Under normal operating conditions, a healthy tankless water heater should maintain a stable water temperature with minimal fluctuation of no more than plus or minus 2°F to 3°F. Modern tankless units utilize sophisticated electronic thermistors and modulating gas control valves to adjust burner output dynamically in response to flow rate changes. If your water temperature fluctuates wildly by 10°F to 20°F or more during a shower, it indicates an underlying flow sensor, scale accumulation, or burner modulation malfunction. Verification: Measure output temperature with a digital thermometer at the tap during steady flow.
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