How to Backwash a Pool Filter — Step-by-Step Guide (Sand & DE)

The One Rule That Prevents Most Backwash Mistakes

Before anything else: always turn the pump off before moving the multiport valve.

Moving the valve while the pump runs cracks the valve body. It happens constantly — and it is a $150–$300 fix that is 100% avoidable. Every step in this guide starts with the pump off.

What Backwashing Actually Does

Your filter traps debris in the sand or DE media as water flows downward from the inlet. Over time, the trapped debris increases resistance — that is why pressure rises.

Backwashing reverses this flow: water comes in from the bottom, travels up through the media, and carries the trapped debris out the waste/backwash line. Think of it as flushing the filter in reverse.

The result: the debris is removed, pressure drops back to your clean baseline, and the filter resumes efficient operation.

Step-by-Step Backwash Procedure

Follow the steps in the How-To section above. Here is what each step looks like in practice:

The Sight Glass: Your Key Indicator

The sight glass is the small clear tube or viewport on the multiport valve. It shows you what the backwash water actually looks like.

When you start the backwash cycle, the sight glass will be dark and cloudy — that is the debris. As backwashing continues, it gradually clears. Stop when it runs fully clear. Running the backwash longer than needed wastes water without additional benefit.

The RINSE Step — Do Not Skip This

RINSE is the step most pool owners skip, and it is why sand ends up in their pool after backwashing.

After backwashing, the sand bed is disturbed — some sand is loosened and floating near the top of the tank. If you go straight from BACKWASH to FILTER, that loose sand returns through the laterals and into your pool via the return jets.

RINSE runs water in the normal filter direction for 30 seconds to re-settle the sand into a compact, level bed before returning to normal operation.

Sand Filter vs DE Filter: One Important Difference

The backwash procedure is identical for sand and DE filters up through the RINSE step. The critical difference is what comes next for DE filters.

After every backwash of a DE filter, you must replenish the DE powder.

DE grids are just fabric frames — the diatomaceous earth powder coated on them is the actual filtering medium. Backwashing strips off most of that coating. Without fresh DE, the water flows through the bare grids and directly back to the pool — barely filtered at all.

After the RINSE cycle on a DE filter:

1. Calculate how much DE your filter needs (approximately 80% of the rated total capacity)
2. Mix DE powder with water in a bucket to form a slurry
3. With the pump running on FILTER, slowly pour the slurry into the skimmer
4. The DE will travel to the filter and coat the grids evenly

See our guide on how much DE for pool filter for exact quantities by filter size.

How Often Should You Backwash?

Backwash when your pressure gauge reads 8–10 PSI above your clean baseline — not on a weekly schedule.

Your clean baseline is the pressure reading immediately after a fresh backwash when your filter is working at full efficiency. For most residential filters, this is 8–12 PSI.

When that reading rises to 18–22 PSI (8–10 PSI above baseline), it is time to backwash.

Why does the calendar schedule fail? Because filter loading depends on bather load, weather, and seasonal factors — not the day of the week. After a summer pool party or a heavy rainstorm, you may need to backwash the next day. During a mild week with no use, you may go 3 weeks without a pressure rise.

Write your baseline PSI on a piece of waterproof tape and stick it on your filter housing.

Related Guides

• How Often to Backwash Pool Filter — the PSI method explained in detail
• How to Backwash a Hayward Pool Filter — model-specific steps
• How to Backwash a DE Filter — DE-specific procedure
• How to Change Pool Filter Sand — when backwashing stops working