Domakihub How to Backwash a Pool Sand Filter the Right Way
A simple, safe routine to clear trapped dirt and restore strong water flow.
Reviewed by Domakihub pool pros · 2026-06-23 · 4 min read
Backwashing reverses the water flow through your sand filter to flush out the dirt it has trapped, and it is one of the easiest jobs in pool care once you know the order of the valve positions. The video above shows the whole process on a real multiport valve, and this article gives you the clear written steps plus the small details that matter for homes, holiday rentals, and hotels across Greece. Follow it and you will keep water flow strong and your pool clear through the long Greek swimming season.
What backwashing actually does
A sand filter works by pushing pool water down through a bed of fine sand that catches dirt, oils, and fine debris. Over time that trapped grime clogs the bed, the filter pressure rises, and water flow to the pool drops. Backwashing simply reverses the flow so clean water lifts the dirt up and out through the waste line, leaving the sand ready to filter again.
Backwashing is not the same as replacing the sand or deep-cleaning it. It is routine housekeeping, and on a busy rental or hotel pool you may need it more often than on a quiet family pool, especially after a heatwave, a pollen burst, or a crowd of guests.
When to backwash and why timing matters
The clearest signal is the pressure gauge on the filter. When it climbs roughly 0.3 to 0.5 bar (about 8 to 10 psi) above the clean starting reading you noted after the last backwash, it is time. Weak returns, cloudy water, or a long run since the last clean are all supporting signs.
Do not backwash more than you need to. The process sends pool water, and the chemicals in it, straight to waste, so over-backwashing wastes water and money, something worth watching closely in Greece where summer water is precious and some islands face restrictions. Backwash on need, not on a fixed daily habit.
The backwash routine, step by step
- 1
Turn the pump off first
Always switch the pump off before you move the multiport valve handle. Turning the valve while the pump is running can tear the internal gasket (the spider gasket) and cause leaks between ports. Note the current pressure reading, then power down and let the system settle for a moment.
- 2
Set the valve to Backwash and prime the waste line
Press down on the multiport handle and rotate it to the Backwash position. If you have a backwash hose, roll it out to your drain or legal discharge point now and secure it, because the flow comes out with force. Make sure any waste valve is open.
- 3
Run the pump until the water runs clear
Switch the pump back on and watch the sight glass on the valve. The water will look dirty or cloudy at first. Keep running, usually about two to three minutes, until the glass runs clear. That clear water is your signal that the sand bed is flushed.
- 4
Stop, switch to Rinse, then run briefly
Turn the pump off again. Move the handle to the Rinse position, then run the pump for about 20 to 30 seconds. Rinse resettles the sand bed and clears loose dirt from the plumbing so it does not blow back into the pool when you resume filtering.
- 5
Return to Filter and top up the water
Turn the pump off one last time, set the handle back to Filter, and restart the pump. Check that the pressure gauge now reads lower, near your clean baseline. Backwashing lowers the pool level, so top up the water and rebalance the chemistry if needed.
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FAQ
- How often should I backwash my pool sand filter?
- Backwash by pressure, not by calendar. When the filter gauge rises about 0.3 to 0.5 bar (8 to 10 psi) above the clean reading you noted last time, it is due. For a busy holiday rental or hotel pool in peak Greek summer that can mean every one to two weeks, while a quiet home pool may go longer. Avoid backwashing more than necessary, as it sends water and chemicals to waste.
- Why is the pool cloudy right after I backwash?
- A brief cloudiness is normal because freshly disturbed sand and loose dirt can blow back into the pool when filtering restarts. The Rinse step is what prevents most of this, so never skip it. If the water stays cloudy for hours, your chemistry may be off or the sand may be old and channelled, in which case it is worth checking the balance or considering a sand change.
- Do I need to add chemicals after backwashing?
- Usually yes, at least a top-up. Backwashing removes a noticeable volume of treated water that you replace with fresh, untreated water, which dilutes your sanitiser and can shift pH and stabiliser levels. After topping the pool back up, test the water and adjust chlorine, pH, and alkalinity so the pool stays balanced and safe for swimmers.