A 20x4.5 filter is the largest housing most households will fit: a big diameter body taking a cartridge twenty inches long. It is chosen for two reasons, low pressure drop at high flow and a long service interval, and it is worth being clear which of those you are buying, because they lead to different cartridge choices.
What it supports on flow
Media area is what determines how much a filter restricts, and this size has the most of any common domestic housing. That means a household running a shower, a washing machine and a dishwasher together loses much less pressure through it than through a slim or a short housing. It also means a tighter cartridge rating becomes survivable that would be impractical elsewhere. Port size is specified separately and constrains flow independently, so match ports to the service line rather than accepting the smallest thread option.
What it buys on interval
Roughly double the capacity of the ten inch version in the same width, so roughly double the time between changes on the same water. On a supply carrying real sediment that is the difference between a manageable schedule and a recurring chore. For carbon the gain is qualitatively different: contact time doubles, and since chlorine removal depends directly on contact, this size takes carbon from partial removal to close to complete at household flow.
Where it will not fit
Clearance is the constraint that decides most installations. The cartridge withdraws downwards, so there must be at least the sump depth in clear space below the housing, which at this length is considerable, and the space must exist at the actual mounting height rather than in principle. Under a low bench, in a crawl space or above a floor mounted appliance, this housing produces a filter that cannot be serviced without cutting pipework. Measure with a tape at the intended position before ordering.
Installing it properly
On the main after the meter and shutoff, before any branch and before the water heater, with the irrigation tap taken off ahead of it so garden water does not consume cartridge capacity. Bracket to a solid surface with fixings suited to the weight of a full sump. Isolation valves either side and a bypass around the assembly. Pressure gauges before and after, which is the only honest way to know when the cartridge is spent. Our whole house filter guide covers staging two of these.
Questions people ask about 20x4.5 water filter
Why choose the largest housing?
Lowest pressure drop at household peak flow and the longest service interval, plus enough contact time for carbon to work properly.
How much clearance does it need below?
At least the sump depth, measured at the actual mounting height, or the cartridge cannot be withdrawn without cutting pipework.
Does a big housing guarantee good flow?
No. Port size is specified separately and small ports throttle the supply regardless of the cartridge diameter inside.
Should the outside tap be filtered through it?
Usually not. Branch irrigation off ahead of the filter so garden water does not consume cartridge capacity for no benefit.