How to Backwash a Sand Filter - Step-by-Step Guide
The short answer
Backwash a sand filter by turning the pump off, setting the multiport valve to BACKWASH, running the pump until the sight glass clears, then running RINSE for 30 seconds before returning to FILTER. This guide covers both multiport and push-pull valves, the right PSI trigger, and the mistakes that send sand back to the pool.
Pentair 1.5-inch Backwash Hose - 50 ft
Heavy-gauge wall that does not balloon under backwash flow.
In This Guide
How to Backwash a Sand Filter
To backwash a sand filter, turn the pump off, set the multiport valve to BACKWASH (or pull a push-pull valve up to its backwash stop), then run the pump until the sight glass runs clear. Turn the pump off again, run RINSE for 30 seconds on a multiport valve, then return the valve to FILTER. The whole job takes about 10 minutes and reverses water flow through the sand bed to flush out the dirt it has trapped.
When to Backwash: Use Pressure, Not a Calendar
The single rule that matters: backwash when the pressure gauge climbs 8 to 10 PSI above your clean baseline. The clean baseline is whatever the gauge reads right after a fresh backwash. Write that number on waterproof tape stuck to the filter housing.
Backwashing on a fixed weekly schedule is a mistake. A sand bed that is lightly loaded with dirt filters smaller particles than freshly cleaned sand does, so backwashing too soon actually leaves your water cloudier. The same logic is covered in more depth in this general guide on how to backwash a pool filter.
Multiport Valve vs Push-Pull Valve
Sand filters use one of two valve types, and the steps differ slightly.
| Valve Type | Backwash Action | RINSE Step? |
|---|---|---|
| Top or side-mount multiport | Rotate handle to BACKWASH | Yes, 30 seconds |
| Push-pull (slide) valve | Pull plunger rod up to backwash stop | No RINSE position |
A multiport valve has six labeled positions: FILTER, BACKWASH, RINSE, WASTE, RECIRCULATE, and CLOSED. A push-pull valve is a simple two-position plunger: down for filter, up for backwash. If your valve only moves up and down, you have a slide valve and there is no RINSE cycle to run.
Step-by-Step Backwash Procedure
- Turn the pump off at the timer or breaker and wait 5 seconds.
- Connect a backwash hose to the WASTE port and run it somewhere that can take dirty, chlorinated water.
- Set the valve to BACKWASH (rotate the multiport handle, or pull the slide plunger up).
- Start the pump and watch the sight glass. Run until the water runs clear, about 2 minutes.
- Turn the pump off, set the multiport valve to RINSE, and run 30 seconds. Skip this on a push-pull valve.
- Return to FILTER and restart. Record the new clean baseline pressure.
The reason you always stop the pump before moving the valve: on a multiport valve, the internal spider gasket gets sheared if the handle moves under pressure, and replacements mean opening the valve. On a push-pull valve, moving the plunger under load can blow the plunger seal.
After Backwashing: Check the Water Level
Backwashing sends a few hundred gallons to waste, so the pool level drops. Top it off and recheck your chemistry, since you have also diluted the water. Backwashing is not a substitute for a proper deep clean either. Every couple of seasons the sand needs a chemical treatment to strip out oils that backwashing cannot reach, which is walked through in this guide on how to clean a pool sand filter.
When Backwashing Stops Working
If the gauge will not drop back near baseline after a thorough backwash, the sand is channeling or coated with grease that reversed flow cannot lift. First try a chemical filter cleaner: with the pump off, pour it into the skimmer, let it sit 8 to 12 hours, then backwash hard. If pressure still will not return to baseline, the media is worn and rounded and needs replacing. Sand typically lasts 3 to 5 years, and the full schedule is laid out in this article on how often to change pool filter sand.
When you do replace it, use 0.45 to 0.55mm #20 silica sand, the grain size sand filters are engineered around. Our pick is in this roundup of the best pool filter sand.
Related Guides
- How to Backwash an Intex Sand Filter - the Intex 6-position valve steps
- How to Backwash a Pool Filter - the complete general procedure for every filter type
- How to Clean a Pool Sand Filter - deep cleaning beyond backwashing
- How Often to Change Pool Filter Sand - the replacement schedule and warning signs
Frequently Asked Questions
How do I know when to backwash a sand filter?
Why is sand going back into my pool after backwashing?
How long should I run the backwash cycle?
Can I backwash a sand filter too often?
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