How many hours a day should the filter run?
A reasonable residential starting point is eight to twelve hours a day, every day the pool is open and the water is above roughly 70 °F. That is a starting point, not a rule: the correct figure is whatever achieves at least one full turnover, and preferably more, given your pool's volume and your pump's flow rate.
Two things push the number up. Warm water consumes sanitiser faster and grows things faster, so a pool at 88 °F needs materially more run time than the same pool at 78 °F. Heavy bather loads do the same. Put the pump on a timer so the decision is made once rather than every morning, and revisit the timer when the season changes.
Should the filter run during the day or at night?
During the day. That is when sunlight is destroying chlorine, when the water is warmest, when swimmers are in it and when debris is blowing into it. Night running is quieter and, on time-of-use electricity tariffs, cheaper — so if the tariff difference matters, split the schedule and keep a meaningful block during daylight rather than moving the whole run to the small hours.
Is running the filter 24 hours a day necessary?
Not necessary, but not wasteful either, and it has two real advantages. Continuous running puts far less stress on the motor than repeated starts, and owners who run continuously consistently report fewer cloudy, hazy and green episodes. With a variable-speed pump, running continuously at low speed is often cheaper than running eight hours at full speed, which makes the question largely moot.
How often should I backwash?
Only when the pressure tells you to — roughly 8 to 10 psi above the pressure the gauge showed when the filter was last properly cleaned. Backwashing to a calendar is one of the most common self-inflicted causes of hazy water.
The reason is counter-intuitive. A perfectly clean sand or DE filter is at its least effective. As the media loads with debris, the passages between grains narrow and the filter begins trapping finer and finer particles. Backwash it every week and you reset it to its worst state every week, and it never gets the chance to polish the water. The filter needs to be allowed to get dirty in order to work well.
There is also no need to backwash after vacuuming as a matter of routine. Vacuum, check the gauge, and act on what it says.
Is chemical cleaning of the filter really necessary?
Yes, and it is the maintenance step most often skipped. Backwashing and hosing move loose debris. They do nothing about the film of sunscreen, body oil, hair conditioner, cosmetics, pollen and tree sap that accumulates on the media and inside cartridge pleats. That film is greasy, water will not lift it, and it progressively blinds the filter.
The useful analogy is laundry. Backwashing is rinsing a shirt under a tap; chemical cleaning is actually washing it. You can rinse a shirt many times and it will still be a dirty shirt.
Symptoms of a chemically fouled filter are distinctive: pressure that returns to a high reading within days of a backwash, water that will not clear no matter how much clarifier is added, a filter that "needs backwashing" constantly, and — in cartridge systems — pleats that feel slick rather than gritty. Twice a season is a common interval; once a season is the minimum for a pool that gets real use.
Why not just use acid to clean the filter?
Because acid dissolves mineral scale and does nothing whatever to oils and organic film — and if you apply acid to a filter that is oil-fouled, the acid seals the oils onto the media and makes the problem permanent. The correct sequence when both scale and oil are present is a degreasing filter cleaner first, then an acid rinse only if scale remains.
Acid also attacks the internal components of many filters, and neat acid on a DE grid or a cartridge fabric will shorten its life considerably. Where a manufacturer permits an acid soak, they specify a dilution — follow it. General handling precautions for concentrated acids are the ones set out in the OSHA overview of chemical hazards: eye protection, ventilation, acid into water and never the reverse.
Sand, cartridge or DE — what is the practical difference?
| Sand | Cartridge | Diatomaceous earth | |
|---|---|---|---|
| Typical particle capture | Coarsest | Middle | Finest |
| Routine service | Backwash | Remove and hose | Backwash and recharge |
| Water used per service | High | Low | High |
| Media replacement | Sand every several years | Cartridge every 1–3 seasons | Powder every backwash; grids every several years |
| Tolerance of undersizing | Moderate | Poor — an undersized cartridge is a common cause of persistent haze | Good |
No filter type rescues a pool with bad chemistry or short run times, and every filter type will keep a well-run pool clear. The most consequential decision is sizing, not type: a filter chosen at the bottom of its range will be marginal on the hottest week of every year for as long as you own it.
The filter did not cause it
When water goes cloudy, the filter is blamed first and is responsible perhaps a third of the time. Before deciding the filter has failed, confirm that the pump is running long enough, that the baskets are clear, that the returns are actually returning, that pH and sanitiser are in range, and that the filter has had a real chemical clean this season. If all of that is true and the water is still hazy, then look at the filter — and look at its size before its condition.
Related: circulation, pump run hours, cloudy water.