Pool filter: how long to run it and how to backwash
How many hours a day to run your pool filter, how to work it out from your pool size, and step-by-step backwashing.
2 min read
Chlorine kills the dirt, but it's the filter that takes it out of the water. Not running the filter long enough is one of the most common causes of cloudy water and high chemical costs.
How many hours a day
The rule is simple: the whole volume of the pool should pass through the filter at least once a day. To work out how long that takes, divide the pool volume (in m³) by the pump's flow rate (in m³ per hour, shown on the label or in the manual).
- Example: a 30 m³ pool (30,000 litres) with a 6 m³/h pump: 30 ÷ 6 = 5 hours a day.
- In summer, with strong sun or lots of swimmers, run it longer: 8 hours is a good number.
- In winter, when nobody swims, you can cut back, but don't leave the water still for days.
With a salt chlorinator, chlorine is only made while the pump runs. Short run times mean low chlorine.
When to backwash
Backwashing reverses the water flow and flushes out the dirt trapped in the filter sand. Do it when the pressure gauge rises about 50 kPa (7 psi) above its reading with a clean filter (or reaches the zone shown in the manual), after vacuuming heavy dirt and after treating green water.
Always turn the pump off before changing the position of the filter valve. Turning the valve with the pump running damages it and can cause accidents.
Step-by-step backwash (sand filter)
Turn the pump off.
Set the valve to BACKWASH.
Run the pump for 2 to 3 minutes, until the water in the sight glass runs clear.
Turn the pump off and set the valve to RINSE.
Run it for 30 to 60 seconds and turn it off again.
Set the valve back to FILTER and turn the pump on.
Backwashing takes water out of the pool: top up the level afterwards if needed. Cartridge filters don't backwash: take the cartridge out and hose it down from top to bottom.
Filter sand wears out too. Follow the replacement interval in your filter's manual.
The right dose depends on the size of your pool and the product you have. The calculator works it out in spoons and cups.
Calculate the dose now