The gauge on top of the filter reads 26. Nobody knows what it read when the cartridges were new, the water is a little hazy, and somebody at the pool store has already said the words “pool filter replacement” with a straight face. Most “bad” filters turn out to be a dirty filter, a tired cartridge or an $18 o-ring, and figuring out which is the first job on any pool equipment repair visit, well before anyone prices a new tank.

The tank itself rarely dies. What wears out is the stuff inside it (cartridges, sand, grids) and the seals holding it shut. So the real question is which cheaper thing is wrong, and whether it’s worth fixing on your pad.

Cartridge, sand or DE: what you have decides what fails

There are three filter types, and each one fails its own way.

Cartridge filters trap dirt in pleated fabric. You hose them off instead of backwashing. They fail slowly: pleats clog with oils and calcium, then fray, then tear.

Sand filters push water through a bed of No. 20 silica sand. Hayward’s ProSeries manual calls for grains of 0.45 to 0.55 mm. You backwash to clean them. They fail by clumping, channeling and broken laterals in the bottom of the tank.

DE filters coat fabric grids with diatomaceous earth powder, which does the actual filtering. They filter the finest, and they fail by grid tears, which you find out about when a gray cloud comes out of the return jets.

The pressure gauge rule every manual agrees on

Every manual we opened uses the same logic: note the pressure when the filter is clean, then act when it rises a set amount.

• Pentair’s Clean & Clear Plus manual: clean at about 10 psi above the “Original Starting Pressure,” or when you notice a big drop in flow.
• Jandy’s CS cartridge manual: clean when pressure “rises 10 psi from the operating pressure of a clean cartridge.”
• Hayward’s SwimClear manual: clean or replace when pressure rises 7 to 10 psi above starting pressure.
• Sand: Pentair says backwash at 10 psi over clean, Hayward’s ProSeries manual says 8 to 10.
• DE: Pentair’s FNS Plus manual says 10 to 12 psi over starting pressure, Hayward’s Pro-Grid manual says 8 to 10 over the pre-coat pressure.

The number that matters is the rise. A filter that runs at 12 clean and reads 22 is due. One that runs at 20 clean and reads 22 is fine. That’s why a gauge reading over the phone tells us almost nothing (and why “it says 20, is that bad?” has no answer without a second number).

Compare readings at the same pump speed, since a variable speed pump moves the needle. And if the gauge sits on zero with the pump running, it’s the gauge. INYO lists a Hayward replacement at $42.49.

How long does a pool filter cartridge last?

HomeGuide puts cartridge life at about 2,000 hours, or 2 to 3 years of regular use. Angi’s 2026 filter cost guide says replace every 1 to 3 years. A Ventura County pool runs all year, so lean toward the short end.

Hayward’s SwimClear manual gives the clearest replace rule we’ve seen. After a cleaning, if the starting pressure is more than 6 psi higher than it was with new cartridges, replace them the next time the gauge climbs to the cleaning point. In plain terms, when a clean cartridge doesn’t act clean anymore, it’s done.

Signs a cartridge is finished, even if the pressure looks OK:

• Torn or split pleats, or pleats that have gone flat and fuzzy.
• Cracked end caps, or a collapsed core in the middle.
• Fabric that stays gray or crusty after a proper soak.
• Short cycles, meaning you clean it and the pressure is back up in a week or two.
• Dirt blowing back into the pool from the returns.

Jandy’s troubleshooting table lists “hole in filter cartridge” and “worn o-ring seal inside filter” as the two causes of dirt returning to the pool. A torn pleat is a hole that lets everything through, so a pool that stays cloudy even with good chlorine is worth a close look at the cartridges.

How to clean a cartridge without wrecking it

Pentair’s Clean & Clear Plus cleaning guide says to hose each cartridge top to bottom with a straight nozzle held at a 45 degree angle. Hayward says flatly: “Do not pressure wash as it can damage the filter element.” A pressure washer is how a $200 cartridge becomes a $200 lesson.

When hosing isn’t enough, soak. Pentair lists three options for at least an hour: a commercial filter cleaner, 1 cup of trisodium phosphate in 5 gallons of water, or 1 cup of dishwasher detergent in 5 gallons. For calcium, Pentair’s guide gives a separate acid soak of one part muriatic acid to twenty parts water, for 10 minutes or until the bubbling stops.

Hard water makes the acid step likely here. California American Water’s 2025 report for Thousand Oaks lists total hardness at 140 to 164 mg/L depending on the source, and pool water gets harder as it evaporates. That’s the same calcium that builds up on calcium scale on your tile line, and it plates onto pleats the same way.

One local rule people miss. The City of Thousand Oaks says never let DE filter material into the street, gutter or storm drain, and the City of Ventura says to hose off filters over a lawn or planter so the water soaks in. Rinse on grass, not the driveway.

