Heater will not start; display shows E03 indicating pressure switch or flow issue.

Safety: Turn off power to the heater. Pump can remain on while checking flow, but do not open any pressurized plumbing.

Steps:

• Verify pump operation: Ensure the pool pump is running at normal speed and primed. Low-speed settings on variable-speed pumps may not provide enough flow for the heater.
• Clean filter and baskets: Backwash or clean the pool filter. Empty skimmer and pump baskets. Check for obstructions in the pump strainer.
• Check valves: Confirm that all valves in the heater’s inlet and outlet lines are fully open. Bypass valves should be set according to Lochinvar’s installation instructions.
• Inspect pressure switch tubing: Locate the water pressure switch on the water manifold. Inspect any small sensing tubes for kinks, cracks, or blockages. Clear debris carefully.
• Test pressure switch: With power off, disconnect the two wires from the pressure switch and temporarily jumper them together. Restore power and call for heat. If the heater now starts, the pressure switch is likely faulty and should be replaced. Remove the jumper immediately after testing and do not operate the heater long-term with the switch bypassed.

If proper flow is confirmed and E03 persists, have a technician check the pressure switch calibration and internal water passages.