The swimming pool sand filter is the least glamorous and most important machine in your backyard: every litre of pool water passes through it, and when it quietly stops doing its job, no amount of chemicals will keep the water right. Here is everything a pool owner actually needs to know — how it works, what sizes and prices look like in South Africa, and the failure signs we diagnose every week.
How a swimming pool sand filter actually works
Inside the tank sits a bed of specially graded silica sand. The pump pushes water in at the top, gravity and pressure pull it down through the bed, and the microscopic gaps between grains trap dirt, algae and debris. Clean water collects in slotted fingers at the base — the laterals — and returns to the pool. Two facts follow that explain almost everything about filter care:
- The sand bed loads up as it works — which is why pressure rises and backwashing exists;
- The laterals are the only barrier between the sand and your pool — which is why one hairline crack puts sand on your pool floor.
Sand filter quick reference:
- Common home sizes: 2, 3 and 4 bag filters (bags of 40kg filter sand)
- Typical SA retail prices: roughly R3,500 – R8,000 by size and brand
- Backwash trigger: 20–25% above your clean baseline pressure — not the calendar
- Sand life: 5–8 years; laterals are checked whenever the tank opens
Which sand filter size does your pool need
Filters are sized to match pump flow and pool volume — the common shorthand is bags of sand. As a practical guide:
- 2-bag filters suit plunge pools and small splashers with 0.6–0.75kW pumps;
- 3-bag filters are the workhorse for the standard family pool on a 0.75kW pump;
- 4-bag filters pair with 1.1kW pumps on large pools — and undersized filters on big pumps are a classic cause of sand blow-through.
Matching matters more than size alone: a big pump forcing water through a small filter channels the bed and shortens sand life. If your system was assembled piecemeal over the years, a mismatch is worth checking — it is a common find on our repair assessments.
Sand filter faults we repair most
- Cracked laterals — sand returning to the pool floor. Repairable; the tank is opened, the lateral replaced, and the sand usually renewed at the same time.
- Exhausted or channelled sand — pressure rebounds within days of backwashing, water never polishes. Rebedding restores it.
- Multiport valve wear — leaks to waste, weak backwash, or water bypassing the bed. Spider gaskets and valve internals are standard repairs.
- Tank and fitting leaks — weeping unions and cracked lids that let air into the system and prime out of it.
Making a sand filter last
Three habits cover it: backwash on pressure, not ritual (our backwashing guide explains the 20–25% rule); keep baskets clean so debris load stays sane; and service the bed before it fails — if your filter is five-plus years old and the pool is harder to keep than it used to be, the sand is the first suspect. Not sure what you are looking at? Book a free pool assessment and we will check the whole filtration circuit — filter, pump, valve and plumbing — in one visit.
Frequently asked questions
Glass media or silica sand?
Glass filters finer and resists channelling; silica is cheaper and completely proven. On a fresh rebed glass is worth the conversation — on a system with other faults, neither fixes the real problem.
Why is there sand in my pool after backwashing?
Almost always a cracked lateral inside the filter, occasionally overfilled sand. It will not fix itself — and escaping sand wears the pump on its way through.
How do I know my filter size?
The tank label usually states the model; failing that, tank diameter is a good proxy. Bring a photo of the tank and label to any assessment and we will identify it on the spot.
Can I change filter sand myself?
It is a physical but doable DIY job — the risk is damaging laterals during the dig-out and refill. If the filter has never been opened, having it done professionally once, with the laterals inspected, is cheap insurance.
