Pentair FNS Plus DE Filter: Diatomaceous Earth Entering the Pool

A milky white cloud billowing out of the return jets is one of the most visible signs that something has gone wrong with a DE filter. When diatomaceous earth bypasses filtration and enters the pool, the water turns hazy and the DE coats the pool floor and walls. The FNS Plus manual identifies two direct causes: torn or damaged grid elements, and a damaged or improperly installed internal air bleed sock. Both require opening the filter for inspection.

What DE Returning to the Pool Looks Like

The symptom typically appears in one of two patterns:

• Milky or white cloud at startup: Right after the pump starts following a backwash or manual cleaning, a white puff appears at the return jets, then clears within 30–60 seconds. A brief initial cloud after startup is noted in the manual as normal for DE filters. If the cloud persists or recurs mid-cycle, there is a filtration bypass problem.
• Continuous DE return: White powder visible at returns throughout the filter run, or DE accumulating on the pool floor in a pattern matching the return jet locations. This indicates an ongoing bypass — the grids or bleed sock are compromised.

Step 1: Shut Down and Open the Filter

1. Turn off the pump at the circuit breaker or timer
2. Open the Manual Air Relief Valve on top of the filter tank — turn counterclockwise until it snaps to the full-open position
3. Wait for the pressure gauge to drop to zero and all air to finish venting
4. Release the tank clamp and carefully lift the lid
5. Grasp the manifold handle and lift the complete grid element assembly straight up and out of the tank
6. Set the element assembly on a flat surface for inspection

Step 2: Inspect the Grid Elements for Tears or Holes

The grid element assembly consists of multiple fabric-covered grids mounted on a central manifold. DE coats the outside of these grids during normal operation; water passes through the fabric into the manifold and exits through the return line. Any breach in the fabric allows DE to pass through with the water into the pool.

1. Hose down the entire element assembly with a garden hose to remove residual DE — this makes tears easier to see
2. Inspect every grid panel on all sides — run your hand across each surface feeling for soft spots, thin areas, holes, or rips
3. Hold individual grids up to a light source and look for pinholes or tears in the fabric
4. Check the grid edges and seams where fabric meets the plastic frame — edge tears are common
5. If any grid has a confirmed tear or hole, that grid must be replaced; if multiple grids are damaged, replacing the complete element assembly is recommended

Replacement grid P/N varies by FNS Plus model size. The FNS Plus 48 and 60 use 8-grid assemblies; the FNS Plus 36 uses 7 grids. Consult the parts diagram in the manual for the correct P/N for your model.

Step 3: Inspect the Internal Air Bleed Sock

The internal air bleed tube and sock is mounted on top of the manifold assembly. Its purpose is to allow trapped air inside the tank to escape through the system, and its sock prevents DE from bypassing the grid surfaces near the manifold top. A torn or dislodged sock creates a direct path for DE to enter the return water.

1. Locate the bleed tube at the top center of the manifold assembly
2. Inspect the sock for tears, holes, or deterioration of the fabric
3. Verify the sock is properly seated over the manifold tube — it must be firmly positioned and not shifted to one side
4. If the sock is torn, replace it before reinstalling the element
5. If the sock has simply shifted out of position, reseat it firmly and reinstall

Step 4: Reinstall and Re-Coat With DE

After completing repairs or replacements:

1. Lower the element assembly back into the tank, seating the manifold standpipe into the bottom fitting
2. Verify the assembly is fully seated and the O-ring on the manifold standpipe is in place
3. Reinstall the tank lid and tighten the clamp
4. Open the Manual Air Relief Valve before starting the pump
5. Start the pump and allow water to fill the tank; close the relief valve when a steady stream of water — not air — flows from the valve
6. Add the correct amount of DE through the skimmer with the pump running:

Record the new starting pressure after the DE charge is complete. This becomes the new baseline for the next service interval.

Related Guides

• FNS Plus: Pressure Remains High After Backwash
• FNS Plus: Air Entering the Filter System
• FNS Plus: Pressure Drops on the Gauge
• FNS Plus: Short Filter Runs