A tankless water heater delivering hot water initially before turning cold is usually caused by exceeding the maximum flow rate capacity, water flow dropping below the activation threshold, or burner short-cycling. If water flow falls below the manufacturer's required GPM minimum, the unit immediately extinguishes the gas burners. Additionally, heavy lime scale buildup on the internal heat exchanger trips high-temperature limit safety switches, shutting down heating operations mid-shower. Verification: Adjust water flow at the tap to a steady, moderate stream and monitor temperature output.
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