Filter for demineralizing water for modern heating systems (C.O.) and many other systems requiring water with low TDS and conductivity. A simply designed filter consisting of a glass-reinforced plastic tank, a flow control head, and a special demineralizing resin. Each unit can only treat a specified amount of demineralized water, which depends on the raw water conductivity and the size of the resin bed. After treating the specified amount of water, the resin must be replaced or the tank exchanged, as the resin is not regenerable.