Pentair FNS Plus Internal Air Bleed - purpose, replacement, modifications ?


Nov 3, 2014
Hi! Can any of you pros explain the purpose of the internal air bleed in Pentair FNS Plus filters, please? Mine is always clogged up with dirty DE and its metal netting is bent. What purpose does this device serve?
I've also seen several modifications to the Internal Air Bleed Assembly like the one in the attached picture. Is it "allowed" or "suggested"? I also inserted a picture of unmodified assembly for your reference.

Any input is greatly appreciated in advance.





TFP Expert
Platinum Supporter
May 23, 2015
Tucson, AZ
The metal screen is supposed get coated with DE and it’s ok if it’s crushed a little. It’s there to allow air inside the filter to find an easy pathway out of the filter body. If there was no internal air bleed and someone forgot to open the external air bleed when purging an empty filter, air could get trapped in the upper head space of the filter body and potentially cause the filter to explode. It’s a safety feature on the filter.

I don’t really see the point of the proposed modification other than it could allow more DE to potentially pass thru the filter and get into the returns. I would not make that modification but just replace any damaged screen with an OEM replacement part.