Sand filters, and the flow rate that is not on the box

What a sand filter actually does

Water is pushed down through a bed of sharp silica sand. Dirt lodges between the grains, and filtered water leaves through laterals at the bottom. When the bed clogs, you reverse the flow and send the trapped dirt out of the waste line. That is backwashing, and it is the entire argument for sand over cartridges: no rinsing a pleated element every fortnight, no buying replacements.

The cost of that convenience is fineness. Leslie’s puts sand at 20 to 40 microns, against 10 to 20 for cartridges and 3 to 5 for DE, and In The Swim publishes the same ranges. Sand is the coarsest of the three. In a properly balanced above-ground pool that is invisible; on a pool fighting fine particulate, it is why the water stays faintly hazy, and it is why our clarifier guide exists.

The number that decides everything: system flow

Every sand filter is advertised on the pump’s flow rate. That figure is measured at the pump, not through a tank full of sand. Intex is unusually straight about this, publishing a 3,000 gph pump flow and a 2,450 gph system flow on the same page. The second number is the one that turns your pool over.

Hayward sidesteps the issue by publishing in GPM and by filter area: 30 GPM through 1.4 square feet on the 16 inch system, 35 GPM through 1.75 square feet on the 18. Bestway publishes a single gallons-per-hour figure and no horsepower at all.

To size a filter you need your pool volume and a target turnover. Our pump sizing guide does that arithmetic properly, and our run-time guide converts it into hours a day. The short version: divide your gallons by the system flow, and if the answer is more than eight hours you are under-filtered.

How much sand, and what grade

This is the difference nobody mentions until delivery day. The Bestway units take about 19 to 20 lb. The Intex takes 120 lb. The Hayward 16 inch takes 100 lb and the 18 inch takes 150 lb. That is the difference between one bag and three, and it repeats every few years.

The grade is specific and it is not optional. Intex specifies No. 20 silica at 0.45 to 0.85 mm with a uniformity coefficient under 1.75; Hayward specifies No. 20 silica at 0.45 to 0.55 mm. Bestway’s manuals go further and state that using the wrong size reduces filtering performance and may damage the filter, voiding the warranty.

Sand does not last forever, because the grains round off and stop catching dirt. Intex publishes four to five years under normal conditions, two to three where the air is dusty, and eight to nine years for glass media. That is a real running cost, and it is covered in our filter media guide.

Alternative media, and who actually approves it

Glass and filter balls are both sold as sand replacements. Whether they are allowed depends entirely on which filter you own, and the three makers here take three different positions.

Intex approves glass explicitly and publishes a capacity for it: 85 lb, against 120 lb of sand, with a stated particle size of 0.4 to 0.8 mm. Bestway’s manuals void the warranty for media other than the specified sand and mention no alternatives. Hayward publishes nothing either way — its manual specifies No. 20 silica and is silent on substitutes.

Filter balls are a separate question again, and one where the marketing has run well ahead of the evidence. Our filter balls roundup goes through the claims in detail; the summary is that no filter manufacturer here publishes an approval for them.

The running cost nobody budgets: backwash water

Every backwash sends filtered pool water down the drain, and it has to be replaced and re-treated. The City of Chandler, Arizona puts a typical backwash at 250 to 1,000 gallons. Intex gives a duration rather than a volume: backwash until the sight glass runs clear, about three minutes.

On a small frame pool, 250 gallons is a meaningful fraction of the whole pool, and it takes your chemistry with it. That is the honest case for a larger tank on a larger pool: more filter area means longer between backwashes, and fewer backwashes means less water and less chemistry replaced. Our backwashing guide covers the rinse step most owners skip, which is what sends dirty water back into the pool.

Which one belongs on your pool

Under about 5,000 gallons: the Bestway 800. It is the only unit here sized for genuinely small pools, and 19 lb of sand suits a pool that comes down each autumn.

5,000 to 11,000 gallons, seasonal frame pool: the Bestway 1,500 if the pool is storable, the Intex if it is bigger or if you want the timer and the glass-media option. Note Bestway’s own restriction: the 1,500 is specified for storable pools, not permanent installations.

A permanent steel-wall pool: a Hayward. The difference is not the flow, it is that the tank is rated to 50 PSI, the valve is serviceable, the parts are stocked, and there is a published two-year parts warranty. On a pool you intend to keep for a decade, that is the relevant specification, and our permanent pools roundup is where those pools live.

Over about 15,000 gallons: the Hayward 18 inch, for the filter area rather than the flow. Our large-pool equipment guide covers what else changes at that size.

How we picked

We have not tested these filters. Scores are our reading of published specifications, with credit for publishing the unflattering numbers: Intex for its system flow, Hayward for its horsepower and pressure rating. Bestway publishes neither horsepower nor wattage for either unit, and no manufacturer here publishes pump wattage at all — so any running-cost figure you see quoted for these filters was not read off a maker’s page.

Backwashing properly, and the step everyone skips

A sand filter only stays effective if the bed is cleaned, and cleaning it is a sequence rather than a switch. Intex’s manual gives the endpoint rather than a duration: backwash until clean, clear water runs through the sight glass or drain outlet, about three minutes.

The step that gets missed is the rinse afterwards. Backwashing lifts and turns the bed; if you go straight back to filter, the first thing the pool receives is the disturbed sand and debris sitting above the laterals. Every six-function valve here has a rinse position for exactly that reason, and our backwashing guide covers the full sequence.

Two habits make the job cheaper. Note the clean pressure when the filter is fresh and write it on the tank, so you are backwashing on a real change rather than a schedule. And backwash after vacuuming rather than before, since vacuuming is what actually loads the bed.

The sand itself: grade, life and the refill job

The grade is specified, not suggested. Intex asks for No. 20 silica at 0.45 to 0.85 mm with a uniformity coefficient under 1.75; Hayward’s ProSeries manual asks for No. 20 silica at 0.45 to 0.55 mm. Bestway’s manuals state that the wrong size reduces performance, may damage the filter and can void the warranty.

Sand does not dissolve, it rounds. Grains that arrive sharp are worn smooth by years of water and backwashing, and smooth grains let dirt through. Intex publishes the interval plainly: four to five years for residential filters under normal conditions, and two to three years in dusty conditions or where frequent backwashing is needed. Hayward and Bestway publish no interval at all.

Replacing it is the job nobody warns you about. Wet sand has to come out of a tank through an opening designed for pouring it in, usually with a scoop and a wet vacuum, and on a Hayward that is 100 to 150 lb of it. That single afternoon is the strongest argument for the lighter media covered in our filter balls roundup, and for Intex’s approved glass option at 85 lb with a published eight to nine year life.

What we would not buy

A filter sized on pump flow. If a listing quotes only the pump’s gallons per hour, assume the figure through the sand is materially lower — Intex publishes 3,000 gph at the pump and 2,450 gph through the system for the same unit.

A model with no live manufacturer page. Two Bestway units here are absent from Bestway’s own US catalog, which makes spares and warranty support a question rather than a given.

A big filter for a small pool. The Intex unit publishes a minimum of 5,500 gallons. Below that, a cartridge pump is the correct answer and our pump roundup covers it — a sand filter on a 2,000 gallon pool is 120 lb of sand solving a problem the pool does not have.