Pool filters: sand, cartridge or glass

Illustration.

The three types

HealthyWA lists sand and cartridge filters as the common types, and says sand and glass filters are cleaned by backwashing while cartridges are taken out and cleaned by hand, always following the maker's instructions.

When to clean it

Many filters have a pressure gauge: as dirt builds up, pressure rises. Note the reading when the filter is clean; when it rises noticeably above that, clean it. Without a gauge, clean it when the flow back into the pool drops.

How long to run it

HealthyWA says the filter should filter all the pool water within 6 to 8 hours and keep running for at least an hour after the last person gets out. More on pump hours.

How to backwash (sand or glass)

1. Turn the pump off.
2. Set the multiport valve to BACKWASH.
3. Run the pump until the sight glass runs clear, usually a minute or two.
4. Pump off, set to RINSE, run 20 to 30 seconds.
5. Pump off, set back to FILTER, pump on.

Always switch the pump off before moving the valve handle.

Maple's tip

Backwash water goes to waste, so in a hard summer or water restrictions a cartridge filter saves water. Check your local water authority's rules on where backwash water can go.

Sand filters in more detail

A sand filter is a tank filled with special filter sand. Water from the pump flows down through the sand, which traps dirt, and clean water returns to the pool. As dirt builds up, the pressure on the gauge rises and flow drops. Backwashing reverses the flow to lift the dirt out to waste. Sand filters are simple, cheap to run and tolerate neglect, but they trap less fine dirt than cartridges and use more water through backwashing. The sand wears smooth over the years and needs replacing, often after several years depending on use.

Glass media filters

Glass media is crushed, recycled glass used in a sand filter tank in place of sand. Its shaped particles trap finer dirt, it resists the biofilm that can build up in sand, and it often needs less backwashing. Many owners switch an existing sand filter to glass when the sand is due for replacement. The tank, valve and backwashing routine stay the same. Follow the media maker's guidance on how much to use and which grade goes in which layer.

Cartridge filters

A cartridge filter uses pleated fabric cartridges inside a canister. The fine fabric traps smaller particles than sand, giving very clear water, and there is no backwashing, which saves water. Instead, you hose the cartridges clean when the pressure rises, and soak them in a cartridge cleaner now and then to remove oils and scale. Cartridges wear out over time and need replacing every few years depending on use and care. Cartridge filters suit areas with water restrictions and owners who want clear water and do not mind hands-on cleaning.

Reading the pressure gauge

The gauge on top of the filter is your best guide to when it needs cleaning. Note the pressure just after a clean: that is your clean reading. When the pressure rises a set amount above it, the filter is dirty enough to backwash or clean; many makers suggest acting when pressure rises by around 50 kPa (about 7 psi) above clean, but follow your filter's manual. A gauge that never moves may be faulty; replace it, because it is cheap and important.

How to clean a cartridge

1. Turn the pump off at the power point.
2. Release the pressure using the air bleed valve on the filter lid.
3. Open the canister and lift out the cartridge.
4. Hose it from the top down, between every pleat, with a garden hose nozzle; do not use a pressure washer, which can tear the fabric.
5. For a deeper clean, soak it in a cartridge cleaning solution as directed, then rinse well.
6. Check the canister o-ring, lubricate it with a silicone lubricant if the maker says to, and refit the cartridge and lid.
7. Open the air bleed, turn the pump on, and close the bleed when water flows steadily.

Some owners keep a spare cartridge to swap in while the dirty one dries and is cleaned.

Choosing a filter size

A filter must be sized to the pump and the pool's volume. Too small and it clogs quickly, raises pressure and can let dirty water through; too big costs more but runs longer between cleans. Pool shops and the filter maker's charts match filter size to pump flow and pool volume. When replacing a pump, check that the filter still suits it.

Signs your filter needs attention

• The water is cloudy even with good chemistry.
• The pressure stays high after backwashing or cleaning.
• Sand appears on the pool floor (a broken lateral in a sand filter).
• Water leaks from the multiport valve or the lid.
• Flow at the returns is weak.

Water use and backwashing

Backwashing sends hundreds of litres of pool water to waste each time, which then needs topping up and re-balancing with chemicals. Backwash only when the gauge says to, rinse for the recommended time, and check your local council's rules on where backwash water may go. Some areas require it to go to sewer, not the stormwater drain or the garden.

Filters and pool clarity

A filter is only half of clear water. Chemistry matters as much: low chlorine, high pH or high alkalinity can cloud water no matter how good the filter is. If the water stays cloudy, test it first, then check the filter. See cloudy pool water.

Sand, glass or cartridge: which to choose

There is no single best filter. Many pool owners keep the type they have and upgrade the media or cartridge rather than replacing the whole unit.

