How to backwash your pool filter
A backwash sends water backwards through the sand and flushes the dirt the filter has trapped to waste. It takes a few minutes, is done when the pressure gauge reading rises, and always with the pump off when you change valve positions.
When it's needed
Your guide is the pressure gauge on the filter. Right after a backwash, note the pressure: that is your clean-filter pressure. As the sand fills with dirt, the pressure rises. When it is about 0.5 bar above the starting figure, or whatever your filter manufacturer specifies, it is time to backwash.
Other signs are weak return inlets and skimmers that pull less. The need comes sooner after a storm, Saharan dust, lots of swimmers, heavy vacuuming or a shock chlorination in cloudy water. A backwash is also done before winterizing and at the spring opening.
But don't do it "just to be safe". A sand filter with a little dirt in it catches very fine particles better than a freshly washed one, and every backwash means lost water and chemicals.
The multiport valve positions
| Position | What it does |
|---|---|
| Filter | Normal operation: water passes through the sand and returns to the pool |
| Backwash | Water passes backwards through the sand and goes to waste along with the dirt |
| Rinse | The sand settles again and the first cloudy water goes to waste |
| Waste | Water goes from the pool straight to waste, bypassing the filter; for lowering the water level or vacuuming heavy silt |
| Recirculate | Water returns to the pool without passing through the sand |
| Closed | Shuts off the flow; never with the pump running |
Some valves also have a winter position. On most valves the positions are labelled in English, as above (Filter, Backwash, Rinse, Waste, Recirculate, Closed).
The steps in order
- Switch off the pump. Never turn the handle with the pump running, because it wears the valve's internal gasket.
- Open the valve to the drain line, if your system has one.
- Press down and turn the handle to "Backwash".
- Start the pump and watch the valve's sight glass. At first the water is cloudy or brownish. As soon as it runs clear, usually after 2–3 minutes, switch off the pump.
- Turn to "Rinse" and run the pump for about half a minute to a minute, so the sand settles again and no cloudy water goes back into the pool. Switch off.
- Return to "Filter", close the drain valve if there is one, start the pump and note the new pressure.
- Top up the water you lost, so the level is back at the middle of the skimmer.
Common mistakes
- Skipping the rinse: the first cloudy water goes back into the pool and shows as a cloud at the return inlets.
- Running it too long: once the water in the sight glass is clear, you gain nothing, you only waste water.
- Sending the water to the wrong place: backwash water must go to a drain point planned for it, not onto the neighbour's property or the street.
- Pressure that stays high even after backwashing: this usually means clumped sand or a blockage in the circuit. It may be time for a filter sand change.
- Sand on the floor after backwashing: the valve gasket or a broken lateral inside the filter may be to blame; it needs checking by a technician.
Glass media, cartridges and water renewal
With glass media the process is exactly the same. Cartridge filters have no backwash: the cartridge is taken out and rinsed with a hose. You'll find the differences between the types in pool filters.
Each backwash also renews a small part of the water, which is useful since a liner pool is never drained (see why we don't drain the pool). If the water stays dull despite backwashing, the reason is usually the chemistry: see cloudy water.
Questions
How much water is lost in a backwash?
It depends on the filter size and the pump's flow rate, but in a few minutes enough water goes out for the level to drop noticeably in a small pool. That is why you backwash when the pressure gauge calls for it, not out of habit.
Why does cloudy water come into the pool right after a backwash?
Usually because the rinse was skipped or was too short. Let the filter run for a few hours and, if you see sand on the floor, have the valve and the filter laterals checked.
See also
Pool filter
Sand, glass media or cartridge pool filters: how each one works, how to choose the size and what the pressure…
Filter sand change
Changing pool filter sand: roughly how often, the signs that call for it, the process step by step, and when…
Cloudy water
Cloudy pool water: how to tell whether filtration, pH, hardness or dust is to blame, and when a clarifier or…