The object of this study is the processes of scale formation in electric water heating systems when using water of increased hardness. The paper reports the design and testing of a magnetic activator to prevent scale formation on tubular electric heaters under hard water conditions without the use of chemical reagents. The device with neodymium magnets is installed on the supply pipeline of the water heater and solves the problem of scale formation without the use of chemical reagents. The experiment was conducted for 90 days on a laboratory bench simulating the operation of a domestic water heater. Magnetic treatment of water reduces the overall hardness by 15–20%, the content of Ca2⁺ and Mg2⁺ ions by 35–50% and forms predominantly an aragonite modification of calcium carbonate, which is less prone to strong adhesion to the heating surface. The thickness of the scale layer on the heating element is about 0.5 mm versus 2 mm in the control sample, which reduced additional energy costs from 13% to 3%. The effect is attributed to the action of magnetic field on the hydrate shells of hardness ions, which contributes to the formation of a less durable aragonite modification of calcium carbonate and disruption of the conditions for the crystallization of durable calcite on the heating surface. A unique feature of the device is a combination of internal and external magnetic activators based on neodymium magnets that generate a stable magnetic field with high induction, as well as the presence of a sump and design solutions that prevent contact of the magnets with water. This enables a long-term and targeted effect of the field on the water flow, which improves the efficiency of treatment. The magnetic activator is effective in domestic and industrial water heating systems with hard water; it increases the service life of the equipment, reduces energy consumption, and decreases the need for chemical reagents