How to backwash a pool filter
Backwashing reverses the flow through a sand or DE filter, lifting trapped dirt out and sending it to waste. It takes a few minutes. The two things to get right are doing it when the pressure says so, not on a fixed schedule, and always switching the pump off before moving the valve.
When to backwash
Write down the filter's pressure right after a thorough cleaning. That's your "clean" pressure. Backwash when the gauge reads about 8–10 psi above it, or as your filter's manual says. Backwashing too often removes the thin layer of dirt that actually helps a sand filter catch fine particles, and wastes water.
Cartridge filters can't be backwashed. Remove the cartridge and hose it off instead, and replace it when it's worn or won't clean up.
Sand filter: step by step
- Turn the pump off. Moving the multiport valve while the pump runs can damage the valve's internal seal.
- Connect and run out the backwash hose, if your system uses one, to where your local rules allow the water to go.
- Set the valve to Backwash. Press the handle down and rotate it.
- Turn the pump on and run it until the water in the sight glass runs clear, typically a couple of minutes.
- Turn the pump off, and set the valve to Rinse.
- Run the pump for about 30 seconds to a minute. This resettles the sand so dirt doesn't blow back into the pool.
- Turn the pump off, set the valve back to Filter, and turn the pump on. Check that the pressure has dropped back near the clean reading.
DE filters
DE (diatomaceous earth) filters backwash the same way, but backwashing washes out the DE powder too. After backwashing, add fresh DE through the skimmer with the pump running, in the amount your filter's manual specifies. Many DE filters also need the grids taken out and cleaned periodically. Follow the manual, and check local rules on discharging DE.
How much water it uses
Water leaves at roughly the pump's flow rate. At 50 gallons a minute, 3 minutes of backwash plus a minute of rinse uses about 200 gallons. In a 15,000-gallon pool, that's a little over 1% of the water, or about 0.8 inch off the level at an average depth of 5 feet. Top up afterwards.
That replacement water slowly dilutes stabilizer, calcium and salt, which helps if they're creeping up. The drain and refill calculator shows how much replacement it takes to make a real difference.
Pressure still high after backwashing?
- Sand is old or caked. Sand wears smooth and can clump with oils and calcium. Many owners replace it every several years. A filter cleaner product can help in between.
- DE grids or a cartridge are clogged with oils or scale. Clean or soak them as the manual describes.
- Return lines are restricted: a partly closed valve, a clogged heater or a blocked return fitting raises pressure.
- The gauge is faulty. Gauges fail. A cheap replacement rules it out.
If pressure is unusually low, look on the suction side: full skimmer or pump baskets, a low water level, or air leaks.
Frequently asked questions
How often should I backwash?
When the pressure rises about 8–10 psi above clean, not by the calendar. For many pools that's every few weeks in season, and more often after storms, algae treatment or heavy use.
Should I backwash after shocking?
If the shock was for algae, the filter will load up with dead algae, so backwash when pressure rises, and once the water is clear.
Can I backwash to lower the water level?
The Waste setting is the right one for that: it sends water out without passing through the filter. See how to vacuum a pool for using Waste.