In short
- A micron rating describes the particle size a cartridge is made for; a smaller number is a finer rating, which in general also means more resistance to flow.
- Spun (melt-blown), pleated and string wound are the common constructions, and each one has its own look, handling and wording to read.
- Two lengths and two diameters are common, so measure the inside of your housing before you choose a cartridge.
- A sediment cartridge normally sits first in a line, ahead of any carbon stage.
- Keep spares in their sealed wrapping, and check which cartridge size the housing's maker intends.
A sediment cartridge is a filter for solid particles that travel in the water, such as grit, silt or rust flakes. It is a simple part, but a listing for one can be full of unfamiliar words: microns, spun, pleated, string wound, nominal, 2.5 by 10. This guide explains those words in plain language, so you can read a listing, compare two cartridges and be sure the one you order will fit your housing. It does not say what is in any particular water supply. For that, see the last section.
Micron ratings
A micron is one millionth of a metre, a unit used for very small things. A cartridge's micron rating describes the size of particle it is made to hold back. The number can seem backwards: a smaller number is a finer rating. A cartridge with a lower number is made for smaller particles than one with a higher number.
The rating comes with a trade-off, which the table sets out as general tendencies.
| Lower number (finer) | Higher number (coarser) | |
|---|---|---|
| Particle size it is made for | Smaller | Larger |
| Resistance to water flow | Generally more | Generally less |
| Filling up with particles | Generally sooner, in the same water | Generally later, in the same water |
These are tendencies, not rules. Two cartridges with the same rating from different makers can behave differently, and your water and your flow rate matter as much as the number. You may also see a rating called nominal or absolute. Roughly, nominal is an approximate rating and absolute is a stricter one, and the two are not measured the same way, so read what the maker says the number means before you compare.
Which rating suits a stage is a question for your system's manual and its maker. If the manual names a rating, use that one. If you are unsure, contact us before you order.
The common constructions
The same rating can be reached in different ways. These are the three constructions you will see most often.
| Construction | In plain words | What to read in the listing |
|---|---|---|
| Spun (melt-blown) | Fibres are bonded together into a thick, solid tube. | Rating, size, and whether the maker describes the density as the same throughout or graded. |
| Pleated | A sheet of filter material is folded into pleats around a core, which gives it more surface in the same space. | Rating and size. Some makers describe their pleated cartridges as washable and some do not, so follow the instructions for the one you buy. |
| String wound | String or thread is wound around a core in layers, like a spool. | Rating, size, and the material of the string and core. |
None of these is better in every situation. They suit different housings, different flow needs and different habits, so choose by what your system's maker describes and by the details in the listing. Browse them under Sediment Cartridges.
Lengths and diameters
Housings are built around a few standard cartridge sizes. Two lengths are common, 10 inches and 20 inches, and two diameters are common, about 2.5 inches and about 4.5 inches. Together that makes four common formats: 2.5 by 10, 2.5 by 20, 4.5 by 10 and 4.5 by 20. Other sizes exist, but these four are the ones most often meant. The figures are nominal, meaning they are the names of the sizes, so a cartridge may measure a little differently from its label.
What matters is the inside of your housing, because the housing decides what fits. A cartridge that is too long will stop the sump from closing. One that is too short, or too slim for the housing, may not sit or seal as designed. Measuring the housing and not just trusting the old cartridge is the safest habit, and our journal piece A five-minute check before you order a whole-house cartridge or housing shows how.
Why the sediment stage goes first
In a multi-stage line, the sediment cartridge is normally the first stage, ahead of any carbon cartridge. Carbon cartridges and later stages are not made to hold large amounts of grit, so putting a sediment stage first is the usual arrangement. If your system has more than one stage, your manual gives the order and the schedule for each. Carbon cartridges have their own section, Carbon Cartridges.
Pack sizes and spares
Sediment cartridges are sold singly and in multi-packs, and a listing says how many are in the pack, so read the count. A multi-pack means a spare is on the shelf when the next change comes due. Many makers give a time and a volume for changing a cartridge, and you change at whichever you reach first. If a cartridge seems to clog much sooner than the maker's schedule suggests, read the manual again and ask the maker, or ask us, about the rating. Do not remove the cartridge to restore flow.
Matching a cartridge to a housing
- Look for the size in the manual or on the housing. Many housings state the cartridge size they take.
- Measure if it is not stated. Turn off the supply, release the pressure as the manual says, open the sump, and measure the inside length and the inside diameter. Write the figures down.
- Match the nominal length and diameter to a cartridge in one of the common formats above.
- Check the details of the fit. The cartridge should sit on its seat as the instructions show, and the sump should close fully with the O-ring seated.
- Look at the housing too. If a housing is cracked, worn or does not match your plumbing, see Housings & Bags.
After you fit the cartridge and turn the water back on, check for drips and look again later in the day.
Storing cartridges
Keep a cartridge in its sealed wrapping until you are ready to fit it, and store it as the packaging says, usually in a clean, dry place. If the wrapping is torn, or a cartridge has been dropped on a dirty floor, do not fit it. Handle cartridges with clean hands and fit them promptly once opened.
If your question is about your water
A cartridge description tells you how a part is built and what size it is. It does not tell you what your water carries or whether it suits you. For that, or for any health concern, contact your local water utility (most publish an annual water quality report), a certified testing laboratory or a health professional. For the parts themselves, browse the Sediment & dirt section.