Here’s how to backwash a pool filter: switch the pump off, turn the multiport valve to Backwash, run the pump for 2 to 3 minutes until the sight glass runs clear, then switch off, rinse for 30 to 60 seconds and return the valve to Filter. The one rule that matters most is switching the pump off before every turn of the handle.
That’s the short version. Below we go through each step, when to backwash (hint: it’s the pressure gauge, not the calendar), what every multiport valve setting does, and how to save water while you’re at it. If your filter isn’t behaving even after a proper backwash, our pool filter repairs team can find out why.
What it means to backwash a pool filter
A sand or glass media filter cleans the water by trapping dirt in the media bed as water flows down through it. Over time that dirt clogs the bed, flow drops and the pressure gauge creeps up.
When you backwash a pool filter, you reverse the flow. Water goes up through the media instead of down, lifts the bed, shakes the trapped dirt loose and sends it out of the waste pipe. The rinse step that follows settles the bed back into place so dirt doesn’t blow back into the pool.
Cartridge filters are different. They don’t backwash at all; you remove the cartridge and hose it clean. Everything below applies to sand and glass media filters with a multiport valve, which is what most South African home pools have.
How to backwash a pool filter, step by step
Before you start, empty the pump basket and skimmer baskets. Make sure the backwash outlet or hose is connected and pointed where it should go. Then follow these steps in order:
- Switch the pump off. Use the switch or isolator, not just the timer. Never turn the multiport handle while the pump is running.
- Note the pressure. Look at the gauge and remember the reading. It helps you judge whether the backwash worked.
- Set the valve to Backwash. Push the handle down and rotate it until it clicks into the Backwash position.
- Switch the pump on. Watch the sight glass on the side of the valve. The water will look cloudy or brown at first.
- Run it for 2 to 3 minutes. Stop when the sight glass runs clear. Going longer just wastes water.
- Switch the pump off again.
- Set the valve to Rinse. Push down and rotate until it clicks.
- Switch on for 30 to 60 seconds. This resettles the bed and flushes loose dirt out to waste, not into the pool.
- Switch off, set the valve to Filter and switch the pump back on.
- Check the gauge. It should be back near its clean reading. Top up the pool if the level has dropped below the middle of the skimmer mouth.
The whole job takes about five minutes. If you’re filling in new media, the refill process in our guide on how much sand for a pool filter ends with exactly this backwash and rinse.
Why the rinse setting matters
Skipping the rinse setting is the most common shortcut, and it shows. Straight after a backwash the media bed is loose and the pipework under it still holds dirty water. Switch straight to Filter and that first slug of dirt goes back into the pool as a puff of cloud at the return jets. Thirty seconds on Rinse prevents it.
Pump off for every turn. Turning the multiport handle with the pump running forces water against the rubber spider gasket inside the valve and tears it. A torn gasket is why so many valves end up dripping water to waste all day.
When to backwash a sand filter
When to backwash depends on the pressure gauge, not the day of the week. Write down the pressure reading straight after a good backwash, when the filter is clean. That’s your baseline. When it rises about 0.5 to 0.7 bar (8 to 10 psi) above that, it’s time.
For many Cape Town pools that works out to every one to two weeks in summer. You’ll need to backwash sooner after:
- A heavy south-easter that blows dust, sand and leaves into the pool.
- Winter storms that wash soil and runoff in from the garden.
- Vacuuming a lot of dirt, or clearing a green pool with flocculant.
- A pool party or a busy holiday week with lots of swimmers and sunscreen.
Don’t backwash too often. A slightly dirty bed actually filters finer than a perfectly clean one, so backwashing every couple of days wastes water and makes the water less clear. If the pressure climbs back up within a day or two, something else is wrong. Our guide to pool filter pressure that is too high covers the usual causes.
A gauge that hardly moves can mislead you too. If the needle is stuck or the reading is very low, read about low pool filter pressure before you assume the filter is clean.
Filter pressure still high after backwashing?
We check the media, valve, gauge and laterals, tell you what is wrong and quote before any work starts.
