Display shows 'HLS' and heater shuts down or will not fire.

Safety first: Turn off power and gas to the heater. Allow the unit to cool completely before touching internal components.

Steps to address HLS error:

• Reset condition: Some high limits are automatic reset; others are manual. If manual, press the red reset button on the high limit switch after the heater has cooled.
• Check water flow: Ensure pool pump is running at full prime. Clean skimmer and pump baskets, and backwash or clean the filter. Low flow can cause localized overheating.
• Inspect bypass and valves: Verify that any external bypass valves are correctly adjusted and that isolation valves to/from the heater are fully open.
• Inspect high limit wiring: Locate the high limit switches on the header (often two: primary and secondary). Check for loose spade connectors or corroded terminals. Reseat connections.
• Test high limit switches: With power OFF, disconnect one lead and check continuity across each switch at ambient temperature. An open switch at normal temperature indicates a failed high limit that must be replaced.
• Consider scale or internal blockage: If HLS recurs despite good flow, the heat exchanger may be scaled or partially blocked, causing hot spots. A professional descaling or heat exchanger inspection is recommended.

Warning: Do not bypass or jumper high limit switches for operation; this is unsafe and can cause severe damage or fire.