CI WT No. 25 is the focused nitrite treatment for closed-loop and hot water systems operating under 900 psi where the water is already softened and the loop does not need a broader multi-mode treatment package. Sodium nitrite builds a passivating oxide film on internal metal surfaces, EDTA chelates trace hardness that gets past the softener, and sodium hydroxide holds pH and alkalinity in the protective range — three jobs, clean chemistry, nothing extra in the drum. Particularly effective in systems with long runs of small-diameter piping where high surface-area-to-volume ratios concentrate corrosion risk and make passivation film integrity the single most important variable in system longevity. Where CI WT No. 20 is the full-program compound for mixed-metal systems that need corrosion, scale, and oxygen management built into one product, No. 25 is for the operator whose water is already conditioned and whose system needs reliable nitrite passivation without carrying chemistry that doesn’t apply to the loop. Compatible with standard boiler water testing — monitor nitrite concentration, pH, alkalinity, and conductivity against control ranges, adjust dosing through normal blowdown procedures, and keep the system protected without adding complexity. Used properly, it helps prevent the internal corrosion that quietly shortens boiler life, reduces heat transfer, and turns stable loops into maintenance problems. Simpler system, simpler treatment, fewer variables — and the corrosion protection still holds.