Pentair FNS Plus DE Filter Guide: Cleaning, Maintenance, and Troubleshooting

The Pentair FNS Plus is a vertical grid diatomaceous earth (D.E.) filter available in four sizes (24, 36, 48, and 60 square feet). D.E. filters use a powder coating on fabric grids to achieve the finest filtration of any filter type. This guide covers DE amounts, how to coat the filter, backwashing, manually cleaning the grid assembly and individual grids, acid soaking, and troubleshooting.

Filter Overview

The FNS Plus uses fabric grids pre-coated with diatomaceous earth as the filter medium. The filter must never be operated for more than 3 minutes without a D.E. coating — doing so will damage the filter elements. The maximum working pressure is 50 psi. The pressure gauge (P/N 53003201) is the primary indicator of filter condition.

Note: A 1-lb coffee can filled level (not packed) with D.E. equals approximately 1/2 pound by weight. Use only D.E. labeled for swimming pool and spa use (typically 34-micron median particle size).

Coating the Filter Elements with D.E.

The filter elements must be pre-coated with D.E. each time the filter is cleaned or the system is started fresh. To coat:

1. For a Push-Pull valve: push the handle down and lock in place (filter position). For a Multiport valve: position to FILTER or VACUUM TO POOL.
2. Open the Manual Air Relief Valve.
3. Mix the required amount of D.E. with water in a bucket until the consistency of milk.
4. Follow System Startup Instructions (below) to start the pump.
5. With the pump running and the pool skimmer valve open, pour the D.E. slurry directly into the skimmer. The pump will draw it through the filter.

Record the original starting pressure after coating. Backwash when pressure reads 10–12 psi above the original starting pressure.

After backwashing: If the starting pressure after backwashing reads 4–5 psi higher than the normal starting pressure, the filter elements need to be removed and manually cleaned.

System Startup

1. Open the Manual Air Relief Valve: turn counterclockwise until it snaps into the full open position (1/4 turn).
2. Stand clear of the filter tank, then start the pump.
3. Close the air relief valve after a steady stream of water appears.
4. If a solid stream of water does not appear within 30 seconds after the pump basket fills, or if the pressure gauge indicates pressure before water outflow, shut off the pump immediately, relieve pressure, and clean the air relief valve. Call (800) 831-7133 if the problem persists.

Manually Cleaning the Filter (Entire Element)

1. Turn off the pump and shut off any automatic controls.
2. Set the skimmer trimmer valve to 100% skimmer (closes main drain), or close the main drain valve.
3. Open the filter air relief valve and, if your system has one, the waste drain valve or plug.
4. Remove and clean the pump strainer basket. Reinstall and secure the lid.
5. Release the tank clamp assembly and remove the tank lid. Lift out the complete element assembly for washing. Do not lose or damage the internal air relief located on top of the manifold.
6. Using a garden hose, thoroughly flush all contaminated D.E. from the filter element surfaces.
7. Coat the standpipe O-ring generously with a silicone-based lubricant before replacing the element assembly. Seat it firmly in place.
8. Ensure the internal air bleed assembly is in place and free of D.E. or debris (see Cleaning the Internal Air Bleed Tube below).
9. Ensure the tank O-ring is in position on the lower tank half. Place the filter lid over the lower tank half.
10. Follow the Filter Clamp Installation instructions below.
11. Reset skimmer valve, close the waste drain, and coat the elements with D.E. per the procedure above.

Filter Clamp Installation

1. Turn off the pump and open the air relief valve. Wait until all pressure is fully relieved.
2. Ensure the O-ring is clean and seated on the lower tank half. Place the filter lid onto the lower tank half.
3. Hold the clamp ends apart and position the clamp over both upper and lower tank flanges. Bring the ends together.
4. Insert the T-bolt. Place small washer on T-bolt, then larger washer and spring on barrel nut, then place onto T-bolt. Hand tighten.
5. Using a 7/8-inch wrench, tighten the nut. Tap around the clamp with a rubber mallet (or similar tool) multiple times around the full circumference to fit the clamp. Continue tightening only until the spring coils touch.
6. The clamp is correctly installed only when spring coils remain touching after tapping.
7. Check spring coil tension at least once a month. If coils do not touch, turn system off, tighten and tap until coils touch, then restart.

Cleaning Individual Grids (Element Disassembly)

When the element is removed from the tank, stand it on the deck exactly as it came out of the filter, with the manifold on top.

Disassembly

1. Remove plastic standoffs from tie rod ends, if equipped.
2. Using a 1/2-inch wrench or deep socket, remove both nuts from the manifold tie rods.
3. Remove the top manifold by holding the lift handles and pulling straight up.
4. Gently remove the plastic template from the elements.
5. Remove elements one at a time from the bottom spreader.

Cleaning with Water

Using a garden hose, thoroughly flush all contaminated D.E. from the element surfaces.

Acid Soaking (for Mineral Deposits / Liming Up)

Mineral deposits (calcium, magnesium) cause the grid fabric to stiffen — this is called "liming up." To remove:

1. Soak elements in a solution of 6 parts water to 1 part hydrochloric acid (muriatic acid) in a plastic tub.
2. Soak for at least 4 hours.
3. Rinse elements thoroughly with tap water.

Reassembly

1. Starting with the "SMALL GRID" element, place it on the bottom spreader in the position marked "SMALL GRID," using the footprint pattern as a guide.
2. Continue placing remaining elements onto the spreader in the same manner.
3. After all elements are positioned, place the plastic template over the element spouts to hold them correctly.
4. Position the top manifold over the elements with the standpipe inlet port pointing toward the small element. Push down firmly to seat it on the elements. Ensure the template is not caught between the manifold and element spouts.
5. Replace the two nuts and plastic standoffs (if equipped).
6. Replace the completed element assembly into the filter tank.

Cleaning the Internal Air Bleed Tube

Inspect and clean the air bleed tube and screen cap routinely — this can be done while the filter lid is off for maintenance.

1. Remove the screen cap from the air bleed tube.
2. Rinse both the screen cap and tube with water to remove built-up debris. This is typically all that is needed.

Cleaning the Manual Air Relief Valve

1. Turn off the pump and automatic controls. Disconnect power at the circuit breaker.
2. Open the air relief valve (1/4 turn CCW) and wait until all pressure is relieved.
3. With the valve still attached to the filter tank, pull out the locking tabs and remove the valve stem and cover assembly with a counterclockwise and lifting motion.
4. Clean debris from the valve stem and body. Insert a 5/16-inch drill bit through the valve body to ensure the filter tank air passage is open.
5. Check that the O-rings are in good condition, properly positioned, and lubricated with a silicone-based lubricant.
6. Reinstall the valve stem and cover with a downward and clockwise motion until it snaps into position.

Troubleshooting

Replacement Part Numbers