How to Backwash a Sand Filter, Step by Step

2026-09-26

Backwashing is the filter job most pool owners do themselves, and the one most often done wrong. Done properly, it takes a few minutes, keeps the water clear and helps your filter sand last longer. Done carelessly, it wastes water, damages the valve and sends dirt straight back into the pool. This guide covers the valve positions, when to backwash, and the exact order of steps.

What backwashing actually does

A sand filter traps dirt in the top layers of its sand bed as pool water is pushed down through it. As that dirt builds up, the water has to push harder to get through and the pressure gauge climbs. Backwashing reverses the flow: water goes up through the sand, lifts and loosens the bed, and carries the trapped dirt out through the backwash line to waste. Rinsing afterwards settles the sand back into place so it filters properly again.

If you want the background on how sand compares with cartridge and DE filters, our guide to pool filter types: sand, cartridge and DE covers it. This article is about the sand filter most Centurion and Pretoria pools already have.

The multiport valve positions explained

The handle on top of, or beside, your filter is the multiport valve. Most have six or seven positions. The names are printed on the valve head, though on older valves the Highveld sun often fades them.

Filter: normal running. Water goes from the pump, down through the sand and back to the pool. The valve should be here almost all the time.
Backwash: the flow is reversed through the sand and sent out to the backwash line. Used to clean the filter.
Rinse: water flows down through the sand as it does on filter, but goes out to waste instead of back to the pool. Used for a short time after backwashing to settle the bed and flush loose dirt from the pipework.
Waste: water bypasses the sand entirely and goes straight out to waste. Used to vacuum heavy dirt or dead algae without clogging the filter, or to lower the water level after heavy rain.
Recirculate: water bypasses the sand and goes straight back to the pool. It keeps water moving while the filter is being worked on, but nothing is being filtered.
Closed: no flow through the valve. Never run the pump in this position.
Winter: found on some valves only. A resting position that takes pressure off the internal gasket when the system is shut down for a long period. Follow the valve manufacturer's instructions on how to use it.

When to backwash: read the gauge, not the calendar

Many people backwash every Saturday because that is what they were told. Backwashing too often wastes water and can make the filter slightly less effective, because a lightly loaded sand bed traps fine particles better than a freshly cleaned one. Backwashing too rarely lets pressure build, reduces flow and leaves the pool dull.

The better guide is the pressure gauge. Note the reading straight after a proper backwash and rinse, with the pump running on filter. That is your clean reading. Write it on the filter lid with a marker if it helps. When the gauge rises roughly 50–70 kPa above that clean reading, it is time to backwash.

Other times to backwash, whatever the gauge says:

After a Highveld thunderstorm has washed leaves, soil and dust into the pool, once the debris is out and the filter has run for a while
After vacuuming a lot of dirt on filter
After treating a green pool, once the dead algae has been filtered out of the water
When the flow at the returns has dropped noticeably and the weir and pump baskets are clean

If your gauge is broken or stuck on zero, replace it. Without it you are guessing.

How to backwash, step by step

Switch the pump off. Never move the multiport handle while the pump is running. Turning it under pressure tears or dislodges the spider gasket inside the valve, and a damaged gasket leaks pool water to waste all the time.
Empty the weir basket and the pump basket while the pump is off, so the backwash has full flow.
Push the handle down and turn it to Backwash. Make sure it seats firmly in the position.
If your backwash line has a valve or a hose, open the valve or roll the hose out to where the water should go.
Switch the pump on and watch the sight glass, the small clear dome on the side of the valve. The water in it will run cloudy or brown at first.
Keep backwashing until the sight glass runs clear. On a lightly loaded filter this is usually a minute or two; after a storm or a green pool clean-up it takes longer.
Switch the pump off again.
Turn the handle to Rinse.
Switch the pump on and rinse for about 30 seconds, until the sight glass is clear. This settles the sand bed and flushes loose dirt out of the pipework instead of blowing a cloud of it back into the pool.
Switch the pump off, turn the handle back to Filter, and switch the pump on.
Check the pressure gauge. This is your new clean reading. If it is much higher than it used to be after a backwash, read the last section below.
Top up the pool. A backwash and rinse remove a fair amount of water, and a low level lets the weir suck air, which can run the pump dry.

Where the backwash water goes

Backwash water carries dirt, chlorine and whatever else the filter was holding. Where it may go depends on your property and your municipality. Some homes pipe it to a garden area, some to a sewer connection. Pool water should not simply be run into the street, and a large volume of chlorinated water on one flower bed is hard on the plants. Check your municipality's by-laws on discharging pool water, because the rules differ from one municipality to the next, and your local municipal office can tell you what applies to your property. If you are trying to save water, backwashing on the gauge rather than on a schedule is the simplest saving there is.

Common mistakes

Turning the handle with the pump running. This is the main cause of worn spider gaskets.
Skipping the rinse. The first water through the sand after a backwash is dirty, and without a rinse it goes straight back into the pool.
Leaving the valve on backwash, rinse or waste and walking away. The pool can drop below the weir faster than you would expect.
Running the pump on Closed, or with a valve shut on the return line. That builds excessive pressure in the filter and can damage the pump.
Vacuuming heavy dirt or dead algae on Filter. Use Waste for heavy loads so the dirt leaves the system instead of clogging the sand.
Forgetting to top up afterwards.

Is your valve leaking?

A healthy multiport valve sends nothing out of the backwash line while it is on Filter. If the backwash pipe is always damp, or water trickles out of it constantly, the spider gasket is probably worn. Other signs are a handle that will not seat firmly, water weeping around the valve head, or a pool that loses water faster with the pump running than with it off. A worn gasket or valve head is a common repair, and our pool repairs service covers leaking multiport valves, weirs and fittings. A call-out to inspect or diagnose is R650. Deducted from your quote when you accept it.

When backwashing stops helping

Backwashing cleans the sand; it does not renew it. Over the years, filter sand wears smooth, clumps together with calcium and body oils, and forms channels that water runs straight through without being filtered. The signs are easy to spot once you know them:

The gauge climbs back up soon after every backwash
The clean reading after a backwash is higher than it used to be
The water stays dull even though the chlorine and pH are correct
Sand appears on the pool floor near the returns, which usually means a broken lateral inside the tank

At that point no amount of backwashing will fix it. Our article on when to change pool filter sand explains what to look for, and our sand change and filter service is priced by your filter's bag count, from R1,450.

Getting help

If you would like us to look at your filter, show you how your valve works, or take the job off your hands, WhatsApp us on 081 577 5051. Send a photo of the filter and valve, the gauge reading after a backwash, and your area. On our weekly maintenance plans the filter is backwashed and rinsed when the pressure says it needs it, so it becomes one job you no longer have to remember.