Pool filter: types, sizing and how to read it

What a filter does and does not do

The pump pushes the water through a bed of material that holds back suspended particles: dust, pollen, sunscreen residue, dead algae. The clean water goes back into the pool through the return inlets.

The filter does not disinfect; that is the job of chlorine or another system. Without good filtration, though, the water stays dull even with the right chlorine level. The filter is one link in the circuit described under pool water circulation, and it only works while the pump is running.

The three types of filter

Sand filter

The most common type in home pools: a tank filled with special quartz sand graded for pools. It is cleaned by backwashing, i.e. reversing the flow, which sends the dirt to the drain. It is simple, durable and forgives quite a bit of neglect.

Glass media filter

The same tank, but instead of sand it holds specially processed crushed glass. It usually traps finer particles, forms fewer "channels" through the media and needs less water to backwash. It can also go into an existing sand filter when it is time for a filter sand change.

Cartridge filter

Instead of granules it has one or more pleated cartridges made of synthetic fabric. It filters finely, needs no backwashing and wastes no water: the cartridges are taken out and rinsed with a hose. It suits small pools and spots without an easy drain connection, but it needs more frequent hands-on care and replacement cartridges.

How to choose the size

The filter is chosen together with the pump, not on its own. For each model the manufacturer gives the maximum flow in m³/h that can pass without spoiling filtration quality. The rule is simple:

• the flow of the pump must be within the filter's limit, preferably with some margin;
• the pump and filter together must turn over the whole pool volume in a reasonable number of hours;
• for a pool with lots of trees, dust or swimmers, a filter one size up is worth it.

An undersized filter is one of the common mistakes: it clogs quickly, needs constant backwashing and the water turns cloudy in hot weather. A slightly larger filter does no harm; the water simply passes more slowly and is filtered better.

The pressure gauge: when it needs a backwash

Sand and glass filters have a pressure gauge. Right after a backwash, note the reading with the filter clean. As the media fills with dirt, the pressure rises. When it rises by usually about 0.5 bar above the "clean" reading, or whatever the manufacturer specifies, it is time for a backwash.

If the pressure drops below normal, the problem lies before the filter: a full skimmer or pre-filter basket, a low water level or air in the circuit.

The multiport valve

On top of or beside the filter is the valve with the handle that sets where the water goes. The usual positions:

1. Filter: normal operation.
2. Backwash: water flows backwards through the media and goes to the drain.
3. Rinse: a few seconds after backwashing, to let the media settle again.
4. Waste: sends water out without passing through the filter, e.g. when vacuuming up a lot of dirt.
5. Recirculate: water returns to the pool, bypassing the filter.
6. Closed and, on some valves, Winter.

Only change the valve position with the pump switched off. Changing it while the pump is running wears the valve's internal gasket and can cause a leak.

Questions

See also