Pentair FNS Plus DE Filter Blowing DE Back Into the Pool? Causes and Fixes (Model 180008)
You backwash, recharge with fresh DE, and within an hour there's a chalky cloud coming out of the returns. Or worse, a white film settling all over the pool floor. If that sounds like your setup, the Pentair 180008 is telling you something inside the tank has failed.
DE blowing back into the pool is one of the most common calls we get on the FNS Plus 48. The fix is almost always mechanical, and you can usually find it yourself in an afternoon.
Why is DE coming back into my pool?
DE powder passes through when it finds a hole. Something in the filter that's supposed to hold the powder on the outside of the grids has torn, split, or come loose, so the powder goes straight through to the return line.
On a Pentair 180008 the usual suspects are:
- A torn grid fabric. Even a small split will pass a surprising amount of powder.
- A cracked manifold on top of the grid assembly.
- A missing or damaged air bleed screen or standpipe o-ring.
- Grids that were reassembled out of position after a cleaning, so they don't seat against the manifold.
- Adding DE too fast, before the pump has fully primed and flow is steady.
That last one catches people. If you dump powder into the skimmer while the pump is still burping air, some of it will get past the grids no matter how good the filter is. Always let the pump run clean for a minute first, then add DE slowly.
How do I find the bad grid?
Open the tank and look at each grid in daylight. Tears are usually obvious once the grid is rinsed clean, and they show up most often along the seams and at the bottom edge where the fabric flexes.
Here's the order that saves time:
- Shut the pump off and open the air relief valve before touching the clamp. There should be zero pressure on the gauge.
- Pull the clamp band, lift the lid, and take the grid assembly out as one piece if you can.
- Hose the grids down completely. DE hides damage, so they need to be clean to inspect.
- Run your hand along each seam. Frayed threads and soft spots mean that grid is done, even without a visible hole.
- Check the top manifold for hairline cracks, especially around the ports.
- Inspect the standpipe o-ring and the tank o-ring while you're in there.
If everything looks fine and DE is still getting through, the manifold is usually the culprit. Cracks there are easy to miss because they hide under the grid tops.
Should I replace one grid or the whole set?
Replace the full set. Grids in a Pentair 180008 age at the same rate, so if one has torn, the others are close behind, and mixing a new grid with worn ones puts uneven load on the new one.
The exception is physical damage. If you dropped a grid or something punctured it while the rest of the set is only a year or two old, a single replacement is fine. Otherwise, do them all at once and you're set for years.
How much DE does the FNS Plus 48 need?
The standard rule for DE filters is about one pound of DE per 10 square feet of grid area. The 180008 is a 48 square foot filter, so you're in the neighborhood of five pounds after a full teardown and cleaning.
After a routine backwash, use roughly 80 percent of that amount, since some of the old coat stays behind. Under-charging is what wears grids out early. Bare fabric with no powder on it takes the dirt directly, and that's how tears start.
Repair the filter or replace it?
Replace the whole Pentair 180008 if the tank itself is cracked, the clamp band is corroded through, or the pressure gauge reads high right after a fresh cleaning and never comes down. Those are signs the filter is at the end of its life.
Repair it if the problem is grids, o-rings, a manifold, or an air relief valve. Those are wear parts on a filter body that easily lasts 10 to 15 years. A full grid set costs a fraction of a new filter, and swapping grids is a basic hand-tool job.
Keeping DE where it belongs
Break the filter down and hand clean the grids at least once a season, twice if your pool sees heavy use. Backwashing alone doesn't remove the oils and fine debris that blind the fabric.
Watch your pressure gauge too. When it climbs 8 to 10 psi over your clean starting pressure, the filter needs attention. Running it dirty is what pushes grids past their limit and puts DE back in your pool.
Not sure this is the right part for your problem? Reach out and we will help you figure out exactly what you need before you spend money on the wrong thing.