Hill Country minerals coat the flame rod with insulating scale
https://pub-a3d0921cc64d4b5c8a08eea958469665.r2.dev/tankless-water-heater-repair/what-358-mgl-hard-water-does-to-boerne-tankless-heaters.html
Hill Country minerals coat the flame rod with insulating scale, weakening microamp signals; the control board reads unstable flame, causing lockouts and intermittent hot water until cleaning or rod replacement.