Pentair FNS Plus

The FNS Plus is Pentair's vertical grid D.E. filter: a tall tank with a clamp band around the middle and a pack of curved fabric grids inside. The grids get coated with diatomaceous earth, and the D.E. does the filtering. It comes in 24, 36, 48 and 60 square feet, all 21.5 inches across.

D.E. filters trap finer particles than sand or cartridge, and they ask more of you in return. Most of this page is about that: how much powder, when to backwash, and what to do when backwashing stops working.

Two flow ratings, and which one to size by

Pentair publishes two different flow figures for the same filter, and they are both correct.

• Residential, 2.5 GPM per square foot. Both owner's manuals use it: 60 GPM on the FNSP24, 90 on the 36, 120 on the 48 and 150 on the 60.
• Commercial maximum, 2.0 GPM per square foot. Pentair's brochure labels its column "Comm." and "maximum flow rate": 48, 72, 96 and 120 GPM. These are also the figures on Pentair's product page, without the label.

If your pump's real flow falls between the two numbers for a given size, the lower commercial figure is the conservative one to size by. Pump flow and filter area are worked through in the sizing guide, which uses the same 2 GPM figure for D.E.

How much D.E. it takes

The rule is 0.1 lb of D.E. per square foot of filter area:

• FNSP24 — 2.4 lb
• FNSP36 — 3.6 lb
• FNSP48 — 4.8 lb
• FNSP60 — 6.0 lb

Weigh it rather than counting scoops. The 2023 manual converts to one-pound coffee cans and says a level can holds half a pound of D.E. — but its own can counts only agree with that on the FNSP60. Six cans for the FNSP24 would be 3 lb, not 2.4.

Use D.E. labeled for pools and spas. Mix it with water in a bucket to a thin, milky slurry, and with the pump running and the skimmer valve open, pour it straight into the skimmer. Wear a NIOSH or MSHA approved dust mask and safety glasses when handling the dry powder — the manual puts that in a warning.

And one rule that protects the grids: never run the filter for more than 3 minutes without D.E. in it. Uncoated elements get damaged.

Backwash at about 10 psi, then recharge

Write down the gauge reading once the filter is freshly coated and running. The 2025 manual says to backwash when it reads about 10 psi above that starting pressure, or when flow drops noticeably. The 2023 manual gave 10 to 12 psi.

If you run a variable-speed pump, the starting pressure depends on speed, so record the pump RPM alongside it. A reading taken at a different speed is not comparable.

The backwash itself, per the manual:

1. Power off at the breaker, open the air relief valve, and wait for the gauge to read zero.
2. Open the skimmer line fully, close the main drain, and clean the pump basket.
3. Set the valve to Backwash, open the air relief valve, stand clear, and start the pump.
4. Close the air relief valve once a steady stream appears, and run until the waste line flows clear.
5. Shut off, set the valve back to Filter — the position the manual uses for normal running — and follow the startup steps.
6. Add a fresh charge of D.E. Every backwash removes the coating, so every backwash ends with a recharge.

Where you are allowed to send D.E. waste depends on local rules, and some installations use a separation tank to catch it. The manual defers to your local codes, and so should you.

When backwashing stops working

The manual gives a fixed order for a filter whose pressure stays high:

1. Backwash again. If it is still high after several,
2. pull the grids and hose them clean. If it is still high after that,
3. soak the elements. And only then,
4. replace them.

The 2023 manual adds a useful tell: if the starting pressure right after a backwash is 4 to 5 psi above your normal starting pressure, the grids need a manual clean rather than another backwash.

D.E. coming back into the pool is a different problem. The manual's causes are a torn grid, a damaged air bleed screen or manifold, or grids not seated vertically with the top manifold secured.

Pulling the grids

The procedure has a few details that matter:

• Relieve the pressure first, drain the tank through the drain plug, and hold the clamp halves together while taking the nut off.
• Never lift the lid by the air relief valve, and never pry a stuck lid with tools — shift it side to side by hand.
• Lift the grid assembly out by the handles on the top manifold, and clean out the air bleed screen cap while it is out.
• Hose the grids at about a 45-degree angle to get the old D.E. off.
• Going back together, the smallest grid goes in the position marked "SMALL GRID" on the locator.
• Do not overtighten the wing nuts. Pentair's warning is that it can damage the manifold or grids and let D.E. into the pool.

The clamp changed in August 2025

Filters built from August 2025 use a cast clamp, 198102. Its nut, 198098z, goes to 16 to 18 ft-lbs, and the clamp is right when the gap between its ends is ½ to ¾ in. Filters built before that use a spring clamp, 190003, and Pentair says the spring parts fit only those earlier filters.

Pentair now publishes two manuals for the filter because of it, and the part numbers moved with the clamp — lids, gauge, tie rod nut and air bleed all carry new numbers in the 2025 manual. Match parts to the filter you have, not to the model number alone. The table below labels which manual each number comes from.

Two things the 2025 manual does not do. It leaves out the FNSP24, which Pentair still lists, so the 24's parts come only from the 2023 manual. And its new lid numbers — 179296, 179258 and 179260 — are the same numbers Pentair's 2025 manual lists for the lids of the Clean & Clear Plus.

What the warranty covers

Two years, parts and labor, from a dealer, store or pool professional, with the filter professionally installed — which the manual requires. It runs from the installation date.

It gets shorter in three ways. Bought online, including every EC- number, it is one year parts only. Not professionally installed, one year parts only. And Pentair's consumables list names D.E. grids and o-rings, which get one year regardless. On a filter where the grids are what wear out, that last one is the term that matters.

Winterizing

In a mild climate with brief freezes, the manual says to keep the system running until the cold passes. Where freezing lasts, drain the filter, pull and clean the grids, and store them and the drain plug somewhere they will not freeze. If you blow out the lines with compressed air, open the air relief valve first.