Pentair FNS Plus: Pressure Remains High After Backwash

A successful backwash on the FNS Plus should return the filter pressure to its original clean starting baseline. When pressure remains elevated after backwashing, the standard backwash cycle has not fully cleared the grids. The FNS Plus manual addresses this directly: first, backwash a second time; if pressure is still high, the grids have limed up and require manual cleaning or a muriatic acid soak.

What Is Liming-Up?

Liming-up is the FNS Plus manual's term for mineral fouling of the grid fabric. Magnesium and calcium deposits from pool water gradually stiffen and embed into the grid material. Unlike organic debris — which flushes out during backwash — mineral scale bonds chemically to the fabric and cannot be removed by water flow alone. A filter in this condition will show chronically elevated pressure that only partially responds to backwashing. The manual also notes that if starting pressure after backwash is 4–5 psi higher than the normal post-backwash baseline, the grids need manual cleaning.

Step 1: Backwash a Second Time

If pressure remains high after the first backwash cycle, the manual's first instruction is to backwash again immediately. Two consecutive backwash cycles sometimes dislodge debris loads that a single pass leaves behind.

1. Rotate the multiport valve to the BACKWASH position
2. Run for a full backwash cycle — typically until the sight glass (if equipped) runs clear, or approximately 2 minutes
3. Return the valve to FILTER
4. Re-coat the grids with the correct DE amount through the skimmer
5. Record the new starting pressure

If pressure is now at or near the normal baseline, the problem is resolved. If pressure is still elevated, proceed to manual cleaning.

Step 2: Manual Element Cleaning (Hosing Down)

Remove the grid element assembly from the tank and hose down all surfaces thoroughly.

1. Turn off the pump at the circuit breaker
2. Open the Manual Air Relief Valve and wait for the pressure gauge to reach zero
3. Release the tank clamp and remove the lid
4. Lift the complete element assembly out of the tank
5. Use a garden hose at moderate pressure to flush every grid surface — work top to bottom on all sides of each grid panel
6. Pay attention to the grid edges and the junction between fabric and plastic frame, where mineral scale concentrates
7. Reinstall the element, close the tank, and restart with a fresh DE charge

If pressure returns to baseline after hosing, the liming-up was mild. If pressure remains elevated, an acid soak is required.

Step 3: Muriatic Acid Soak

When hosing alone does not restore pressure, the FNS Plus manual prescribes a muriatic acid soak to dissolve calcium and magnesium deposits embedded in the grid fabric.

1. Prepare a soak solution in a plastic container large enough to submerge the element assembly: mix 6 parts water to 1 part muriatic acid (add acid to water)
2. Submerge the element assembly fully in the solution
3. Allow to soak for approximately 1 hour — you will see fizzing as the acid reacts with mineral deposits; this is normal
4. Remove the element and rinse thoroughly with clean water until all acid residue is gone
5. Allow the element to drain before reinstalling
6. Reinstall in the tank, charge with the correct DE amount, and record the new starting pressure

Step 4: Replace Grid Elements

If pressure remains high after a complete acid soak and thorough hosing, the grid fabric has been permanently compromised by mineral or chemical damage and cannot be recovered. Grid element replacement is required.

Replacement grid P/N varies by model:

• FNS Plus 24, 36, 48, 60 — consult the parts diagram in your manual for the correct element assembly P/N for your specific model; the number of grids varies (7, 8, or 9 depending on size)

The pressure gauge P/N is 53003201. If the gauge does not read zero when the system is off or appears damaged, replace it as part of this service.

DE Re-Coat Reference

Related Guides

• FNS Plus: Diatomaceous Earth Entering the Pool
• FNS Plus: Air Entering the Filter System
• FNS Plus: Pressure Drops on the Gauge
• FNS Plus: Short Filter Runs