Pool heater will not start; display shows F1 shortly after a call for heat.

Safety first: Turn off electrical supply to the heater at the isolator and switch off the circulation pump before opening any equipment. Do not run the heater without adequate water flow; heat exchanger damage and scalding risk are possible.

• Check pool circulation: Ensure the pool pump is running and that all isolation valves on the suction and return lines are fully open.
• Clean pump strainer: Open the pump strainer lid, remove debris from the basket, and re-prime the pump if necessary.
• Backwash or clean filter: If you have a Certikin sand filter, perform a full backwash and rinse. For cartridge filters, remove and hose down the cartridge thoroughly.
• Inspect flow switch: Locate the flow switch/pressure switch on the Certikin heater water manifold. Check for obvious leaks or damaged wiring. Gently tap the body to free a stuck paddle (if paddle type). Do not bend or force the paddle.
• Reset and test: Restore power, start the pump, bleed air from the system if needed, then call for heat and see if the F1 code clears.

If F1 persists with good flow and clean filtration, the flow switch or pressure switch circuit may be faulty and should be tested and replaced by a qualified technician.