Hot water turning cold during use on a tankless system is frequently caused by water flow dropping below the manufacturer's minimum activation threshold, the "cold water sandwich" effect, or burner short-cycling triggered by scale buildup. If water flow falls below the required GPM threshold, the unit automatically extinguishes the gas burners. Additionally, heavy mineral scale on the heat exchanger trips high-temperature safety limit switches. Verification: Maintain a steady, moderate water flow rate at your tap to prevent rapid burner deactivation.
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