Experiencing temperature fluctuations where a tankless water heater cycles between cold and hot water is often caused by the "cold water sandwich" effect, low water flow rate causing burner cycling, or a failing flow control valve. When you turn off hot water briefly and turn it back on, residual cold water trapped in the pipes passes through before newly heated water arrives. Verification: Maintain a steady, moderate water flow rate at the tap to prevent rapid burner short-cycling.
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