Sand filter problems: channeling, clumps and sand in the pool

Sand doesn’t last forever. Leslie’s, which sells #20 silica at $19.99 per 50 pound bag, says to replace pool filter sand every 5 to 8 years. Angi’s 2026 guide says 5 to 7 years and $50 to $150 for the job.

The failure to know about is channeling. Oils and calcium glue the sand into clumps, water finds a path around them, and it rushes through without being filtered. Orenda, the pool chemical maker, describes the signs: you backwash regularly but the pressure barely changes, you see ridges of raised sand around the inside edge of the tank, and the water goes cloudy. Low, steady pressure with cloudy water is not good news.

Sand in the pool is a different problem. Pentair’s sand filter manual lists two causes: a broken underdrain lateral, or a backwash rate that’s too high. Laterals are the slotted fingers at the bottom of the tank. Replacing one means digging out all the sand, so new sand goes in too, in the exact amount on the nameplate, per Pentair’s manual.

DE filters: grid tears and DE in the pool

A small puff of DE during recharge can be normal (Hayward’s Pro-Grid manual calls it a “pre-coat puff”). Powder settling on the floor in front of the returns is not.

Pentair’s FNS Plus manual says that when DE returns to the pool, “Inspect the elements for any tears or holes. Inspect the internal air bleed sock for tears and proper installation.” Translation: pull the grids, hold each one up to the light, and check the little sock on the air bleed.

Poolweb lists a single full grid for the 60 square foot Nautilus/FNS filter at $96.81, and a complete FNS Plus 60 grid assembly at $552.13. Leslie’s lists that same complete assembly at $639.99. One torn grid, replace the one. Several stiff, stained grids, buy the set.

O-rings, clamps and the lid: small parts, big consequences

A drip at the band clamp or a weep around the tank seam is usually the tank o-ring, not the tank. INYO lists Pentair’s Clean & Clear tank body o-ring (87300400) at $17.99. That’s the $18 fix in the first paragraph.

Lube matters. Pentair’s manuals say to use only PTFE or silicone based lubricant, because petroleum products will damage the equipment. Hayward says the opposite for the main tank seal on SwimClear and Pro-Grid filters: “Do not lubricate DEX2422Z2 seal.” Read your own manual before you grab the tube.

Clamps are where this stops being about water quality. Every filter manual we opened lists a 50 psi maximum working pressure, and Pentair’s install guide warns that “trapped air can launch the lid with deadly force.” Pentair’s clamp is right only when the spring coils stay touching. Hayward’s SwimClear manual says never rely on hand tightening the clamp nut, and to torque it to 150 inch-pounds with a 3/4 inch socket. And open the air relief valve before you open anything else.

Clamps cost more than people expect. ePool Supply has Hayward’s SwimClear and Pro-Grid clamp kit on sale at $399.95 (INYO lists it at $542.99), and Poolweb has Pentair’s clamp kit at $333.97. On an old, sun-bleached tank, clamp plus cartridges starts to argue for a new filter.

2026 prices: pool filter replacement vs. new media

Part prices below are 2026 retail listings from the sellers named. Installed figures are Angi’s 2026 national numbers, not ours.

By type, Angi puts installed cost at $500 to $1,200 for sand, $700 to $1,600 for cartridge and $1,200 to $2,500 for DE.

Look at the first and third rows together. At about $204 apiece, a set of four OEM cartridges runs about half the price of the whole filter. On a filter that’s still sound, that’s fine. On a tank with a weeping seam, a tired clamp and a faded lid, it’s the point where full pool filter replacement starts to make more sense than feeding the old one.

A clogged filter costs you elsewhere too. Poor flow starves the heater (a heater that keeps tripping often has a filter behind it), and unfiltered water is how hazy turns into green water.

Asked at the pad, every week

How often should I clean a cartridge filter? When the gauge hits about 10 psi over clean, not on a calendar. Pentair’s own support page says it depends on your season, your debris and your pool. A Santa Ana wind that fills the pool with leaves moves the gauge faster than any calendar.

Should I switch from DE to cartridge? Often, yes. There’s no powder to buy, dose or keep out of the storm drain, and you give up a little fine filtration in exchange.

My pressure dropped after I cleaned it but my water is still cloudy. Why? On a cartridge, look for a torn pleat or a bad o-ring. On sand, suspect channeling. Low pressure isn’t always good pressure.

Write down one number this week

Next time your filter is clean, write the gauge reading on the tank. That’s the single most useful thing you can do before anyone talks to you about pool filter replacement. If you’re on weekly pool service, your tech should already know that number. Ask them.

If the pressure won’t come down, DE or sand keeps showing up in the pool, or the tank is weeping at the seam, send us the details through the site: filter brand and model off the label, clean pressure if you have it, current pressure, and a photo of the cartridges or the tank. Pentair’s filter cleaning and safety guidance is worth a read in the meantime.

We service Pentair, Jandy, Zodiac, Hayward and Sta-Rite equipment. Call (805) 586-2055 and we’ll get a tech out to the pad.