Heater will not start; display shows F01 shortly after a call for heat.

Safety first: Turn off power to the heater at the breaker before opening any panels. Do not run the heater without proper water flow; heat exchanger damage and scalding risk are possible.

• Check pump operation: Verify the pool pump is running at normal speed and primed. If using a variable-speed pump, temporarily set it to a higher RPM to increase flow.
• Inspect valves: Ensure all valves on the suction and return lines to the Thermeau heater are fully open. Confirm any bypass valve around the heater is not diverting most of the flow away from the unit.
• Clean filtration system: Backwash or clean the pool filter. A dirty sand, cartridge, or DE filter can reduce flow enough to trigger F01.
• Check skimmer and pump basket: Remove leaves and debris from skimmer baskets and the pump strainer basket.
• Inspect Thermeau flow switch: With power off, locate the flow switch in the heater’s water manifold. Check for scale or debris around the paddle or sensor. Clean gently and ensure wiring is firmly connected.
• Reset and test: Restore power, restart the pump, then reset the heater and call for heat. If F01 returns despite good flow, the flow switch or control board may need replacement.