Heater displays F17; unit may start but then shuts down when water temperature is near setpoint.

Safety first: Turn off power before opening panels. Avoid running the heater with known flow issues.

• Check temperature readings: Compare inlet and outlet temperatures on the display. A very high difference indicates low flow or internal restriction.
• Improve flow: Clean filters, open valves fully, and increase pump speed if using a variable-speed pump.
• Inspect for scale: If the heater is older or used with hard water, internal scale may be restricting flow. A professional descaling of the Thermeau heat exchanger may be required.
• Sensor verification: Ensure both inlet and outlet sensors are reading accurately; a bad sensor can falsely indicate a high differential.
• Board evaluation: If hydraulics and sensors are correct but F17 persists, the control board’s logic may be faulty and require replacement.