A method of automatically ensuring against chloramine contamination in purified product water includes supplying input water to the system and purifying the water to generate the purified product water. The purifying includes removing chlorine and chloramine contamination from the water using a carbon filter and supplying chlorine-depleted water to a deionization filter, and deionizing the chlorine-depleted water using said deionization filter. The product water is supplied to a sensor for continuous monitoring of the resistivity of the purified product water by the first sensor, and an alarm is generated indicating possible chloramine breakthrough when the resistivity of the product water falls below a predetermined resistivity level, which is selected to provide a reserve filter capacity before breakthrough would occur. The carbon filter is replaced at least responsively to the alarm to ensure excess capacity of said carbon filter sufficient to prevent chloramine breakthrough in said product water.