Backwashing is how a sand filter cleans itself: you reverse the water flow to flush trapped dirt out to waste. It's a two-minute job once you know the steps, and doing it at the right time — not too often — keeps your filter working and your water clear. Here's how and when. Always follow your specific filter's manual.
When to backwash
Let the pressure gauge tell you, not the calendar. As the sand bed traps dirt, the filter pressure rises; when it climbs roughly 8–10 PSI above the clean starting pressure you noted after the last backwash, it's time. Backwashing more often than that is counterproductive — a slightly dirty sand bed actually filters finer than a spotless one, so chasing a perfectly clean gauge just wastes water. Note your clean baseline pressure so you know your target.
Important note: always turn the pump off before moving the multiport valve. Turning the handle with the pump running can damage the valve's internal gasket. Follow your filter's manual for specifics.
How to backwash, step by step
Work in order with the pump off between valve changes. First, turn the pump off. Set the multiport valve to "Backwash." Turn the pump on and run it until the sight glass runs clear — usually a couple of minutes. Turn the pump off, set the valve to "Rinse," run for 20–30 seconds to resettle the sand, then turn off again. Finally return the valve to "Filter" and restart. You'll have lowered the water level by backwashing, so top the pool back up afterward.
After backwashing
Recheck and rebalance. Backwashing sends pool water to waste, so add fresh water to bring the level back to the middle of the skimmer, and retest your chemistry since you've diluted it. Note the new clean pressure on the gauge as your fresh baseline. If you run a sand filter, periodically adding a specialized sand-filter cleaner and replacing the sand every few years keeps it filtering well — old, channeled sand won't clean no matter how often you backwash.
Don't over-backwash
The most common mistake is backwashing too often. It wastes water, wastes the chemicals in that water, and keeps the sand too clean to filter its finest. Backwash on pressure, not habit, and your filter does more with less water. If you find yourself backwashing constantly, the real problem is usually upstream — an algae bloom or heavy debris load overwhelming the filter — and fixing that is the answer, not more backwashing.
Frequently asked questions
How often should you backwash a sand filter?
By pressure, not by calendar. Backwash when the filter's pressure gauge reads about 8-10 psi above its clean starting pressure — that's the sign the sand bed is loaded with debris and flow is choked. Depending on your pool and season that might be every couple of weeks, but let the gauge decide.
How do I backwash a sand filter step by step?
Turn the pump off, set the multiport valve to Backwash, turn the pump on and run until the sight glass runs clear (about 2-3 minutes), turn the pump off, set the valve to Rinse and run 20-30 seconds, turn off again, then return the valve to Filter and restart. Never move the multiport valve while the pump is running.
Can you backwash a sand filter too much?
Yes. A slightly dirty sand bed actually filters finer than perfectly clean sand, so backwashing too often keeps the filter in its least-effective state and wastes water. Wait for the pressure to climb 8-10 psi rather than backwashing on a fixed schedule or 'just because.'
Why is my sand filter pressure high right after backwashing?
If pressure stays high after a proper backwash and rinse, the issue usually isn't debris — suspect channeling or hardened/aged sand, a clogged return side, or that the sand needs replacing (typically every several years). Very high pressure with low flow is a filter problem, not a reason to keep backwashing.
Do cartridge and DE filters backwash the same way?
No — backwashing is specific to sand (and DE) filters with a multiport valve. Cartridge filters don't backwash; you remove and rinse the cartridge instead. This guide is for sand filters; if you're weighing filter types, see our sand vs cartridge comparison.
