Fluctuating water temperatures in a tankless water heater are typically caused by water flow dropping below the minimum activation threshold, simultaneous multi-fixture hot water usage, or heavy lime scale accumulation on the heat exchanger. When water flow drops too low, burners cycle on and off repeatedly. Additionally, hard water scale buildup triggers internal high-temperature safety limit switches, causing the unit to shut down burners temporarily to prevent overheating. Verification: Maintain a steady, moderate water flow rate at your tap and test temperature stability.
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