Display shows FL or FLO; unit will not start heating and may show a water flow icon or message.

Safety first: Turn off the heater before adjusting valves or opening any plumbing.

• Verify pump operation: Ensure the pool pump is running at normal speed and pressure. Check for air in the pump basket and prime if needed.
• Open all valves: Confirm suction and return valves to the heater are fully open. If there is a bypass, set it per AquaCal's instructions so adequate flow passes through the heater.
• Clean the filter: Backwash sand/DE filters or clean cartridge filters to restore normal flow.
• Inspect the flow switch: Locate the flow switch on the heater's water manifold. Check for kinks in the wiring and ensure the switch body is not cracked or leaking.
• Check for debris: With pump off and water isolated, remove any accessible strainer screens or unions at the heater inlet to clear debris that may be blocking flow.
• Replace flow switch if needed: If FL persists with strong flow at the returns, the flow switch may be stuck open. Replace with an AquaCal‑approved flow switch and ensure correct orientation (arrow in direction of flow).