A Rinnai tankless water heater cycling between hot and cold water 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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