assemblies (3) considerably contributes to it. It also allows relocating localization of sliming processes including on the electrodes alone. The suggested location of the electrodes (1) and electrode assemblies (3) enables the current path choosing and its density distribution varying. All the above provides optimization of the boiler efficiency both in static and dynamic modes of operation of multi-phase water boilers in all suggested configurations.