Raypak controller shows FS or FLOW SW error; unit will not start heating even though the pump is running.

Safety first: Shut off power at the breaker before touching wiring or opening the Raypak cabinet.

• Bleed air from system: If the system was recently opened, air may be trapped. Open any manual air bleeders on the filter or heater manifold until water flows steadily.
• Inspect plumbing orientation: Confirm that water is entering and leaving the Raypak unit according to the arrows on the manifold. Reverse plumbing can prevent the flow switch from closing.
• Check wiring: Locate the flow switch on the water manifold and trace its wires back to the control board. Reseat the connector and look for damaged insulation or corrosion. Repair or replace damaged wiring.
• Test the switch (advanced DIY): With power off, disconnect the flow switch and check continuity across the switch terminals while water is flowing (you may need a helper to run the pump). If the switch does not close with good flow, replace it.
• Do not bypass permanently: Never leave the flow switch jumped out; it is a safety device to prevent overheating.