Heater will not start; display shows F01 shortly after a call for heat.

Safety first: Turn off power to the heater at the breaker and shut off the gas supply before opening any panels. Do not run the heater with no water flow; this can damage the heat exchanger.

• Check pool circulation: Ensure the pool pump is running at normal speed and is fully primed (no air in pump basket). Verify skimmer and pump baskets are clean.
• Inspect valves: Confirm all valves on the suction and return lines are fully open. Make sure any heater bypass valve is not diverting too much water away from the heater.
• Clean filter: If filter pressure is high, backwash or clean the cartridge/DE filter to restore normal flow.
• Check heater inlet: Remove the front service panel of the Nirvana pool heater and inspect the water inlet union and internal inlet screen (if equipped). Clean out debris, leaves, or scale.
• Test flow switch: Locate the Nirvana flow switch assembly on the water manifold. Inspect wiring for loose or corroded connections. With power off, disconnect the flow switch plug, check continuity with a multimeter while water is flowing (pump on). If the switch does not close under good flow, replace the flow switch.
• Reset and test: Restore power and gas, restart the pump, then call for heat. If F01 persists after confirming good flow, contact a qualified pool heater technician.