Backwashing is the cleaning process for sand and DE filters — it reverses the flow of water through the filter tank to flush accumulated debris out through the waste line. It's a straightforward procedure but one that's often done incorrectly or at the wrong time. Here's how to do it right.
When to backwash — use the pressure gauge, not the calendar
Backwash based on filter pressure, not a fixed schedule. When your filter is clean, note the operating pressure on the gauge — this is your baseline "clean pressure." Backwash when the gauge reads 8–10 PSI above that baseline. This is the reliable indicator that the filter media has captured enough debris to restrict flow.
Backwashing too frequently (before pressure rises to the 8–10 PSI threshold) is counterproductive — a small amount of debris in the filter media actually improves its ability to capture finer particles. Backwashing too infrequently (waiting until pressure is 15+ PSI above baseline) puts strain on the pump and reduces filtration efficiency significantly.
After major debris events — heavy storms, algae treatment, significant backwash — check pressure and backwash if needed rather than waiting for the next scheduled service.
Step-by-step backwash procedure (sand filter)
- Turn the pump off. Never move the multiport valve while the pump is running — this can tear the valve's internal spider gasket, which is an expensive repair.
- Move the multiport valve to "Backwash." The valve handle should click firmly into position.
- Confirm the backwash line is clear. The backwash line routes water to a waste drain, yard drainage, or a dedicated drain point. Make sure it's not blocked and that the discharge area can handle the flow.
- Turn the pump on and run for 2–3 minutes, or until the sight glass (the small clear window on the filter) runs clear. Initially the water in the sight glass will appear brown or cloudy — this is the captured debris flushing out.
- Turn the pump off.
- Move the valve to "Rinse." This brief rinse cycle resettles the sand after backwashing — without it, fine sand can be pushed into the pool through the returns when you return to filter mode.
- Turn the pump on and run the Rinse cycle for 30–60 seconds.
- Turn the pump off, move the valve to "Filter," turn the pump back on. Note the new clean pressure on the gauge — it should be back near baseline.
Backwashing a DE filter
The procedure for a DE filter is the same sequence, with one additional step: after backwashing, you need to replenish the DE that was flushed out. Add DE through the skimmer while the pump is running on filter mode — use approximately 1 pound of DE per 10 square feet of filter area (check your filter's specifications for the correct amount). Mix DE with water in a bucket before adding to avoid a cloud of fine powder.
Note that backwashing alone doesn't fully clean a DE filter — DE media and grids need a full breakdown cleaning 1–2 times per year to remove oils and deposits that backwashing doesn't address.
Signs you've waited too long to backwash
- Filter pressure is 15+ PSI above baseline — the pump is working significantly harder than it should be
- Return jet pressure is noticeably reduced — restricted flow through the filter is reducing circulation
- Water clarity is declining despite adequate chemistry — the filter can't capture fine particles effectively when it's overloaded
- The pump is running hot or making more noise than usual — both signs of strain from restricted flow
How much water does backwashing use?
A typical backwash cycle uses 200–300 gallons of water. This is one advantage cartridge filters have over sand and DE — they don't require backwashing and therefore don't waste water. In Florida, water costs aren't usually a primary concern for pool owners, but during drought restrictions or for environmentally conscious pool owners, the water consumption of regular backwashing is worth factoring into filter type decisions.
After backwashing: check pool water level
Backwashing lowers pool water level — sometimes significantly after multiple cycles. After backwashing, check that the water level is still at mid-skimmer height. If it's dropped below the skimmer opening, add water before running the pump again to prevent the skimmer from drawing air.