Setting your pool filtration properly
The question comes up at every commissioning: how many hours? The rule you hear everywhere — the temperature divided by two — gives a correct starting point, and stops there. It ignores your volume and your pump’s real flow rate. Those two figures are enough to set the adjustment.
The starting reference: the temperature divided by two
The best-known rule is to filter for a number of hours equal to half the water temperature. Water at 24 °C: about 12 hours a day. Water at 30 °C: about 15 hours.
It is a good starting point, and that is all. This rule takes account neither of your pool’s volume, nor of your pump’s real flow rate, nor of how much the pool is used. In the Moroccan climate, where the water easily exceeds 28 °C in summer, it mechanically pushes towards long durations that are better checked.
The real criterion: turning the volume over in 4 hours
Professional sizing rests on the filtration cycle: the most widespread convention is that the installation should be able to pass the whole volume of the pool through the filter in about four hours. Your installer may adopt another basis depending on the pool and the equipment.
The calculation is simple:
Required flow rate (m³/h) = Pool volume (m³) ÷ 4
A 40 m³ pool therefore calls for a pump capable of 10 m³/h. A 30 m³ pool, 7.5 m³/h. And conversely, if you know your pump’s flow rate, the duration of a full cycle is:
Cycle duration (h) = Volume (m³) ÷ Flow rate (m³/h)
The trap: the stated flow rate is not the real flow rate
This is the most common mistake. The flow rate given by the manufacturer is a nominal rate, measured with no pipework. Once installed, the pump is subject to the head losses of the pipes, bends, valves and filter: it is not unusual to lose a significant share between the stated flow rate and the real one.
In other words, in practice a pump delivers less than its stated flow rate. This has to be taken into account when choosing it, not afterwards.
Filter during the day, and preferably continuously
Filtration must cover the hottest hours and the bathing hours, that is to say the middle of the day. A continuous cycle is better than several short split cycles: every restart puts part of what had settled back into suspension.
The special case of the variable-speed pump
A variable-speed pump changes the trade-off completely. The affinity laws, which govern centrifugal pumps, mean that halving the flow rate divides consumption by eight. In a real installation the gain is smaller — head and motor efficiency come into play — but it remains considerable. It therefore becomes more economical to filter for a long time at low speed than for a short time at full power, with finer filtration as a bonus, since the water passes through the medium more slowly.
Be careful, however, if you have a salt chlorinator: it needs a minimum flow rate to produce, which depends on the cell installed. A badly chosen low speed cuts off chlorine production with nothing to indicate it. This is a setting to make with both units in front of you: see the minimum flow rate required by an electrolysis cell.
What should make you adjust
- Water that clouds after a busy day: extend temporarily.
- After a storm or a sandstorm: run the filtration continuously for 24 to 48 hours — see the maintenance schedule in a hot climate.
- Filter pressure risen more than 0.3 bar above its clean value: what is needed is a backwash, not more filtration time.
- Clear, stable water: try cutting one hour and watch for a week.
The right setting is found with two figures in front of you: the exact volume of the pool and the pump’s real flow rate, with the pipework connected. If you do not have them, they can be measured in one visit — including on a pool we did not build.