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Clean a pool filter cartridge when the pressure gauge reads 8 to 10 psi above its clean baseline, which in practice is every 4 to 6 weeks during swim season for a correctly sized filter. Give it a full chemical soak every three months to remove oils and sunscreen that a hose rinse cannot touch, and replace the cartridge every two to three years or roughly 2,000 to 3,000 running hours. Heavy bather loads, pollen season, storms and algae recovery all shorten the interval, sometimes to weekly.
Why is the pressure gauge the real answer?
Because a calendar cannot know how dirty your water is. As the media loads with debris, resistance rises and the gauge climbs while flow falls. That 8 to 10 psi rise is the point at which the flow reduction starts to matter — circulation drops, the skimmer weakens, the heater may trip its flow switch, and on a variable-speed pump running at low RPM the flow can fall below what the system needs entirely.
Establish the baseline once, with a clean cartridge, at your normal pump speed, and write it on the housing. From then on the decision is arithmetic rather than guesswork. Counter-intuitively, a slightly dirty cartridge filters finer than a spotless one, because the loaded debris itself acts as a filter layer, so cleaning too early is mildly counterproductive as well as a waste of a Saturday.
| Condition | Typical rinse interval | Deep soak |
|---|---|---|
| Light use, screened or covered pool | Every 6 - 8 weeks | Twice a year |
| Normal residential, in season | Every 4 - 6 weeks | Every 3 months |
| Heavy bather load | Every 2 - 3 weeks | Every 6 - 8 weeks |
| Pollen season, spring | Weekly to fortnightly | After the pollen drop |
| After a storm | As soon as pressure rises | If debris was heavy |
| Algae recovery | Daily until clear | Once the water clears |
| Spa cartridge | Every 1 - 2 weeks | Every 3 months |
| Off-season, pool open | Every 8 - 12 weeks | At closing |
What is the correct cleaning method?
- Shut the pump off and release the air relief valve before opening the housing. A pressurized filter lid is genuinely dangerous.
- Note the pressure before shutdown so you are tracking the trend.
- Lift the cartridge out and hose it with a fan nozzle, working top to bottom down each pleat at an angle so debris is pushed out rather than driven in. Do both faces. This takes 15 to 20 minutes on a large cartridge.
- Every third or fourth clean, soak overnight in a commercial filter cleaner. Oils and sunscreen do not rinse out, and they are what permanently blinds the media.
- Rinse until no foam appears after any chemical soak, and let the cartridge dry in the shade.
- Inspect before reinstalling: cracked end caps, split pleats, a collapsed core or fabric that stays gray after a soak all mean replacement.
- Lubricate the housing O-ring with silicone grease, never petroleum jelly, and reassemble. Note the new clean pressure.
Owning a second cartridge changes the job entirely. Swap the clean one in, soak the dirty one at your leisure, and the pool is down for five minutes instead of a day. It also means each cartridge gets a full dry cycle, which extends both of their lives.
How fine does a cartridge actually filter?
Finer than sand, coarser than diatomaceous earth. That gap explains a lot of pool problems, particularly why dead algae stays suspended after shocking.
| Filter type | Approx. particle size removed | Maintenance |
|---|---|---|
| Sand | 20 - 40 microns | Backwash; media change every 5 - 7 years |
| Cartridge | 10 - 20 microns | Rinse and soak; replace every 2 - 3 years |
| Diatomaceous earth | 3 - 5 microns | Backwash and recharge; teardown annually |
| Cartridge with a clarifier | Effectively finer | Clarifier binds fine particles into filterable clumps |
Dead algae cells are roughly 2 to 5 microns, which is why a pool can stay milky for days after a successful shock on a sand or cartridge filter. The fix is a clarifier to clump the particles, plus continuous filtration — explained in why pool water goes cloudy after shocking.
When does a cartridge need replacing rather than cleaning?
Two to three years is the normal working life for a residential pool cartridge cleaned on schedule, and 12 to 24 months for a spa cartridge, which handles a far heavier organic load per gallon. The signs are the same in both cases: the clean baseline pressure creeps up after every cleaning and never returns to where it was; the pleats stay flat instead of springing back; the fabric is gray-brown deep in the fold after a full soak; or there is physical damage to the core, seams or end caps.
Running a spent cartridge is a false economy. It costs pump energy through added resistance, it costs chemicals because poor filtration is compensated for with sanitizer, and it costs run time. Longer, slower circulation partly offsets a loaded filter but does not replace a working one — see how many hours a day to run a pool pump.
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