Jandy LXi FAULT-HIGH LIMIT: Overheating Troubleshooting

What FAULT-HIGH LIMIT Actually Means

The LXi has two temperature limit switches wired in series in the safety circuit. The 150°F switch is installed in the lower well of the header (marked with a red dot), and the 130°F switch is in the upper well at the header outlet (no dot). If either switch opens due to excessive water temperature, the safety circuit breaks and the gas valve closes immediately.

This is a critical safety feature. Without adequate water flow to absorb heat from the combustion process, the heat exchanger can overheat, potentially cracking headers or causing dangerous conditions. The limit switches prevent this by shutting down the burner before temperatures reach destructive levels.

Step-by-Step Troubleshooting

Owner-Level Checks

Verify water flow

1. Confirm the filter pump is running and fully primed. Check return jets for strong flow.
2. Check the filter pressure gauge. If pressure is significantly above your clean baseline, clean or backwash the filter. A dirty filter is the number one cause of high limit faults.
3. Ensure all suction and return valves are fully open. A partially closed valve restricts flow through the heater.

Check for obvious restrictions

1. Empty the skimmer basket and pump strainer basket.
2. Verify no winterizing plugs are still in place.
3. If the system has solar panels or water features, ensure those valves are not diverting flow away from the heater.

Tech-Level Checks

Perform a temperature rise test

1. Place a thermometer in the return header pipe. Record the pool water temperature with the heater off.
2. Start the heater and let it run for 5 minutes. Record the temperature again. The difference is the temperature rise.
3. For the LXi 250: target is 8–12°F rise. For the LXi 400: target is 14–21°F rise.
4. If the temperature rise exceeds the maximum, flow is too low. Investigate the cause.

Inspect the internal bypass

1. The LXi has an adjustable internal bypass on the right-hand side of the header with a screw adjustment.
2. Turn the screw counterclockwise to decrease temperature rise (increase flow through the exchanger), clockwise to increase temperature rise.
3. If temperature rise is too high and the bypass screw is already fully open (counterclockwise), the problem is external: inadequate system flow, undersized piping, or weak pump.

Test the limit switches directly

1. Check voltage at test points 8 and 9 on the wiring diagram. With pump running and water cool, you should see 20–28 VAC at both points.
2. If voltage is present at test point 8 but not at test point 9, one of the limit switches is open.
3. Access the switches through the limit switch plugs on the side of the heater where water pipes connect. Remove the plugs and gently extract the switches.
4. The 150°F switch (red dot) goes in the lower well. The 130°F switch (no dot) goes in the upper well. Verify correct placement.
5. Test each switch with a multimeter for continuity when cool. A switch that stays open at room temperature has failed and needs replacement.

Common Parts That Fix This Problem

• 150°F (65°C) high limit switch (red dot, lower well)
• 130°F (55°C) high limit switch (no dot, upper well)
• Internal bypass assembly or bypass adjustment screw
• Filter cartridge or DE grids if flow restriction is at the filter

Model-Specific Notes

• The LXi 400 produces significantly more heat than the 250, so it is more sensitive to flow restrictions. Even a partially dirty filter can push the 400 into high limit territory.
• Both models require a minimum of 30 GPM. Variable speed pumps must be programmed to run at adequate speed during heater calls.
• Over-firing due to incorrect gas orifices (e.g., supplying propane to a natural gas configured heater) can also trigger high limit. Verify the gas type matches the rating plate.
• If the heater is plumbed backwards (inlet and outlet reversed), the limit switches will trip because water flow through the heat exchanger is disrupted.

How to Prevent High Limit Faults

• Maintain clean filters. Check filter pressure monthly and clean when it rises 8–10 PSI above the clean baseline.
• Perform an annual temperature rise test during pre-season startup to confirm flow is adequate.
• Program variable speed pumps to run at sufficient speed during heater operation — do not rely on the lowest energy-saving speed.
• Inspect the internal bypass annually and adjust if needed.
• Never operate below 70°F water temperature to avoid condensation and corrosion that can restrict internal flow paths over time.