How to backwash a pool
Backwashing reverses the flow through a sand or DE filter and flushes the trapped debris out to waste. The rule that matters most is that you backwash because the pressure gauge tells you to — not because it has been a week.
Find your clean baseline first
The pressure gauge only means something relative to a clean filter. Note the reading right after a thorough clean — that number is your baseline, and everything else is measured against it.
Write it on the filter housing with a marker. It takes ten seconds and saves you guessing for the next three seasons.
When to backwash
- When the gauge sits roughly 8–10 psi above the clean baseline. That is the standard trigger and it is far more reliable than a calendar.
- When the return flow is visibly weaker than it was, even if the gauge has not moved much.
- After a heavy debris event — a storm, a big leaf drop, or a vacuum that picked up a lot.
- During and after an algae treatment, because the filter is catching dead algae continuously.
When not to backwash
- If you have a cartridge filter, there is nothing to backwash — the cartridge comes out and gets hosed or soaked.
- Do not backwash a brand-new sand filter repeatedly in its first weeks. The bed needs to settle and stratify before it filters properly.
- Do not backwash because the water is cloudy but the pressure is normal. Clean pressure means the filter is not the bottleneck — read the cloudy water diagnostic order instead.
- Do not backwash every week out of habit. See the trap at the end of this page.
Step by step
- Turn the pump off. Never move the multiport valve while the pump is running. The valve is holding back pressurised water and moving it under load can damage the valve, the plumbing, or you.
- Point the waste hose somewhere safe. You are about to send a few hundred gallons of chlorinated (possibly salty) water out. Not onto a flower bed, and not into a storm drain if your area regulates pool discharge.
- Set the valve to Backwash. Then open the waste line if your plumbing has a separate valve for it.
- Turn the pump on and watch the sight glass. Run it until the water in the sight glass turns clear. That is typically one to three minutes, but the glass decides — not the clock.
- Pump off. Set the valve to Rinse. Run the pump for 15–30 seconds. This settles the sand bed back down so it does not dump dirt into the pool.
- Pump off. Set the valve to Filter. Pump on. Note the new pressure reading — that is your refreshed baseline. Then top the pool back up, because you just sent water to waste.
Sand, DE and cartridge are not the same
| Filter type | How you clean it | What people get wrong |
|---|---|---|
| Sand | Backwash, then rinse | Backwashing too often, so the bed never matures and never catches the fines |
| DE (diatomaceous earth) | Backwash, then rinse, then recharge with DE | Forgetting the recharge — the grids then run bare, and filtration collapses |
| Cartridge | Remove, hose down or soak, replace when worn | Trying to backwash one, or refitting a torn cartridge |
After you backwash
- Top the pool back up to its normal level.
- Retest, because the water you added is fresh water: backwashing and refilling slowly dilutes stabiliser (CYA) and, in a salt pool, salinity. Both creep down over a season without anything being obviously wrong. Our CYA calculator and salt calculator take your current reading as the starting point.
- Check the sight glass and the waste line for sand. Sand in the waste means something inside the filter is broken.
The over-backwashing trap
A sand filter actually filters better once it has a modest load of dirt in it — the trapped debris is part of what catches the fine particles. Backwash too eagerly and the filter never reaches that state, so the water stays slightly hazy no matter how often you clean.
Meanwhile each backwash sends water to waste, which means refilling, which means fresh water that dilutes your stabiliser, and in a salt pool your salinity too. You end up paying three times: in water, in chemicals and in clarity.
Cartridge filters: what to do instead of backwashing
A cartridge filter has no backwash valve and no waste port, so there is nothing to reverse. People search for how to backflush one, and the answer is the same every time: the cartridge comes out and gets washed by hand.
- Switch off and release the pressure. Turn the pump off at the breaker, close the suction and return valves, then open the air-relief valve on the tank so the housing is not under pressure when you open it.
- Lift the lid and take the cartridge out. Note which way up it sits. Most drop straight back in, but some have a locating tab or an arrow moulded into the end cap.
- Hose it from the top down, between the pleats. A garden hose with a fan nozzle, working down each fold and from the inside out. Most of the debris sits in the outer pleats, because that is where the water hits first.
- Soak it if the pleats feel oily. Sunscreen and body oils bind to the pleats and will not rinse off with water alone. A cartridge cleaner, used exactly as its label instructs, lifts them.
- Check the pleats and the end caps before refitting. Tears, collapsed pleats and split end caps mean replace, not clean. A torn cartridge passes debris straight through, so cleaning it is wasted effort.
- Reinstall, and write down the clean pressure. That gauge reading is the baseline for every future cleaning decision — the same rule that governs sand and DE.
Backwash questions
How often should I backwash my pool?
Whenever the pressure gauge reads about 8–10 psi above its clean baseline. For some pools that is twice a month in summer; for others it is once every six weeks. The gauge is the schedule.
How long do I backwash for?
Until the sight glass runs clear — commonly one to three minutes. Running it for a fixed time either wastes water or stops too early.
Why is my pool cloudy right after backwashing?
Usually because the valve was returned straight to Filter without a Rinse step, so debris in the filter was pushed into the pool. It can also be an air lock, or sand getting disturbed. Rinse for 15–30 seconds every time.
Can I backwash a cartridge filter?
No. There is no backwash path and no waste port. The cartridge is removed and cleaned, or replaced when the pleats are worn or torn.
Should I backwash after shocking the pool?
Very often yes, because the filter loads up quickly with dead algae and the pressure will tell you. Backwash when it is needed, which during an algae treatment may be daily.
How do I clean a cartridge filter?
Take it out, hose it down between the pleats from the top, soak it in a cartridge cleaner if the pleats feel oily, then refit it once you have checked for tears. There is no backwashing step, because there is nowhere for the waste water to go.
What does backflushing a cartridge filter mean?
It means cleaning the cartridge by hand. The word is borrowed from sand and DE filters, which really are cleaned by reversing the flow. A cartridge has no such path, so nothing gets flushed anywhere.
Related
- Cloudy pool water — the diagnostic order
- How long to run a pool pump
- Pool maintenance schedule — where backwashing fits
- How to winterize an above ground pool
Review status: Equipment and consumables. Last reviewed 2026-09-13. See the source and review status of every formula →