You brush the floor, it clouds, it settles, and the grit is back tomorrow.
Everyone will tell you it's a cracked lateral. Usually it is. But two of the three causes Pentair prints in its own troubleshooting table are not that, and one of them costs nothing to fix.
Key takeaways
- Pentair's own troubleshooting lists three causes, and a broken lateral is only the first of them.
- Filter sand is No. 20 silica, roughly 0.40 to 0.55 mm depending on the maker, and gritty. DE is white dust.
- A new sand bed sheds fines. Hayward and Pentair both start a new filter with a two-minute backwash.
- Opening the filter is the only way to inspect laterals, and the sand has to come out to do it.
- Vent the tank before you open it and again before you restart. That is where these jobs hurt people.
First, make sure it's sand
Worth two minutes before you drain a filter you didn't need to open.
Filter sand is coarse enough to feel. The makers all specify the same grade — No. 20 silica, which Hayward gives as 0.45 to 0.55 mm and Jandy as 0.40 to 0.55 mm. That's grit that rolls between your fingers, not powder.
White dust is DE, not sand. If you have a DE filter, fine white powder in the pool is diatomaceous earth. Pentair's FNS Plus manual blames a torn element, a damaged air bleed cap screen or manifold, or elements that were put back in wrong. DE in the pool has its own causes and its own fix.
Grit with a rough patch on the wall is the surface. Plaster is cement mixed with white sands, and the National Plasterers Council says aggressive water leaches the calcium hydroxide out of it, leaving etched areas that show up as small spots around the aggregate. If the wall nearby feels like sandpaper where it used to feel smooth, you're picking up your own pool. That's a chemistry problem, and the saturation index is where it starts.
Sand that blew in during a storm is just sand. Look at where it sits: in my experience filter sand shows up as a fan downstream of the returns, while storm sand lands wherever the wind put it. That's a hint rather than a test, so use it alongside the checks below.
What the makers actually say
Pentair prints a row for it. In both the Triton side-mount manual and the Tagelus top-mount manual, the problem reads "sand entering pool through return lines" and the causes are a damaged filter lateral, a backwash flow rate too high, and a valve air strainer damaged or missing. Its Sand Dollar manual lists "sand returning to pool" with the first two.
Jandy puts it in a caution. Cracked or broken laterals "will cause sand to be discharged to the pool." It tells you to add media slowly until the laterals are covered, for exactly that reason.
Hayward publishes nothing on it. Its ProSeries manuals carry a three-problem troubleshooting list — low flow, short cycles, water won't clear — and this isn't on it. Its knowledge base has an article on DE leaking into a pool and no equivalent for sand.
So start with the cause you can check for free, which is the backwash rate, then the valve, and open the tank last.
The cracked lateral
Laterals are the underdrain. Hayward describes them as wagon-wheel spokes with tiny holes along the top edge, letting clean water back to the pool while the sand and dirt stay in the filter. Crack one and sand leaves with the water.
They break for ordinary reasons. Sand poured onto bare laterals lands hard, which is why Hayward, Pentair and Jandy all tell you to put water in the tank first. A tank left full through a hard freeze is the other one I see, which is why every winterizing section starts by draining it. Old ones give up on their own, too — brittle plastic that's spent years in chlorinated water.
The parts are specific to your filter, and nobody sells you one. Hayward's top-mount manual lists laterals only in packs of ten, in three part numbers by tank size, plus a folding lateral assembly with standpipe for each size if the whole underdrain is going. Pentair splits the Triton's by size and build date — the TR100 takes eight of part 152202, or eight quarter-turn 157105 laterals on filters built from November 2025. Jandy's SFTM22-1.5 takes a full set of eight, R0745300, while the SFTM24-2.0's laterals and hub are one part, R0746000. Work from your own filter's manual.
Each maker's catalog page lists them: Hayward's ProSeries top-mount and side-mount, Pentair's Triton II and Tagelus, and Jandy's SFTM.
Backwash flow that's too high
Pentair's fix for this one is a sentence: reduce the backwash flow rate. No parts, no tank opening.
It's the cause nobody mentions, and it follows a pump upgrade more often than anything else. Pentair doesn't explain the mechanism in its residential manuals — it lists the cause and moves on — so I won't invent one for it either. What's worth knowing is that it's on the list, it's free to test, and a variable-speed pump can simply be backwashed at a lower speed.
An oversized pump causes a separate fault. Hayward says a pump running above the filter's recommended flow can channel the sand bed, cutting small paths that let debris through unfiltered. That's debris getting past the filter, not sand leaving it, and it's still worth checking against what your filter is rated for.
The valve, and the pipe under it
The air strainer. Pentair's third cause reads "valve air strainer damaged or missing," and Pentair never explains the part in prose. Its Triton parts lists name P/N 150035 as a strainer on the filter's internal air relief tube, sold per model. Missing is a real possibility, because parts get left out after somebody services a filter.
The standpipe. On a top-mount, one pipe runs from the valve down to the lateral hub, and Hayward's instruction when you fit it is that the pipe be securely in place in the bottom underdrain hub. No maker lists a cracked or unseated standpipe as a cause, so this one is mine rather than theirs: the pipe is the only other thing standing between the sand bed and your returns, so it gets looked at while the tank is open.
Sand poured into the pipe during a refill. Hayward supplies a sand shield to cap the standpipe, Pentair a sand guide, and Jandy a clear stand pipe shield that it says should completely cover the opening. Skip it and you pour sand directly into the pipe that feeds your returns.
