Display shows “FLO” or “FL” and the unit will not start the compressor; fan may run briefly or not at all.

Safety first: Turn off power to the Heat Siphon and the pool pump before opening any plumbing or electrical compartments.

• Verify pump operation: Ensure the pool pump is fully primed and running with a solid stream of water in the pump basket (no air cavitation).
• Open all valves: Confirm suction and return valves to and from the heater are fully open. Bypass any water features that may be stealing flow.
• Clean the filter: Backwash sand or DE filters, or clean cartridge filters thoroughly to restore flow.
• Bleed air from the system: Use the filter air relief valve to purge trapped air until a steady stream of water appears.
• Inspect the flow switch: Locate the flow switch in the heater’s water manifold. Check for scale or debris around the paddle or sensor. If accessible, gently clean and ensure it moves freely.
• Test operation: Restore power and run the pump. If FLO persists with good flow, the flow switch or its wiring may be faulty and should be replaced.