Water return tapers off to nothing w/ FNS 48 DE filter

Inground 15k gallons pool.
Nautilus FNS 48 DE filter, with cellulose fiber

The problem, what I have observed and what I have done so far....

Pool runs every night from 8pm to 5am. The other day I walked out at midnight to find the pump running but no water was returning to the pool-reading was close to 30psi. Next day I did a back flush, 3x, and it was running ok-psi was 15. What I noticed is that after about 3 hours of operation return flow quantity starts to tapper off, pressure starts to trend up. If I let it continue running by midnight or one the return is just trickling back into the pool, next to nothing really-psi is about 25-30ish.

Pool used to run great even with a filter much dirtier than I found it this past spring. It used to take about a year for psi to build up from 10 to say 35-37ish, then I would clean the filter. Sometimes this last spring I noticed the psi climbed up faster than in previous years so I cleaned it. That is why when I opened it again this week it was still clean.

I attempted to vacuum the bottom, with selector set to waste. I hose-fill, then vacuum as I have always done annually. This time suction pressure was gone after a short while. It came back after a backwash. Did the same thing so I stopped that. I estimate it would take 4 more of this action to complete the vacuuming process. It used to be one go and finished.

I tried to put a bladder on a hose through the skimmer but it couldn't make the 90 angle turn, to go through.

IG 15k gal pool => PF=8

I've never worked with cellulose fiber replacing DE, but from what I've read, it is an inferior product marketed to people afraid of DE.

The actual risk from DE is quite low, but if it scares you, just don't breathe in, when you are dumping DE. Of course, if you are over 40, you can breathe in DE all you want, since something ELSE will kill you long before the DE does!

This whole pattern, of preying on people's chemical superstitions irritates the crap out of me. I know it is not just the pool industry that does so, but many products are marketed to fearful people who lack the understanding of practical statistics and toxicology needed to distinguish the significant risks from the trivial ones. And unfortunately, even most technically educated folk don't have a good enough practical understanding of chemistry, statistics, or toxicology to make those distinctions.
[ Rant over! ]

However, one problem with cellulose is that it DOES break down, and chlorine accelerates that deterioration. Reportedly, it can leave "a gummy mess" on your filter, that requires a full tear down to clean properly. I think this is the manual you need: fns_plus_owners_manual_english.pdf

I'd recommend a full cleaning as your first step. BTW, if the manual doesn't forbid it, I'd clean the grids with these 6 steps:

1. Power wash, with a hose and nozzle (NOT: a pressure washer!!!)
2. Soak in a solution of bleach + TSP (1 gallon of bleach + 1 lb of TSP + 4 gallons of water)
3. Rinse COMPLETELY.
4. Soak in acid (1 gallon of muriatic + 4 gallons of water)
5. Rinse COMPLETELY.
6. Replace grids in filter.

When you put the filter back into service, USE DE POWDER, at least initially.

Live algae -- even if it's barely visible -- RAPIDLY stops up DE filters. The same algae, once dead, behaves like normal dirt and doesn't stop up the filter the same way.

If the filter clogs again, you may have barely visible algae. Easy test? Assuming you have a chlorine pool, with the pump running, add 3 gallons of Walmart 10% pool bleach ( Pool Essentials Chlorinating Liquid ) via a skimmer. That's about 25 ppm of chlorine, and will kill any live algae on the filter, and should kill most live algae in the pool.[B] (Do NOT do this with cellulose: adding chlorine via the skimmer can mess up cellulose!)

Unfortunately, if you STILL have problems, you may need to replace the grids, or at least the fabric on them. It is possible to foul them in ways that cannot be cleaned effectively. On the giant commercial vacuum DE filters I've worked with, you could see when this happened, because the DE would not 'coat' the fouled areas properly. Unfortunately, I don't know any way to 'see inside' consumer pressure DE filters to detect this.

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(Note: post has been revised since the notification email you received.)