FS
Heat Siphon
Water Flow Switch Circuit Fault
Display shows “FS” or “FS ERR”; unit will not heat or cycles off prematurely.
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Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
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Possible Causes
Loose or corroded flow switch wiring, Failed flow switch, Faulty control board input for flow sensor
How to Fix / Troubleshooting
Safety first: Shut off power at the breaker before accessing the control compartment.
- Inspect wiring: Open the electrical/control panel. Locate the flow switch harness on the main control board. Check for loose connectors, corrosion, or damaged wires. Reseat the connector firmly.
- Check continuity: With power off and wires disconnected, use a multimeter to check continuity across the flow switch terminals while the pump is running (simulated flow) and stopped. It should change state (open/closed) with flow.
- Clean or replace: If the switch does not change state or is intermittent, replace the flow switch assembly with a Heat Siphon–compatible part.
- Control board check: If the switch tests good but FS remains, the control board input may be defective. Contact a qualified technician to confirm and replace the board if needed.
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Repair Difficulty
Medium
3/5
Required Part
Water Flow Switch
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