Most pool filters
don’t fail dramatically. There’s no moment where everything stops working and the problem becomes obvious. What happens instead is a gradual decline that’s easy to explain away; the water’s been a little cloudy but it rained last week, the pressure is higher than it used to be but it’s always been finicky, and the chemicals aren’t holding the way they did last summer but maybe the bather load has been heavier. By the time the filter is genuinely failing the pool owner has usually been compensating for it for an entire season without connecting the dots.
The signals are clear if you pay attention and know what to look for.
Pressure Readings
The pressure gauge is the most direct diagnostic tool available and most pool owners glance at it without knowing what they’re actually seeing. Every filter has a normal operating pressure, a baseline reading when the filter is clean and the system is running properly. That number matters more than any specific PSI figure, which is why the first thing to do with a new or freshly serviced filter is note what it reads under normal conditions.
Eight to ten PSI above baseline means the filter needs cleaning. That’s normal and expected. The problem is a filter that climbs back to high pressure faster after each cleaning than it did before. One that needed backwashing every two weeks and now needs it every four days isn’t just dirty. The media is degraded or the filter is being asked to handle more than it can process.
High pressure that doesn’t drop after cleaning is the replacement signal rather than the maintenance one. Backwash the sand, rinse the cartridge, reassemble everything, and if the pressure is still elevated that filter is past the point where cleaning resolves anything. Low pressure is the less obvious signal and gets missed more often. A reading significantly below baseline suggests water is bypassing the filter media entirely through a crack or tear, moving through the system without being cleaned. The pool chemistry deteriorating despite correct chemical treatment is often the only visible sign.
Clarity Issues
Persistent cloudy water that doesn’t respond to chemical adjustment is the symptom most pool owners notice first and misdiagnose most consistently. When the chemistry tests correctly, pH balanced, chlorine at appropriate levels, and the water is still cloudy, the filter is the likely explanation rather than a chemistry problem.
A filter that’s processing water but not cleaning it returns cloudy water to the pool continuously regardless of what goes in chemically. This is the pattern that leads pool owners to spend money on clarifiers, shock treatments, and enzymes, all of which address symptoms rather than the filtration failure producing them.
Green water that returns within days of being treated is a related signal. Algae that keeps coming back despite adequate sanitizer levels often traces back to filtration that isn’t removing dead algae cells efficiently enough to prevent regrowth. A filter working properly breaks the cycle. A marginal filter recirculates enough organic material to keep feeding whatever is trying to grow back.
Lifespan
Sand filter media degrades over time regardless of how well it’s maintained. Sand that’s been in service five to seven years has broken down enough through constant water movement and backwashing that it’s passing debris newer sand would capture. The filter housing may be fine while the media inside it has quietly become a fraction of what it was. Replacing the sand every five to seven years is maintenance rather than repair and changes filtration performance immediately in a pool that’s been struggling.
Cartridge filters have a shorter media lifespan than most manufacturers suggest under real conditions. Two to three seasons in a heavily used pool, three to five under average conditions. A cartridge that’s been cleaned so many times the pleats have compacted isn’t filtering at the level of a new one regardless of how intact it looks from the outside.
Repair or Replace Your Pool Filter
A filter more than ten years old requiring repeated attention is usually at the point where replacement produces a better outcome than continued maintenance. The cumulative cost of media replacement, parts, and service on aging equipment often approaches the cost of a new unit with a warranty and better efficiency.
The moment that tips the decision is a repair estimate running more than half the replacement cost. Cracked housing, broken manifold, unavailable parts. These are situations where continued repair doesn’t make financial sense regardless of how functional the filter was before the problem developed.