A 20 x 4.5 cartridge is the long big diameter element, twenty inches by four and a half, and it holds roughly twice the media of the ten inch version in the same width. That extra volume changes which media types make sense in it, because some duties depend directly on how much media the water passes through.
Where the extra length matters most
Carbon, decisively. Chlorine removal depends on contact time, which is bed volume divided by flow, so doubling the length doubles the contact and takes a carbon cartridge from partial removal to something close to complete at household flow. Sediment cartridges gain differently: the removal is much the same but the capacity doubles, so the interval doubles. For specialist media such as copper zinc or polyphosphate, more media means a longer working life rather than a better result.
The element types available
Melt blown polypropylene is the cheap high capacity sediment default. Pleated cartridges give lower pressure drop for their rating and some can be rinsed once. String wound behaves like melt blown and sheds fibers as it ages. Granular activated carbon suits this size well, since the length gives contact without excessive pressure drop. Carbon block gives better contact still at a higher drop. Catalytic carbon is the version to fit where the utility doses chloramine rather than free chlorine.
Handling and clearance
This is the practical cost of the size. A full sump is heavy and awkward and needs the bracket fixed to something solid with appropriate fixings, never to plasterboard and never supported on the pipework. Clearance below the housing must be at least the sump depth or the cartridge cannot be withdrawn without cutting pipe, and for this length that is considerable. Have a bucket ready, since the volume released when the sump comes off surprises people the first time.
Getting the order right
Measure the old cartridge rather than the housing, length end to end and diameter across the body, since housings vary in wall thickness. Sellers list the numbers in either order, so 20x4.5 and 4.5x20 are identical. Then choose the rating and media for the water rather than buying the tightest rating available, which costs pressure and blinds early. Where two problems exist, stage two housings rather than seeking one cartridge claiming both. Our replacement cartridge guide covers intervals by media.
Questions people ask about 20 x 4.5 water filter cartridge
What does the twenty inch length gain over ten?
Roughly double the media: double the contact time for carbon and double the capacity and interval for sediment cartridges.
Which media suits this size best?
Carbon benefits most, since chlorine removal depends directly on contact time. Sediment gains capacity rather than performance.
Is 20x4.5 the same as 4.5x20?
Yes. Sellers list the figures either way. The larger number is always the length and the smaller the diameter.
What clearance does it need?
At least the sump depth below the housing, which for this length is considerable, or the cartridge cannot be withdrawn.