A tankless water heater randomly turning hot water cold is typically caused by water flow dropping below the unit's minimum activation threshold, flow sensor calibration errors, or scale buildup tripping high-limit safety switches. If a low-flow showerhead restricts water movement too much, the internal flow switch assumes water has been turned off and immediately shuts down the gas burners. Verification: Remove flow-restrictive showerhead inserts and monitor temperature consistency during extended showers.
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