Your filter gauge is climbing fast, the pressure is near zero, the tank is leaking, or sand or DE is showing up on the pool floor. Most pool filter problems come from a dirty filter, a clog or air leak before the filter, a worn O-ring or gasket, or a broken part inside the tank. This page covers sand, cartridge and DE filters: how they work, which parts fail, when to backwash or clean, and when to call a pro.
How does a pool filter work?
- Water enters under pressure. The pump forces pool water into the filter tank.
- The media traps dirt. Sand catches dirt between its grains, and a cartridge catches it in fabric pleats. A DE filter uses DE (diatomaceous earth, a fine white powder) coated onto fabric grids; Hayward says DE filters trap particles down to 1 to 3 microns.
- Clean water leaves at the bottom. In a sand filter, laterals (slotted plastic fingers at the bottom of the tank) let water out and keep the sand in.
- Pressure rises as dirt builds up. The gauge reads in psi (pounds per square inch). As the filter loads up, pressure climbs and flow at the jets drops.
- Backwashing or cleaning resets it. To backwash, the valve reverses the flow and sends dirt out the waste line. Sand and DE filters backwash; a cartridge comes out and gets hosed off.
- The air relief valve lets trapped air out. Air collects at the top of the tank, and this small valve releases it.
Which pool filter parts fail, and what are the signs?
| Part | What it does | Sign it is failing |
|---|---|---|
| Pressure gauge | Shows how loaded the filter is | Needle stuck, reads zero with the pump running, or a fogged face |
| Multiport valve gasket | Routes water to Filter, Backwash, Rinse or Waste | Water runs out the backwash line while on Filter; Hayward points to a worn valve seat gasket |
| Tank O-ring and clamp | Seal the tank halves or the lid | Leak at the clamp or lid, often right after cleaning; Pentair lists a pinched O-ring first |
| Sand bed | Traps dirt in a sand filter | Short runs between backwashes, pressure stays high after backwashing, a crust on the sand |
| Laterals and standpipe | Hold the sand inside the tank | Sand on the pool floor near the return jets |
| Cartridge | Pleated fabric that traps dirt | Pressure stays high even after cleaning; torn or flattened pleats |
| DE grids | Fabric frames that hold the DE coat | DE puffs back into the pool; Hayward lists torn grid fabric and too much DE |
What should I check first when my filter pressure is too high or too low?
- Your clean pressure. Write the gauge reading on the tank right after a backwash or cleaning, and use it as your baseline.
- Baskets and water level. Low pressure with weak flow usually means the problem is before the filter. Hayward lists dirty baskets, blocked lines and air leaks (bubbles at the return jets) for low flow.
- Valve position. The multiport handle must sit fully on Filter, not between settings. Hayward says always shut the pump off before you move it.
- Air in the tank. With the pump running, open the air relief valve until a steady stream of water comes out.
- Clean or backwash. High pressure with weak flow means a dirty filter.
- Water chemistry. Hayward lists algae as a cause of short filter cycles, so test and balance the water. See our pool chemistry guide.
- Where the leak is. A drip at a union usually needs a new O-ring. A leak at the clamp points to the tank O-ring. Water from the backwash line on Filter points to the valve gasket.
Safety first. Turn the pump off at the breaker and open the air relief valve until the gauge reads zero before you loosen any clamp or lid. Pentair warns that trapped air can make the filter lid separate with deadly force, and never to tighten a clamp while the system is under pressure. Never pressure-test above the 50 psi limit, and stop the pump before you turn a multiport handle. If you acid soak a cartridge, wear gloves and eye protection, pour the acid into the water, and never mix acid with chlorine or other pool chemicals.
