Jandy LXi NO FLOW: Water Pressure Switch Troubleshooting

What NO FLOW Actually Means

The LXi uses an internal water pressure switch to verify that enough water is flowing through the heat exchanger before it allows the burner to fire. The switch senses back pressure inside the heat exchanger. When the switch is open, the controller displays NO FLOW on the LCD. The heater will not attempt ignition in this state.

This is a critical safety feature. Firing the burner without adequate water flow would overheat the heat exchanger, potentially cracking the headers or triggering the high limit switches. The pressure switch must close before the ignition sequence begins.

Unlike some fault codes that require a manual reset, NO FLOW will auto-clear as soon as the pressure switch detects sufficient flow. If you restore flow and the heater fires normally, the problem was purely hydraulic.

Step-by-Step Troubleshooting

Owner-Level Checks

Verify pump operation and flow

1. Confirm the filter pump is running and fully primed. Look at return jets for strong flow.
2. Check that all suction and return valves are fully open. A partially closed valve after the heater is a common oversight.
3. Look at the filter pressure gauge. If pressure is significantly above your clean baseline, clean or backwash the filter.
4. Check the time clock settings — is the pump actually scheduled to run at this time?

Check for flow restrictions

1. Empty the skimmer basket and pump strainer basket.
2. Make sure no suction or return lines are blocked by debris, closed valves, or winterizing plugs left in place.
3. If the pool has a solar heating system or water features, verify those valves are not diverting flow away from the heater.

Tech-Level Checks

Test the pressure switch directly

1. With the pump running at full speed and adequate flow confirmed, check voltage at test points 4 and 5 on the wiring diagram.
2. You should see 20–28 VAC at test point 4 (violet wire from pressure switch to controller). If voltage is present at point 4 but not at point 5 (gray wire), the pressure switch contacts are open.
3. Disconnect the two wires from the switch and check continuity with a multimeter. With flow present, the switch should be closed (near zero ohms).
4. If the switch stays open even with strong flow, it may need adjustment or replacement.

Adjust or inspect the pressure switch

1. The LXi pressure switch is adjustable. Refer to Section 7.7 of the manual for the adjustment procedure.
2. The switch must be set so the heater will not fire unless the pump is running, but sensitive enough not to false-trip during normal operation.
3. Check the copper siphon loop tube for blockage. A clogged tube prevents the switch from sensing back pressure correctly.

Check for air locks or header blockage

1. Air trapped in the heat exchanger can prevent the pressure switch from closing. Open the drain plug briefly to bleed air.
2. On older units, inspect the heat exchanger headers for scale or debris buildup that restricts flow internally.

Common Parts That Fix This Problem

• Water pressure switch
• Copper siphon loop tube
• Internal bypass assembly
• Filter cartridge or DE grids if flow restriction is at the filter

Model-Specific Notes

• Both the LXi 250 and LXi 400 share the same 30 GPM minimum flow requirement and maximum of 125 GPM.
• Variable speed pumps at low RPM may not produce enough flow to satisfy the pressure switch. Program a dedicated heater speed into your automation.
• If using AquaLink RS automation, ensure the pump ramps up to adequate speed before the heater call is sent.
• FAULT-PUMP is a separate display that appears when the heater is in Maintain Temp mode and the pump is not running. This is normal and does not indicate a fault.

How to Prevent NO FLOW From Coming Back

• Maintain clean filters so flow stays above 30 GPM at all times during heater operation.
• Program variable speed pumps to run at an adequate speed during heater calls.
• Keep suction and return valves fully open when the heater is expected to run.
• Inspect the pressure switch and siphon tube annually during pre-season startup.