PENTAIR

FNS® Plus 36

SKU: EC-180007

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Pool Filter Size Guide

Choosing the correct pool filter ensures clear water, proper flow, and longer equipment life. An undersized filter will clog quickly and reduce circulation, while an oversized filter improves filtration and reduces maintenance.

Filter Types Explained

Cartridge Filters

  • No backwashing required
  • Excellent filtration (10-20 microns)
  • Lower water usage
  • Cleaned manually

Best for: Most residential pools, water-conscious owners

Sand Filters

  • Easy to operate
  • Backwash to clean
  • Lower filtration (20-40 microns)

Best for: Simplicity, minimal hands-on maintenance

DE (Diatomaceous Earth) Filters

  • Finest filtration (2-5 microns)
  • Backwashing + DE recharge required
  • Highest water clarity

Best for: Maximum water clarity, experienced pool owners

Filter Size by Pool Volume

Rule of thumb: Bigger is always better with filters.

Cartridge Filters (Total Square Footage)

Pool Size (Gallons) Cartridge Filter Size
Up to 15,000 100 – 150 sq. ft.
15,000 – 25,000 150 – 300 sq. ft.
25,000 – 35,000 300 – 500 sq. ft.
35,000+ 500+ sq. ft.

✔ Larger cartridges = longer time between cleanings

Sand Filters

Pool Size (Gallons) Sand Filter Size
Up to 15,000 200 lb
15,000 – 25,000 300 lb
25,000 – 35,000 350 lb
35,000+ 500+ lb

DE Filters

Pool Size (Gallons) DE Filter Size
Up to 15,000 24 sq. ft.
15,000 – 25,000 36 sq. ft.
25,000 – 35,000 48 sq. ft.
35,000+ 60 – 72 sq. ft.

Match the Filter to Your Pump

Your filter must handle the maximum flow rate (GPM) of your pump.

Plumbing Size Typical Max Flow
1 1/2″ PVC ~45 GPM
2″ PVC ~65 GPM
2 1/2″ PVC 80+ GPM

⚠️ Running a pump that exceeds the filter’s flow rating can:

  • Damage internal components
  • Reduce filtration efficiency
  • Void manufacturer warranties

Why Oversizing a Filter Is Good

✔ Cleaner water
✔ Lower pressure
✔ Longer time between cleanings/backwashing
✔ Longer filter life
✔ Better compatibility with variable-speed pumps

A larger filter paired with a lower pump speed is the most efficient setup.

Special System Considerations

You may need a larger filter if your pool has:

  • Variable-speed pump running long hours
  • In-floor cleaning system
  • Heavy debris (trees, leaves)
  • Spa spillover
  • High swimmer load

Common Filter Sizing Mistakes

❌ Matching filter size to pump HP only
❌ Choosing the minimum recommended size
❌ Ignoring maximum flow rating
❌ Underestimating debris load

✅ Always size for flow rate and pool volume

Replacing an Existing Filter?

Check the filter label for:

  • Filter type
  • Square footage (cartridge / DE)
  • Tank diameter (sand)
  • Maximum flow rate (GPM)

If you’re unsure, contact us and we’ll help you choose the correct filter.

MORE SURFACE AREA FOR BETTER POOL FILTRATION AND LESS MAINTENANCE

FNS Plus Pool Filters are designed with curved grids to maximize filtration area. More filter area means better performance and longer periods between cleanings. The internal design has been refined to minimize flow restrictions and, therefore, requires minimal pumping power to achieve a needed flow rate. This vertical-grid, D.E. filter is made of fiberglass-reinforced polypropylene for superior strength and reliability.

  • Integrated Continuous High Flow™ Internal Air Relief*
  • Top-to-bottom backwash
  • High Flow Manual Air Relief Valve

PRODUCT COMPONENTS

  • Corrosion-resistant fiberglass tank for longer life
  • 2″ plumbing connections for maximum flow
  • Conveniently located 1″ drain for a more efficient cleanout and 100% draining

DELIVERS ON CLARITY

D.E. filters work extra hard to help trap dirt and debris particles resulting in a clean and clearer pool.

MAXIMIZED FILTRATION

FNS Plus filters have been designed with curved grids to maximize filtration area. More filter area means better performance and longer periods between cleanings. Our internal design has been refined to minimize flow restrictions and, therefore, requires minimal pumping power to achieve a needed flow rate to help reduce utility bill costs.

Additional information

Weight 70.00 lbs
Dimensions 22.50 × 22.50 × 34.50 in
Max Water Flow

Port Size

Filter Area

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