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A failing pool filter announces itself in several ways at once. Catch one signal and you save a cleaning; catch several and you're heading off a bigger problem. Here's the complete diagnostic.
Pressure gauge signs
Your filter's pressure gauge is the single best diagnostic tool you have. Mark your clean baseline pressure (typically 10-15 psi for a freshly cleaned filter) and watch for:
- Pressure 8-10 psi above baseline: Normal. Time to backwash or clean.
- Pressure rising faster than usual: Heavier debris load. Check for source (pollen, swim load, pet).
- Pressure won't drop to baseline after cleaning: Media is failing. Cartridges need replacing; sand needs replacing; DE grids may be damaged.
- Pressure very low (below 8-10 psi): Bypass happening. Check for cracked standpipe, loose multiport valve, or broken manifold inside the tank.
- Pressure fluctuating wildly: Air in the system, possibly from leaking suction-side seals or the skimmer. Not strictly a filter issue but often discovered via the gauge.
Water quality signs
- Persistent cloudiness despite good chemistry — the filter isn't catching fine particles
- Debris settling on the pool floor — either poor circulation or filter not catching particles it's pulling in
- Green water appearing overnight — filter isn't removing algae spores
- Chlorine burning through faster than normal — filter isn't reducing organic load
- Water "looks right" but feels wrong — often high dissolved organics that a filter should be capturing
Equipment behavior signs
- Weaker return jets — hold your hand in front of eyeball fittings. Strong flow = healthy system.
- Pump straining noise — working against filter resistance
- Pump running hot — motor straining
- Suction-side air bubbles — not always filter-related but often discovered during filter troubleshooting
- Heater or salt cell errors — modern equipment often throws flow errors when the filter restricts too much
Visual inspection signs
During your next routine cleaning, open the tank and inspect:
Cartridge filter
- Torn, damaged, or collapsed pleats
- End caps separating from the fabric body
- Discoloration that won't wash off
- Pleats permanently flat (lost springiness)
Sand filter
- Sand level far below original (sand loss through broken laterals)
- Sand caked into hard chunks (calcification or oil saturation)
- Channels or craters in the sand bed (uneven flow)
- Sand appearing in the pool through returns
DE filter
- DE powder appearing in the pool
- Visible damage to grid fabric
- Grids warped or bent
- Manifold cracks or damage
Elevated filter pressure puts mechanical strain on your pump. Running a pump against a clogged filter for weeks can cut pump life in half. Clean promptly when pressure rises.
What to do when you see signs
- Start with the cheapest fix. Clean or backwash first. If that resolves pressure, you've diagnosed.
- If cleaning doesn't fix it: chemical cleaning (overnight soak for cartridges, chemical backwash for sand, deep clean for DE).
- If chemical cleaning doesn't fix it: replacement is likely required. Order replacement media and inspect for damage during the swap.
- If water quality doesn't recover: you may have a separate circulation or chemistry issue compounding the filter problem.
Frequently asked questions
My pressure gauge itself could be wrong — how do I know?
Gauges do fail. If behavior suggests pressure problems but the gauge reads normal, replace the gauge ($15-$25) and re-establish baseline.
Is DE in the pool always a filter failure?
Usually. Small amounts during startup after a service are normal; ongoing DE in the pool means a torn grid or damaged manifold.
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