Replacing filter sand

Over the years, filter sand rounds off, clumps and lets more dirt through, and the filter may need backwashing more often or give cloudy water. Replacing sand involves turning off the pump, removing the multiport valve, scooping out the old sand, checking the laterals at the bottom for cracks, and filling with new media to the maker's level. It is a heavy job; many owners pay a pool shop or technician. It is also the natural time to switch to glass media.

Multiport valve settings explained

• Filter: normal running.
• Backwash: reverses flow to clean the media; water goes to waste.
• Rinse: resettles the media after backwashing; water goes to waste.
• Waste: sends water straight to waste, used for vacuuming heavy debris or lowering the water level.
• Recirculate: bypasses the filter, used for mixing chemicals or when the filter is being repaired.
• Closed: stops flow; never run the pump on closed.

Always turn the pump off before changing the valve setting, to protect the valve's seals.

Filter maintenance calendar

Filters and pool cleaners

Suction cleaners send all their dirt into the filter, so pools with suction cleaners need more frequent backwashing or cartridge cleaning. A robotic cleaner keeps that dirt out of the filter. See robotic vs suction pool cleaners.

Filter problems and fixes

Pressure high right after backwashing. The media may be clogged with oils or calcium, or the sand may be old and clumped. A chemical filter cleaner can help; old sand needs replacing.

Pressure low. A blockage before the filter, such as a full skimmer or pump basket, a blocked impeller or an air leak on the suction side, is reducing flow. Check baskets and listen for air in the pump.

Sand in the pool. A broken lateral or standpipe inside a sand filter lets sand through to the pool. The filter needs opening and repairing.

Water leaking from the valve. Worn spider gaskets or seals in the multiport valve; replace them.

Dirty water returning after cleaning a cartridge. The cartridge may be torn, not seated properly, or the o-ring missing.

How long a filter lasts

Filter tanks and canisters last many years. Sand typically needs replacing every several years, glass media lasts longer, and cartridges last a few years with good care. Multiport valve seals and gauges wear and are cheap to replace. When the tank itself cracks or the valve fails beyond repair, replace the whole unit, matched to the pump and pool.

Filter care in winter

In winter, a pool's filter load drops, but keep the routine going: run the pump enough to keep water clear, check the pressure, and clean the filter before the spring warm-up, when algae can take hold quickly. A clean filter in spring makes the first swim of the season far easier.

Pool filter costs

Replacement sand and glass media, cartridges and parts such as gauges, o-rings and valve seals are the regular costs of a filter. Cartridges and glass media cost more than sand but can last longer and give clearer water. A whole new filter is a larger cost, best timed with a pump upgrade so the two match. A pool shop can quote parts for your exact model.

Cleaning a filter after a green pool

After a green-pool rescue, the filter is full of dead algae. Backwash a sand or glass filter thoroughly and consider a chemical filter clean; soak a cartridge in a cleaner overnight and rinse well. A dirty filter after algae can cloud the water and use up chlorine for weeks.

DE filters

Diatomaceous earth (DE) filters use a fine powder coated on grids to trap very small particles, giving very clear water. They are less common in Australian backyard pools, need the powder replaced after backwashing, and need careful handling of the powder. If your pool has one, follow the maker's instructions closely.

Our short answer

Sand is simple and cheap, glass media gives clearer water in a sand filter, and cartridges give the clearest water with no backwash. Clean when the pressure rises, and replace media or cartridges as they wear.

Pool filters and water restrictions

In dry seasons, some areas restrict filling and topping up pools. Backwashing a sand or glass filter uses a lot of water, so owners in those areas often prefer cartridge filters, or backwash only when the gauge says it is truly needed. Check your water authority's current rules, and capture rainwater for top-ups where allowed. A pool cover also saves water by cutting evaporation.

Buying a new filter

Ask the pool shop to match the filter to your pump's flow, your pool's volume and your plumbing size, and to explain how to clean and maintain it. Check the warranty on the tank and valve, how easy cartridges or media are to buy locally, and whether the filter fits the existing pad and pipework without major changes.

Cartridge filter care in detail

Cartridges collect oils from sunscreen and skin as well as dirt, and oils are not removed by hosing alone. Every few months, or after a busy summer, soak the cartridge overnight in a cartridge cleaner or degreaser as the label directs, then rinse it thoroughly before refitting. Never use acid on a cartridge that has oils on it, because it can set them into the fabric. Let a cartridge dry completely between uses if you rotate a spare, which also helps kill bacteria in the pleats.

Sand filter care in detail

Backwash until the sight glass runs clear, then rinse to resettle the sand before returning to filter. Over-backwashing wastes water without making the sand cleaner. Once or twice a year, a sand filter cleaner can break down oils and scale that backwashing leaves behind.

How long to run the filter

The filter runs whenever the pump runs, so the pump schedule is the filter schedule. In summer, aim for at least one full turnover of the pool's water each day, and longer after heavy use. See how long to run your pool pump for working out the hours from your pool's volume and flow rate, and how variable speed pumps change the answer.

Common questions