A tankless water heater temperature fluctuating wildly is usually triggered by burner short-cycling, a malfunctioning thermistor sensor, or restricted water flow caused by lime scale buildup. When mineral scale cakes the interior of the heat exchanger, localized overheating occurs rapidly, forcing safety sensors to shut down burners. Once the water cools slightly, burners reignite, creating a continuous loop of drastic temperature swings. Verification: Flush the heat exchanger with a descaling solution and test thermistor electrical resistance values.
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