Intermittent hot water operation from a tankless unit is frequently caused by water flow dropping below the minimum activation threshold, fluctuating water pressure, or scaling on the heat exchanger causing the unit to short-cycle on safety overheating limits. If your showerhead has a water-saving trickle valve, flow may drop too low for the heater to stay on. Verification: Remove flow-restrictive showerhead inserts and monitor temperature stability.
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