Pool filter fix: troubleshooting by symptom
Almost every filter fault announces itself in one of three places: the pressure gauge, the water coming back to the pool, or the waste line. Work backwards from the symptom and you will usually narrow it to one part in a couple of minutes.
Start with the gauge, always
A pressure reading only means something against a clean baseline. Note it after a thorough clean and write it on the tank. Then:
- High pressure means resistance after the pump — almost always a loaded filter, sometimes a partly closed return valve or a blocked return line.
- Low pressure means the pump is not getting water — a suction blockage, an air leak, a low water level, a closed valve or an impeller clogged with debris.
- Normal pressure but no filtration points at the media itself: channelled sand, a torn grid, or a worn cartridge.
Diagnose by symptom
| Symptom | Most likely cause | What to do first |
|---|---|---|
| Pressure 8–10 psi above clean | Filter loaded with debris | Backwash or clean the filter |
| Pressure very low, weak returns | Suction blockage, low water, closed valve | Check water level, skimmer and pump baskets, then the impeller |
| Bubbles coming from the returns | Air being drawn in on the suction side | Pump lid o-ring, union, valve stem, skimmer throat |
| Sand in the pool | Broken lateral or cracked standpipe inside the tank | Stop running until the tank is opened — the pump is pushing sand into the pool |
| DE powder in the pool | Torn grid or a damaged manifold | Same: open the tank and inspect the grid assembly |
| Cloudy water with normal pressure | Channelling, worn media, or a chemistry problem | Deep clean or replace the media, then read the cloudy-water order |
| Filter will not prime | Air leak, low water, clogged impeller, worn pump seal | Prime manually, check the lid o-ring and the water level |
| Water leaking at the band or clamp | Failed o-ring or gasket, or uneven clamping | Isolate, open, replace the o-ring, reseat evenly |
| Water will not clear after backwashing | You are backwashing too often, or the sand is channelled | Stop backwashing on a schedule and deep clean the bed |
Sand filters: the usual suspects
- Laterals (the slotted fingers under the sand) crack or come loose, and sand migrates into the pool.
- The sand bed channels: water finds one path and the rest of the bed stops filtering. Deep cleaning, or replacing the sand, fixes it.
- Sand gets old and rounded. Most manufacturers suggest replacing it every few years — check your own manual for the interval.
- Over-backwashing never lets the bed mature, so it never catches the fines.
If you are seeing sand in the pool, stop running the filter. You are pumping the bed into the water. See how to backwash a pool filter for the routine side, and cloudy pool water if the problem is clarity rather than hardware.
DE filters: the usual suspects
- A torn fabric grid lets DE straight through into the pool.
- Forgetting to recharge with DE after a backwash leaves the grids running bare — filtration collapses and the grids can be damaged.
- Overcharging with DE raises the pressure immediately and shortens the cycle.
DE is a fine powder that must be handled carefully: follow the label, avoid inhaling it, and keep it away from children and pets.
Cartridge filters: the usual suspects
- Pleats clogged with oils and sunscreen. Hosing is not enough — a proper soak in cartridge cleaner restores flow.
- A torn or crushed pleat passes debris straight through.
- Running a damaged cartridge raises the pressure and can stress the housing.
Cartridges are consumables. Cleaning extends life; nothing makes a torn one work again.
Air in the system
Bubbles from the returns always mean air is entering on the suction side — never the pressure side. Work along that path: pump lid o-ring, unions, valve stems, the skimmer throat, and the water level itself.
A pump that is drawing air loses prime, runs hot and delivers less flow. It is one of the easiest faults to fix and one of the most often missed.
Replace rather than repair when…
- A sand filter has a cracked lateral or a split standpipe — the repair is real work but it is worth doing properly.
- A DE grid is torn: replace the grid, not the whole filter.
- A cartridge is torn or the pleats are matted flat.
- A tank itself is cracked. That is a replacement, and it is not a repair to improvise.
When to call someone
- Any cracked filter tank or cracked pump housing — pressure vessels are not a patch job.
- Electrical faults at the pump, the timer or the bonding.
- A filter that needs full disassembly and you are not confident reseating the internals. A poorly rebuilt filter fails again quickly.
Filter questions
Why is sand coming back into my pool?
Usually a cracked or loose lateral, or a split standpipe inside the sand filter. Stop running the filter until it is opened and inspected — otherwise you are pumping the bed into the pool.
Why is my filter pressure low instead of high?
Low pressure means the pump is starved. Check the water level, the skimmer and pump baskets, the impeller, and then look for an air leak on the suction side.
How long does filter sand last?
Most manufacturers suggest replacing it every few years, and the real signals are channelling and water that will not clear despite a clean filter. Check your own manual for the interval rather than guessing.
Why does my filter pressure stay high right after backwashing?
Either the backwash did not clear the bed, or the problem is downstream: a partly closed return valve or a blocked return line. If it happens immediately every time, the bed may be channelled and need a deep clean.
Can I run a pool without a filter?
Not for long. Circulation without filtration just moves debris around the pool. The filter is what removes it.
Related
- How to backwash a pool filter
- Cloudy pool water — the diagnostic order
- Pool pump timer — how long to run and when
- Pool maintenance schedule
Review status: Equipment and consumables. Last reviewed 2026-09-13. See the source and review status of every formula →