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Backwash: How & When to Backwash Your Pool Filter

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Backwashing is the process of reversing water flow through a sand or DE (diatomaceous earth) filter to flush out trapped dirt, debris, and contaminants. It's a routine maintenance task that restores filter pressure and circulation efficiency.

How It Works

During normal filtration, water flows down through the filter media (sand or DE), which traps particles. Over time, the media becomes clogged and pressure rises. Backwashing reverses the flow — water enters from the bottom, lifts and agitates the media, and flushes captured debris out through the waste line.

When to Backwash

Backwash when pressure rises 20–25% above your clean baseline (often around 8–10 psi, depending on your system). Note your filter's clean pressure right after a fresh backwash — that number is your reference point.
  • Pressure rise is the primary indicator; also watch for reduced return flow and weaker circulation
  • After treating a green pool or algae outbreak
  • After heavy use or a storm (check pressure — these events often cause it to spike)
Pro Tip: Don't backwash on a fixed schedule. Pressure is your guide. Backwashing too often wastes water and chemicals — and for sand filters, can prevent the development of a beneficial filter cake that improves fine particle capture.

How to Backwash (Sand Filters with Multiport Valve)

Important: Never move the multiport valve handle while the pump is running. Always turn the pump off first.
  1. Turn off the pump
  2. Set the multiport valve to Backwash
  3. Turn the pump on and run until the sight glass runs clear (typically 2–3 minutes)
  4. Turn the pump off
  5. Set the valve to Rinse and run for 30 seconds — this re-settles the sand bed and prevents residual dirty water from returning to the pool
  6. Turn the pump off, return the valve to Filter, and resume normal operation

If your DE filter uses a push-pull (slide) valve instead of a multiport, follow your manufacturer's procedure — the Rinse position may not be available.

DE Filters: One Extra Step

After backwashing a DE filter, you must recharge it with fresh DE powder. Without it, filtration performance drops and the grids can clog with oils and fine debris. Add the correct amount of DE through the skimmer per your filter's square footage and manufacturer specs.

Cartridge Filters Don't Backwash

Cartridge filters are cleaned by removing the cartridge and hosing it down — they have no backwash valve and aren't designed for reverse-flow cleaning. Follow your manufacturer's cleaning procedure.

Water Loss

Backwashing discharges water to waste. The amount depends on your pump's flow rate and how long you run it — at 50 GPM for 3 minutes, that's roughly 150 gallons. Top off the pool afterward and recheck your water chemistry, especially if you have a saltwater system. Check local regulations, as some municipalities restrict where backwash water can be discharged.

Pro Tip: If pressure rises quickly again shortly after backwashing, the filter may be undersized, channeling (water finding a path around the media), or dealing with a very dirty pool. That's a sign to investigate further, not just backwash again.