correct gasket for FNS Plus tank

satpoolguy

Active member
Sep 16, 2014
36
San Antonio, TX
I have found conflicting info on the web, so I am hoping someone here knows for sure -- should the gasket that seals between the top and the bottom of the filter tank on a Pentair FNS Plus be trapezoidal shaped, or round? I've found people selling supposedly the same Pentair part number, with some claiming it's round and others claiming it's trapezoidal.
 

HermanTX

Gold Supporter
May 20, 2020
772
Katy TX
Mine is trapezoidal. I would only buy a OEM Pentair replacement for this seal as it is very critical to a good seal.
One of the few items I purchase at pool stores.
 

JamesW

TFP Expert
Mar 2, 2011
21,091
The original one was trapezoidal, but they changed to a round design.

I'm not sure when the change happened. Possibly in 1992.

How old is your system?

Do you have a picture of the filter and the label?

I think that the fns plus uses the round o-ring.

I think that the original fns filter used a trapezoidal and switched to a round.

Get the round. Do you know for sure that you need a new o-ring?
 

JamesW

TFP Expert
Mar 2, 2011
21,091
I think that the "Plus" model uses the round O-ring part number 39010200.

The trapezoidal was used on the original FNS filter. The original would say Pac Fab, Purex, Nautilus or Triton on the label.
 

satpoolguy

Active member
Sep 16, 2014
36
San Antonio, TX
Thanks for the replies so far. I think you can probably see why I'm confused -- we already have conflicting info on this thread! My filter was installed in 2005. It is definitely an FNS Plus model, the 48 to be specific. I can get pictures of the filter and label later.

I "hope" I need a new o-ring. I have a (relatively minor) leak where the top and bottom of the filter meet, so if it's not the o-ring, it's going to be something much more costly.

Herman's says filter is an FNS Plus and is a similar age to mine, and he says his is trapezoidal.
 

HermanTX

Gold Supporter
May 20, 2020
772
Katy TX
I think that the "Plus" model uses the round O-ring part number 39010200.

The trapezoidal was used on the original FNS filter. The original would say Pac Fab, Purex, Nautilus or Triton on the label.
39010200 O-ring, tank clamp (.470 O.D.) is what I found in the manual for a new FNS Plus model.

I had an old (2003 model) that had a trapezoidal seal.
 

JamesW

TFP Expert
Mar 2, 2011
21,091
I had an old (2003 model) that had a trapezoidal seal.
Do you have a picture of the filter and o-ring?

I don't think that the "plus" ever had a trapezoidal oring.

The trapezoidal design is really old. It's not going to be on a 2003 model.

The filter that takes a trapezoidal oring has flat surfaces that match the flat parts of the oring.

The filter that takes the round O-ring has a rounded edge where the oring and filter top connect.
 

HermanTX

Gold Supporter
May 20, 2020
772
Katy TX
Thanks for the replies so far. I think you can probably see why I'm confused -- we already have conflicting info on this thread! My filter was installed in 2005. It is definitely an FNS Plus model, the 48 to be specific. I can get pictures of the filter and label later.

I "hope" I need a new o-ring. I have a (relatively minor) leak where the top and bottom of the filter meet, so if it's not the o-ring, it's going to be something much more costly.

Herman's says filter is an FNS Plus and is a similar age to mine, and he says his is trapezoidal.
@satpoolguy I want to clarify. My old FNS (2003 model) used a trapezoidal. I recently purchased a new FNS Plus (2020 model) and I have not taken it apart so I dug out the manual and it states oring - 0.47OD so sorry for confusion.

I use to have small leaks as well - it is all about tightened the band uniformly (just like you need to tighten your lug nuts on your wheel uniformly). All I did was lube it real good, move it around a little so it does not sit in the same place, ensure the support bracket is in place.
 

JamesW

TFP Expert
Mar 2, 2011
21,091
The FNS had a trapezoidal oring for a while.

The FNS Plus never had a trapezoidal oring as far as I know.

I'm pretty sure that the FNS Plus only had the round O-ring.
 

HermanTX

Gold Supporter
May 20, 2020
772
Katy TX
The FNS had a trapezoidal oring for a while.

The FNS Plus never had a trapezoidal oring as far as I know.

I'm pretty sure that the FNS Plus only had the round O-ring.
I would suggest this is correct. My original FNS definitely used a trapezoidal seal as I did many of filter clean outs. So it was probably not a PLUS model. My current filter (only several months old) is a FNS Plus and the manual calls out a O-ring with 0.47 diameter so that indicates a circle - not a trapezoid. Unfortunately I have not disassembled my filter yet so I cannot confirm it but I do believe it is an O-ring based on the description in the manual.
However, I do believe, regardless if o-ring or trapezoid that unless you cut it or clip a section or it is just cracked/hardened, it just needs to be lubed and repositioned with proper tighten of the ring to create a seal.
 

satpoolguy

Active member
Sep 16, 2014
36
San Antonio, TX
Thanks for all the discussion. It sounds like I need the round o-ring, so the one I have (trapezoidal) won't work.

As for the cause of the leak, I do believe that I have the band properly tightened. I've been maintaining this filter since it was installed, so I've opened it and reassembled it many times, and never had a leaking problem until recently. I already tried relubing it, with no change.

BTW, some of you may be wondering why I am asking what o-ring is right if I've serviced my filter so much. 1) I wasn't sure if both styles work, and 2) although it seemed unlikely, I wanted to make sure that the trapezoidal o-ring doesn't deform into a circle after installed.
 
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