Multiport valve settings explained
Most South African sand filters use a six-position multiport valve. Here’s what each of the multiport valve settings does and when you’d use it.
| Setting | Water flow | When to use it |
|---|---|---|
| Filter | Down through the media and back to the pool | Normal running, most of the time |
| Backwash | Up through the media and out to waste | When pressure is 0.5 to 0.7 bar above clean |
| Rinse | Down through the media and out to waste | 30 to 60 seconds after every backwash |
| Waste | Bypasses the filter, straight out to waste | Vacuuming heavy dirt or lowering the water level |
| Recirculate | Bypasses the filter, back to the pool | Mixing chemicals if the filter is broken |
| Closed | No flow at all | Servicing the pump. Never run the pump on Closed. |
Some older filters have a slide or push-pull valve instead of a multiport. The principle is the same: pump off, pull the plunger up for backwash, push it down for filter. Check your manual if you’re unsure.
How to backwash a pool filter with less water, and Cape Town rules
A typical home backwash and rinse uses a few hundred litres of pool water, more on bigger filters or if you run it too long. That water has to be replaced, which is why water restrictions in Cape Town have always made backwashing a talking point.
Ways to use less:
- Backwash on pressure, not on a schedule.
- Stop as soon as the sight glass clears, and keep the rinse short.
- Keep a solar cover on to cut down the dust and leaves that clog the filter.
- Consider glass media, which tends to backwash clean faster than sand. Compare them in our glass filter media vs sand guide.
Where the water goes matters as much as how much. The City of Cape Town has said backwash water is high in chlorine and must go into the sewer system, not the stormwater drains, the street or a neighbour’s garden, and owners who discharge into stormwater can be fined. Some households also run backwash recycling tanks to reuse the water.
When backwashing doesn’t fix the filter
Sometimes you backwash a pool filter properly and nothing improves. These are the faults we find most often:
- Water trickling out of the backwash pipe all the time. That’s a worn spider gasket. See our guide to a multiport valve leaking to waste.
- Sand in the pool after backwashing. Usually a cracked lateral or standpipe in the filter, or too much media.
- Pressure won’t come down. The media may be caked with oils or calcium and need replacing.
- Cloudy water despite good chemistry. Worn media that no longer traps fine particles.
- Handle stiff or won’t seat in position. A damaged valve that needs a service or replacement.
These are repair jobs rather than backwash jobs. Our team handles pool equipment repair on the full filter station, and can add filter checks to regular pool maintenance visits.
How to backwash a pool filter: backwash pool filter questions
How to backwash a pool filter without damaging it?
Always switch the pump off before you move the multiport handle, every single time. Backwash for 2 to 3 minutes until the sight glass runs clear, rinse for 30 to 60 seconds, then switch off and return the valve to Filter. Turning the handle under pressure is what breaks the spider gasket inside the valve.
How long should I backwash a pool filter?
Usually 2 to 3 minutes, or until the water in the sight glass changes from cloudy to clear. Going much longer just wastes water. Follow it with 30 to 60 seconds on Rinse.
How often should you backwash a sand filter?
Backwash when the pressure gauge reads about 0.5 to 0.7 bar (8 to 10 psi) above its clean reading, rather than on a fixed day. For many Cape Town pools that works out to every one to two weeks in summer, and after storms or a dusty south-easter. Backwashing too often actually makes filtering worse.
Can you backwash a cartridge filter?
No. Cartridge filters don’t have a backwash setting. You switch the pump off, take the cartridge out and hose it clean, then soak it in a filter cleaner every few months.
Where should pool backwash water go in Cape Town?
Into the sewer system, not the stormwater drain, street or a neighbour’s property. The City of Cape Town has said backwash is high in chlorine and must not enter stormwater, and owners can be fined. If your backwash pipe runs to a stormwater grid, ask us about re-routing it.
If your filter keeps clogging, leaks to waste or sends sand back into the pool, book our sand filter repair service and we’ll sort out the cause rather than the symptom.
Get your filter checked
Send us a photo of your filter, valve and gauge reading with your suburb. We will tell you whether it needs a service, new media or a new valve.