A worn valve seat gasket does something else. Hayward blames a leak out of the backwash line on a worn or loose valve seat gasket. It doesn't connect that gasket to sand in the pool, and neither does Pentair's or Jandy's troubleshooting. The rule that protects it is the one from backwashing: the pump goes off before the handle moves.
Fines after a sand change
New sand sheds fines for a few days, and that's expected rather than broken.
Hayward's startup procedure puts the valve in Backwash for at least two minutes, to flush impurities and fine sand particles out of the new media. Pentair's Tagelus manual adds that shipping can abrade the sand enough that the first backwash needs to run longer than usual — until the waste water is as clear as the pool.
One exception worth knowing: Hayward says to start in Filter rather than Backwash on a new concrete or gunite pool, or anywhere there's a lot of plaster dust, to keep that dust out of the underdrain laterals.
Vacuum what reached the pool to waste rather than through the filter. What you're watching for is whether it stops. Grit still arriving a week later isn't fines.
Wrong sand, or too much of it
Two self-inflicted versions of this problem.
The wrong media. Every maker specifies No. 20 silica pool filter sand, and Pentair adds a uniformity coefficient of 1.75 or less, which is a measure of how evenly graded the grains are. Answering owners on its own product pages, Hayward's support says it recommends only No. 20 silica pool sand for its filters, including when it was asked about glass media and about a 20/40 grade. Glass media is a genuine option on filters whose makers allow it, which the filter comparison covers — just don't assume yours does.
Too much sand. Pentair's Triton manual says the number to hold to isn't the weight at all. It's freeboard: the gap between the top of the bed and the top of the tank, 8.25 inches on a TR40 and 13.5 on a TR140. Sand density varies, so Pentair calls the pound figures reference only, while its Sand Dollar manual sends you to the filter's name plate instead. Hayward's top-mount manual asks for the sand to stay at least 10 inches from the top. Fill past that and the bed has nowhere to expand during a backwash. The sand change guide lists the amount and freeboard for each filter.
Jandy's brochure is worth knowing about here. The manual says .40–.55 mm, and the 2019 brochure drops the decimal point and prints 40-55mm, which would be gravel. Use the manual's number.
Getting it fixed
There's no way around the sand. The laterals are underneath all of it.
The sand change guide has the whole procedure, filter by filter: draining down to a slurry rather than dry, getting the old sand out, and refilling to the freeboard your manual gives. The parts that matter most for this repair are these.
Look at every lateral, not just the one you suspect. Cracks can be hairline. Pentair's remedy is to replace the damaged ones, but whether you can buy one is a catalog question — see the part numbers above before you order. Check that the standpipe is seated in the hub while you're down there.
Protect the new ones when the sand goes back in. Cap the standpipe, fill the tank a third to half full of water first, and pour slowly. Every maker's fill instructions say the water is there to protect the laterals.
Vent before you restart. Fully open the air relief valve and stand clear of the filter before the pump starts. Leave it open until a steady stream of water — not air, and not air and water — comes out. Never tighten a clamp on a pressurized tank. Then backwash, rinse, and go back to Filter.
Then watch the floor for a week. That's the only test that matters.
When to hand it over
Honestly, a lot of people stop at getting the sand out. It's a grim job, and a filter put back together wrong is dangerous rather than merely leaky.
If the tank itself is cracked, if the standpipe won't seat, or if you finish the job and sand still turns up, that's the point to call a service company. The filter cleaning guide covers what else is worth doing while the tank is open.
Go deeperThe valve rules that keep it healthyThe full backwash sequence, why the pump goes off before the handle moves, and how often to do it.Frequently asked questions
Why is my sand filter blowing sand into the pool?
Pentair's Triton and Tagelus manuals both carry a troubleshooting row for sand entering the pool through the return lines, and list three causes: a damaged filter lateral, a backwash flow rate that's too high, and a damaged or missing valve air strainer. Its Sand Dollar manual gives the first two. Jandy's manual warns that cracked or broken laterals will discharge sand to the pool. Hayward publishes no troubleshooting entry for it at all.
Can I fix a broken lateral without emptying the filter?
No. The laterals sit at the bottom of the tank under the whole sand bed, so the sand has to come out before you can see them. That's the real cost of the repair — not the parts, but a few hours of lifting out wet sand and the price of new sand to refill it. None of the three makers sells a single lateral either: Hayward's come in packs of ten, Jandy's as a set of eight, Pentair's by the piece with a count per filter.
Is a little sand in the pool after a sand change normal?
Yes. A new bed carries fine particles that wash through until they're gone. Hayward's startup procedure runs a new filter in Backwash for at least two minutes to flush those fines out, and Pentair's Tagelus manual notes that shipping can abrade the sand enough to need a longer backwash than usual. If grit keeps arriving a week later, that's not new-sand fines any more.
What kind of sand goes in a pool filter?
Every maker specifies No. 20 silica pool filter sand. Hayward gives 0.45 to 0.55 mm, Jandy gives 0.40 to 0.55 mm, and Pentair gives 0.018 to 0.022 inches with a uniformity coefficient of 1.75 or less. Answering owners on its own product pages, Hayward's support says it recommends only No. 20 silica pool sand for its filters, including when asked about glass media and a 20/40 grade.
Could it be something other than filter sand?
Often. Diatomaceous earth from a DE filter is a fine white powder, and Pentair lists a torn element, a damaged air bleed cap screen or manifold, or elements installed wrong as the causes. Grit that comes with a rough or pitted patch on the plaster is the surface breaking down, not the filter. Wind and storms deliver real sand too.
