Pentair FNS Plus: Air Entering the Filter System
The FNS Plus manual treats air entering the filter as a safety issue, not just an operational one. Trapped air inside a pressurized filter tank can build to dangerous levels and cause the lid to separate from the base with significant force. The manual lists this scenario under a specific warning: air entering the filter is dangerous. Before diagnosing the source of the air, the system must be shut down safely.
⚠ Air in the Filter Is a Safety Hazard
Never attempt to service the filter clamp, lid, or tank while the system is under pressure. Air trapped in the tank can cause explosive lid separation. Always shut off the pump, open the Manual Air Relief Valve, and wait for the pressure gauge to read zero before loosening any tank hardware.
Symptoms of Air in the System
- Air bubbles at pool return lines: Fine or large bubbles appearing at the return jets while the pump is running
- Low water level or air in pump hair and lint pot: Visible air pockets inside the transparent pump strainer lid
- Air discharged from Manual Air Relief Valve when opened while pump is running: When the relief valve is opened during operation and air — rather than a steady water stream — discharges, air is accumulating in the filter tank
- Low or erratic filter pressure: Air interrupts normal hydraulic pressure and produces unstable gauge readings
Five Sources of Air Entry (From the Manual)
Source 1: Low Pool Water Level
When the pool or spa water level drops below the skimmer throat, the skimmer begins drawing air instead of water. The pump transmits this air directly through the system and into the filter tank. This is the most common and most easily corrected air source.
Correction: Add water to the pool until the water level is at the midpoint of the skimmer opening. Check that the skimmer is pulling a steady water vortex with no gurgling.
Source 2: Clogged Skimmer Basket
A packed skimmer basket restricts water flow to the point where the pump partially starves and begins drawing air around the basket edges or through any available gap. Even a basket that is not completely full can introduce air if debris has matted across the basket opening.
Correction: Remove and empty the skimmer basket. Check the skimmer throat below the basket for leaves or debris. Reinstall and verify steady water flow.
Source 3: Split Suction Cleaner Hose
An automatic pool cleaner connected to the suction side of the system introduces air directly into the pump suction whenever the cleaner hose has a crack, hole, or split section. The pump pulls air through the hose defect rather than (or in addition to) water from the pool.
Correction: With the pump off, disconnect the automatic cleaner and inspect all hose sections individually. Run your hand along each section feeling for cracks. Flex each section — cracks that are invisible at rest open when the hose bends. Replace any damaged sections before reconnecting the cleaner.
Source 4: Leak in Pump Hair and Lint Pot Lid
The pump strainer pot lid is sealed with an O-ring. If the lid is cracked, the O-ring is worn, or the lid is not fully seated, the suction created by the pump draws air in through the lid joint. Air ingested here enters the filter tank with every pump cycle.
Correction: Shut off the pump. Inspect the lid for visible cracks. Remove the lid and inspect the O-ring — it should be smooth, pliable, and free of nicks or flat spots. Apply a thin coat of approved O-ring lubricant and ensure the O-ring is fully seated in its groove before reinstalling the lid.
Source 5: Leak in Pump Suction Line
Any suction-side plumbing connection that is not fully sealed will draw air under the negative pressure created by the pump. Common failure points include unions (the threaded collar may be loose), valve stems (packing gland seals dry out), and pipe joints at buried fittings that shift over time.
Correction: With the pump running, systematically check every accessible union, valve, and fitting on the suction line between the pool and the pump. Apply soapy water to each joint — bubbles being drawn in toward the fitting confirm an air leak. Tighten unions, replace valve packing, or re-glue loose fittings as needed.
Verifying the Fix
After correcting the identified air source, restart the system and observe:
- Open the Manual Air Relief Valve before starting the pump; close it when a steady, continuous water stream (not air or intermittent spurts) flows from the valve
- With the pump running, watch the pump strainer pot — no air bubbles should be visible in the water stream
- Check the return jets — no bubbles should appear after the initial startup purge
- Open the Manual Air Relief Valve briefly a second time to confirm no residual air remains in the tank