Filtration time: the temperature ÷ 2 rule and its traps
- Starting point: water temperature ÷ 2 = hours of filtration per day (water at 24 °C → ~12 h).
- The rule has limits: above ~28 °C you filter much more (even continuously in a heatwave), and never below a minimum floor.
- During a treatment: continuous filtration, full stop.
- Filter during the day (when UV + heat + swimmers attack the water), not at night "for the cheap tariff".
How many hours to filter per day? The starting point everyone should know: water temperature ÷ 2. Water at 24 °C → about 12 hours of daily filtration. Simple, effective… and misleading if you stop there, because the rule has three blind spots: heatwaves (above ~28 °C, biology goes into overdrive and you need to filter much more, sometimes continuously), treatments (continuous filtration, whatever the thermometer says), and the floor (even in cold water, you don’t drop below a daily minimum). The right filtration time therefore literally changes every day — which is why the CliniPool app recalculates it continuously with YOUR pool’s weather.
Why filtration is so decisive
Chemical sanitisation gets the headlines, but filtration does the groundwork: it’s what physically removes particles, dead algae and debris, and it’s what circulates the water to carry the sanitiser everywhere. Still water is water that stratifies: chlorine at the injection point, nothing in the dead corners. Most “mysterious” cloudy water is a filtration deficit, not a product problem.
The ÷ 2 rule and its three corrections
Correction 1 — heat. The rule is roughly linear; biology isn’t. Above ~28 °C, algae and bacteria accelerate sharply while chlorine burns out faster: extend considerably, and at the peak of a heatwave go continuous. (The heatwave guide, under “Also worth reading”, details this survival mode.)
Correction 2 — treatments. Shock, green water rescue, flocculation: continuous filtration until the process is over. It’s the filter that removes what the chemistry destroyed.
Correction 3 — the floor. Water at 8 °C in active winterising? You still don’t switch everything off: a daily minimum keeps the water moving and the circuit healthy.
When to filter: during the day — and it matters
Water degrades when it’s under attack: UV, heat, swimmers — all of it happens in the daytime. So daytime is when the water needs to circulate and be sanitised. Night-time filtration “for the off-peak tariff” sanitises water nothing is attacking, then leaves the pool motionless through the afternoon assault. If the tariff matters, split it: the bulk during the day, the top-up at night.
Time isn’t everything
Three checks complete the picture: the filter pressure (clogged, it filters poorly even 24/7 — clean it when the pressure climbs above its normal), the media condition (worn sand or a tired cartridge lets the fines through), and the sizing (ideally, the pool’s volume should pass through the filter in 4 to 6 hours). The app cross-references it all: continuous temperature, weather, treatments in progress — and shows you each day’s right filtration time, with a live countdown.
Frequently asked questions
How many hours should a pool at 26 °C be filtered?
The basic rule says ~13 h (26 ÷ 2). It's a good starting point; adjust for bathing load and weather. Approaching 28 °C, don't hesitate to extend considerably.
Should you filter at night or during the day?
During the day: it's when the water is under attack (UV, heat, swimmers) that it needs to circulate and be sanitised. Filtering at night for the electricity tariff means sanitising when there's nothing to sanitise.
Does filtering continuously wear out the pump?
No, pumps are built for it — many pools (salt, variable-speed) filter almost continuously. What wears a pump out is very frequent starts and running dry.
Why does my water turn even though I filter "enough"?
Time is only part of the equation: a clogged or worn filter filters poorly however long it runs, and unbalanced chemistry turns even with perfect filtration. Time + filter condition + water balance form a whole.