Water failing to stay consistently hot with a tankless water heater is usually caused by exceeding the unit's maximum Gallons Per Minute (GPM) flow capacity, heavy lime scale accumulation insulating the heat exchanger, or a failing water flow sensor. When multiple high-demand fixtures run simultaneously, the incoming cold water volume overwhelms the burner heating capacity. Verification: Turn off secondary hot water fixtures and test if the primary shower or faucet returns to a stable, hot temperature.
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