Question about Pentair Fns Plus 48 sqft De Pool Filter (In Ground Pool)

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Ready to Burst?

I have a working 13 year old FNS Plus. Recent inpection by pool guy showed that the surface could be indented by a fingernail. He said that indicated the filter could burst as the material is breaking down. Filter gets the Florida sun ~4 hours/day. Is there any truth here or is he peddling a new filter? Or what are the indications of imminent failure, if any.

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  • Pentair Master
  • 1,448 Answers

Sunlight does weaken fiberglass you might be able to just buy the tank or shell a pool retailer should have a parts breakdown and availbility

Posted on Jul 12, 2014


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Posted on Jan 02, 2017

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SOURCE: I just dissambled my Pentair FNS Plus 48 filter

first: Low pressure is actually good. It means that every thing is clean and water is flowing freely. Three things can cause the filter to pass things thru. First is a rip or tear that you might have missed in a filter grid. Second is a cracked manifold.(The manifold is what the grids plug into) Third is the spider gasket may be worn or broken. (This is the gasket in the valve)

Posted on Mar 14, 2010

  • 310 Answers

SOURCE: Our fns plus pool filter

You may either replace the air relief valve, or plumb in a check valve so that the water cant backflow

Posted on Apr 05, 2011

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SOURCE: Pentair 48 FNS filter, all new grids and top still won't backwash....pool is cloudy....what's the trouble???


Posted on Jul 14, 2012

  • 64 Answers

SOURCE: Weak return, weak suction when vacuuming

It may be the hose going from the pool to the pump intake is partially clogged. I had the same problem,, removed the hose and shoved a broom handle through it to remove tree debris.

Posted on Jul 24, 2012

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I got a pool with a fns plus 60 DE filtering system thats depositing diatomaceous earth all over my pool.Took it apart cleaned the grids bought 2 new ones to replace the bad ones,put it back together...

The FNS PLUS was originally built by American Products under a different name and was absorbed into Pentair's line, recently replacing the PAC-FAB FNS (fiberglass nautilus) also absorbed by Pentair. The two filters have no common background.
The FNS PLUS has been cursed at by more pool techs than I can count. All I can say is practice helps but these are always a pain to get right. Make sure that the narrower of the elements is in the correct spot, the rest do not matter. Also check the manifold on all sides for cracks and make sure the air relief screen is set properly. I wish I had an easy solution for you, but this filter is just a pain, even for the guys that work on them every day.

Aug 23, 2013 | Pool & Spa


Changing from a Pentair FNS DE Filter To A Pentair FNS Plus DE Filter

If you are the proud owner of a Penatir FNS DE filter, it may be nearing the time when you will be upgrading to a new filter. Since your first filter has probably been a great performer, you will be considering keeping the same brand of filter.
Ironically, since Pentair is trying to do away with the FNS filter, you may find that the FNS Plus is actually more affordable, and with the minor changes that have been made, is a an even better filter.
So, how do you avoid costly plumbing expenditures in changing out this filter? The FNS actually has the bulkheads (the connections where the backwash valve union nuts) that sit 1/4" lower than the standard FNS filter. In order to avoid having to change the plumbing for the backwash valve (although it is always recommended you change the valve when you install a new filter), you need to bring the filter up that 1/4" to make the bulkhead fittings line up.
Pictured below is a filter and the two kinds of valves that are used with the filter. On the left, the two black "dots" on the front of the filter are the bulkheads. On the right, you will see the two connection ports that the valves use to connect to the filter. Either valve works with this filter (i.e. multiport (left valve) or standard push/pull (right valve).
When changing the filter you should need only a large pair of channel locks and some lube. The other important component is a 18" X 18" Ceramic tile. When you install the new filter, first lay down the ceramic tile. This will make up for that 1/4" difference on the bulkhead location and ensure that your filter lines up with your valve! Thats additional plumbing required.
Always remember, before you attempt to do any installation on your own, with anything you are unfamilar with, do your best to find the people who have been doing this for awhile. These little tips and tricks will make the job much easier, and save some aggravation!
The Ugly Pool Guy

on Dec 14, 2010 | Pool & Spa

1 Answer

Got a fns plus 60 filter grids all cleaned replaced bad ones what is the secret to putting it all put together right can't line the the grids on the bottom template so they match help!!!!!!!!!!!!!!

Make sure that the space in the spreader is aligned with the big hole in the manifold, and that the small grid is 2 spaces right of the big hole. I curse like a sailor almost every time I work on these (FNS Plus), even after 18 years in the business. Your local pool shop should be able to do this for you at no charge.

Aug 23, 2013 | Pentair FILTER TANK BOTTOM, 48 SQFT

1 Answer

Fns plus vertical grid de filter plumbing diagram from pump

the front is the inlet from pool the top is out and goes into the filter the filter out goes to the pool returns

Dec 30, 2012 | Pentair Fns Plus 48 sqft De Pool Filter...

1 Answer

Pentair sm2-PP3 multiport valve assembly

note position or take picture before taking apart then remove top screws take off top remove old gasket install new one careful some have to be cemented into place

Mar 18, 2012 | Pentair 112 Multiport Valve Kit for FNS...

1 Answer

The filter was backwashed the filter and tnen set on filter adding the powder and the powed er started coming out the return line? what is wrong/ I would like to know how do you clean the filter it was...

You probably have holes in your grids and will need to replace them. You can find a manual at
Just follow the menu's to get to filters and manual's be sure to have your model number.

Sep 05, 2010 | Pentair Fns Plus 48 sqft De Pool Filter...

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