Display shows F05 and heater will not fire; may occur immediately on startup.

Safety first: Turn off power and allow the heater to cool completely before touching internal components. Overheating can cause scalding and equipment damage.

• Check water flow: Confirm the pool pump is running at adequate speed and that filters and baskets are clean. Low flow can cause localized overheating in the heat exchanger.
• Inspect valves and bypass: Ensure water is actually flowing through the Thermeau heater and not fully bypassed. A partially closed valve on the heater inlet or outlet can trigger F05.
• Reset high limit (if manual): Some Thermeau models use a manual-reset high limit switch. With power off and the unit cooled, locate the high limit on the water header and press the reset button if present.
• Check for scale buildup: Hard water can cause scale inside the heat exchanger, restricting flow and causing hot spots. Look for signs of reduced performance and unusual noises. Descaling should be done by a qualified technician.
• Inspect wiring: Verify the high limit sensor wiring harness is intact and firmly connected to the control board.