When a tankless water heater produces only lukewarm water instead of fully hot water, the root cause is typically exceeding the unit's maximum flow rate capacity (Gallons Per Minute), an incorrect digital temperature controller setting, or heavy lime scale accumulation on the heat exchanger. If multiple hot water fixtures run simultaneously, the incoming cold water volume overwhelms the burner heating capacity. Verification: Reduce simultaneous hot water fixture usage and test output temperature at a single tap.
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