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The pressure gauge on your filter is climbing, the water is losing its sparkle and the flow from the return jets feels weaker than it used to. Nine times out of ten, the answer is a good backwash. If you are wondering how to backwash a pool filter properly, and how often it should be done, this guide takes you through it in plain English.
At HiLife Pools we install and look after pool filtration, and backwashing is one of the simplest jobs that makes the biggest difference to water quality.
What does backwashing a pool filter do?
Your filter traps dirt, oil, hair and tiny particles as the water passes through sand or glass media. Over time the media becomes clogged and the pressure builds. Backwashing reverses the flow of water through the filter so that the trapped dirt is flushed out to waste. It cleans the media without needing to open the tank.
It applies to sand and glass media filters. Cartridge filters are cleaned differently, and our guide on how to clean a pool filter covers those.
How do you know when to backwash?
The best clue is the pressure gauge. Note the reading when the filter is clean, and backwash when it rises by about 7 to 10 psi (0.5 to 0.7 bar) above that starting point. Other signs include cloudy water, a drop in flow, or a pool that takes longer to clear after heavy use. As a rule, most domestic pools need a backwash every one to two weeks in the swimming season, and more often after a storm, a party or an algae problem.
What you need before you start
- A multiport valve, or a push-pull valve on your filter.
- Access to a waste outlet or a backwash hose that runs to a suitable drain.
- A few minutes, and a little extra water to top up afterwards.
How to backwash a pool filter, step by step
- Switch off the pump. Never move the valve while the pump is running, as it can damage the valve seals.
- Set the valve to backwash. On a multiport valve, turn the handle to the backwash position. Check your manual if the labels are unclear.
- Switch the pump on. Water now flows backwards through the filter and out through the waste line. You may see dirty water at first.
- Watch the sight glass. Run the backwash for around two to three minutes, until the water in the sight glass runs clear.
- Switch off the pump again. Then turn the valve to rinse.
- Rinse for about 30 seconds. This settles the media and clears any loose dirt from the pipework so it does not return to the pool.
- Switch off, set to filter, and restart. Move the valve back to the normal filter position, switch the pump on and check the pressure has dropped.
Common mistakes to avoid
- Backwashing too often. A slightly dirty filter actually traps finer particles, so there is no need to backwash on a fixed day if the pressure is still fine.
- Running it for too long. This wastes water and can lower the pool level more than you expect.
- Moving the valve with the pump on. Always switch off first.
- Forgetting to rinse. Skipping this step can send dirty water straight back into your pool.
What if the pressure stays high after backwashing?
If the gauge is still high, the media may be caked with oils or scale and need a chemical filter clean. Sand also wears out and usually needs replacing every five to seven years. If the water remains cloudy, check out our guide on cloudy pool water, and review the rest of your routine using our pool maintenance checklist.
Where does the backwash water go?
Backwash water is usually chlorinated, so it should go to a suitable waste outlet rather than a garden bed or stream. Check with your local water company if you are unsure, and avoid sending it into a septic tank.
Frequently asked questions
How often should I backwash my pool filter?
Typically every one to two weeks in the season, or when the pressure rises 7 to 10 psi above the clean reading.
Can I backwash a cartridge filter?
No. Cartridge filters are removed and rinsed instead. Backwashing is for sand and glass media filters.
How long should I backwash for?
Usually two to three minutes, or until the water in the sight glass is clear.
Should I add chlorine after backwashing?
Not normally, but test the water and top up the pool level, as backwashing removes some water.
Questions about your filtration?
If your filter is struggling or you are not sure what type you have, get in touch with the HiLife team. We are happy to help.