Pool filter numbers to know
These numbers come from the makers’ manuals for the Hayward ProSeries sand filter, the Pentair Clean and Clear Plus cartridge filter and the Hayward Pro-Grid DE filter. Check your own filter’s label and manual, because models differ.
| Spec | Sand (Hayward ProSeries) | Cartridge (Pentair Clean and Clear Plus) | DE (Hayward Pro-Grid) |
|---|---|---|---|
| When to clean | 8 to 10 psi above the clean starting pressure | About 10 psi above the original starting pressure, or a big drop in flow | 8 to 10 psi above the pressure right after adding DE |
| Maximum working pressure | 50 psi | 50 psi | 50 psi |
| How to clean | Backwash about 2 minutes or less, until the sight glass runs clear | No backwash; hose each cartridge at a 45-degree angle, top to bottom | Backwash about 2 minutes, until the waste water looks clean |
| After backwash | Rinse about 1/2 to 1 minute, then back to Filter | Does not apply | Rinse about 20 seconds, then add fresh DE |
| Media | Filter sand graded 0.45 to 0.55 mm, kept at least 10 inches below the top | The replacement cartridge listed in your manual | 3.0 lb of DE for the 24 sq ft model up to 9.0 lb for the 72 sq ft model |
| Running without media | Does not apply | Does not apply | No more than a minute or two without a DE coat, or the grids can be damaged |
Sand vs cartridge vs DE: what changes when you troubleshoot?
All three follow the same rule: watch the gauge and clean when it climbs. What changes is how you clean them and what wears out.
A sand filter is the simplest. Its weak spots are the valve gasket and the laterals, and sand in the pool usually points to a cracked lateral or standpipe. A cartridge filter has no backwash; its usual problems are a pinched O-ring and a worn-out cartridge. A DE filter cleans the finest, but it needs fresh DE after every backwash, and Hayward says a backwash removes only about 80 to 90% of the DE, so the grids need a hand cleaning from time to time. Grids, laterals, cartridges and O-rings are sized to each model, so match the model number on the tank label when you buy parts.
When should you call a pool or spa pro?
- A cracked tank or valve body. Do not run a cracked tank under pressure.
- Sand or DE in the pool after the basic checks. Fixing laterals or grids means emptying and taking apart the tank.
- Pressure stays high after a full cleaning. Pentair says that if it stays high after a hand cleaning and an acid soak, the cartridges need replacing.
Pool pros bill by the visit or by the hour, so have your filter model and clean pressure ready.
Common questions
What pressure should a pool filter run at?
There is no single right number, because each pool has its own clean pressure. Hayward says to backwash a sand or DE filter at 8 to 10 psi above that clean pressure, and Pentair says to clean its cartridge filter at about 10 psi above. So 20 or 25 psi can be fine on one pool and too high on another; never let it reach the 50 psi limit.
What causes low pressure in a pool filter?
Low pressure usually means less water is reaching the filter. Check for full baskets, low water, a closed suction valve, a pump that has lost prime, or an air leak on the suction side. A broken gauge can also read low, so compare it with the flow at the return jets.
Why is my pool filter leaking from the top?
On a sand or DE filter with a top-mount valve, it is usually the O-ring between the valve and the tank, the valve gasket, or a loose clamp. On a cartridge filter, it is usually the lid or tank O-ring. Turn the pump off, release the pressure, then clean and reseat the O-ring, or replace it if it is flat, cracked or pinched.
Can I shock my pool if the filter is not working?
You can, but chlorine will not mix well without circulation, and the filter is what removes the dead algae afterward. We recommend fixing the circulation first. If you must shock with the pump down, brush the pool to spread the chlorine and keep everyone out until levels are back in range.
How many hours a day should a pool filter run?
Hayward says to run the filter long enough each day for at least one full turnover. Turnover means pumping the whole pool’s volume of water through the filter once. The hours depend on pool size and pump flow.
How long does a pool filter last, and how often should the sand or cartridge be replaced?
Hayward and Pentair do not publish a lifespan for tanks, sand or cartridges in these manuals. Pentair says to replace cartridges when pressure stays high after a hand cleaning and an acid soak. For a crusted sand bed, Hayward says to remove the top inch of sand; if pressure still stays high after backwashing, we would replace the sand.
Why is my pool cloudy after backwashing?
A common reason is skipping the Rinse step. Hayward says to run the valve on Rinse for about 1/2 to 1 minute after a backwash, so dirty water left in the tank goes to waste instead of back into the pool. After a DE backwash, cloudy water can also mean no fresh DE was added, or a grid is torn.