How to Backwash a Pool Filter

Jan 28, 2026

Backwashing cleans sand, glass media, and DE pool filters by reversing water flow — but only when pressure rises 8–10 psi above normal. Cartridge filters are never backwashed. Done correctly, backwashing restores flow and filtration; done too often, it wastes water and shortens filter life.

Backwashing sounds dramatic, but it’s really just your pool filter asking for a clean-out before it gives up on you. Ignore it for too long, and you’ll notice cloudy water, weak circulation, and a filter that’s working harder than it should.

Do it too often (or incorrectly), and you’re wasting water, chemicals, and filter life.

Let’s clear up what backwashing actually is, which filters need it, and how to do it properly — without overthinking it.

Key Takeaways

Backwashing reverses water flow to flush debris from the filter media
Only sand, glass media, and DE filters are backwashed
Cartridge filters are never backwashed — they’re removed and hosed clean
Always backwash based on pressure rise, not a fixed schedule
DE filters require fresh DE powder after every backwash
Skipping the rinse cycle sends dirty water straight back into the pool
Over-backwashing wastes water and reduces filtration efficiency
If pressure doesn’t drop after backwashing, the filter media may need replacing

What Is Pool Filter Backwashing?

Backwashing is the process of reversing the water flow through your pool filter to flush out trapped dirt, debris, oils, and fine particles.

Instead of pushing water through the filter media like normal, backwashing sends water backwards, lifting and rinsing the filter bed so all the junk exits through the waste line.

💡Think of it like shaking the crumbs out of a rug instead of vacuuming it harder.

Types of Pool Filters and Backwashing Differences

Not all pool filters work the same — and not all of them should be backwashed.

How to Backwash a Sand Pool Filter

Sand filters are the classic backwash champions.

💡Why backwashing matters: Over time, sand traps oils, sunscreen residue, and fine debris. When it clogs, pressure rises and flow drops.

When sand filters need backwashing:

Pressure gauge rises 8–10 psi above clean pressure
Water flow feels weaker
Pool starts looking dull even with balanced chemistry

Backwashing restores flow and keeps the sand effective.

How to Backwash a Glass Media Pool Filter

Glass media filters behave similarly to sand filters — but with a few advantages.

Key differences:

Glass media traps finer particles
Requires less frequent backwashing
Uses less water during the process

💡Expert insight: Glass media still needs backwashing — just not as often. If you backwash it weekly “just in case,” you’re doing it a disservice.

How to Backwash a Cartridge Pool Filter

Short answer: you don’t.

Cartridge filters are not backwashed.

Instead:

Remove the cartridge
Hose it down thoroughly
Occasionally, soak it in a cartridge cleaner

💡Trying to backwash a cartridge filter can damage it or do absolutely nothing.

Do DE Pool Filters Need Backwash?

Yes — DE (diatomaceous earth) pool filters do need backwashing, but with an important extra step most pool owners miss.

DE filters use a fine powder coating on internal grids to trap extremely small particles. When the filter loads up, pressure rises and flow drops — just like with sand filters — so backwashing is required.

💡Here’s the catch: Backwashing a DE filter removes the DE powder itself, not just the dirt.

What that means in practice:

You must backwash when pressure rises 8–10 psi
You must recharge the filter with fresh DE powder afterward
Running a DE filter without recharging reduces filtration efficiency dramatically

💡Expert insight: Backwashing without recharging is one of the fastest ways to turn a premium DE filter into an overpriced strainer.

When Should You Backwash Your Pool Filter?

Ignore fixed schedules. Watch the filter.

The right time to backwash:

Pressure gauge rises 8–10 psi above clean baseline
Flow visibly reduces
After heavy debris loads (storms, pool parties)
When vacuuming large amounts of dirt to waste

💡Pro tip: Write down your “clean filter pressure” after installation or servicing. That number is your reference point forever.

What You’ll Need Before Backwashing

You don’t need much, but skipping basics causes mistakes.

Multiport valve (with BACKWASH setting)
Functional pressure gauge
Waste hose or plumbed waste line
Pump turned OFF before valve changes

That last one matters more than people think.

Step-by-Step Guide: How to Backwash a Pool Filter

This applies to sand and glass media filters.

1. Turn off the pump

Never move the multiport valve while the pump is running. Ever.

2. Set valve to BACKWASH

Firmly click into place.

3. Turn the pump back on

Water will now flow backward and exit through the waste line.

4. Backwash for 2–3 minutes

Or until the sight glass runs clear.

5. Turn pump off again

6. Set valve to RINSE

This settles the filter bed and prevents dirty water returning to the pool.

7. Run RINSE for 20–30 seconds

8. Turn pump off

9. Set valve back to FILTER

10. Restart pump and check pressure

Your pressure should now be close to the clean baseline.

After Backwashing: What to Do Next

Backwashing removes water — and water contains chemicals.

After backwashing:

Top up pool water if needed
Retest pH and chlorine
Add chemicals if levels dropped
Check for leaks at the waste line

💡Expert insight: If pressure doesn’t drop after backwashing, your filter media may be channelled, calcified, or overdue for replacement.

Common Backwashing Mistakes to Avoid

These are the ones professionals see constantly:

Backwashing too often “just in case”
Skipping the rinse cycle
Changing valve positions with pump running
Ignoring pressure gauge readings
Forgetting to rebalance chemicals afterward
Assuming cloudy water = backwash needed (often chemistry-related)

💡Pro rule: Backwash when pressure tells you to — not when vibes do.