Water filter components fall into two groups: the parts that hold water in and the parts that do the treating. Knowing which is which makes ordering spares straightforward and makes diagnosis obvious, since a leak is nearly always a sealing part and a taste complaint is nearly always the media. Here is the whole assembly, piece by piece.
The pressure vessel: cap, sump and O-ring
A cartridge filter is a two-part vessel. The cap carries the inlet and outlet ports and stays attached to the plumbing, and the sump is the bowl that screws onto it and holds the cartridge. Between them sits an O-ring in a machined groove, and that O-ring is the only thing keeping household pressure inside. It needs to be clean, undamaged, lightly greased with a food-safe silicone lubricant and fully seated. Nearly every leak traced to a filter is this part, not a crack in the housing.
The mounting and service parts
A mounting bracket carries the weight of the assembly and lets you unscrew a sump without twisting the pipework, which is why hanging a housing on its own connections eventually causes a leak. The housing wrench is sized to that specific sump and is not optional once mineral has built on the threads. Many caps include a pressure relief button so you can vent trapped pressure before opening. Some systems add shutoff valves either side of the housing so it can be isolated without turning off the whole house.
The media: where treatment actually happens
The cartridge is the working part. Spun or pleated polypropylene catches particles by size according to its micron rating. Carbon, granular or as a compressed block, adsorbs chlorine, taste, odor and organic compounds. Specialty media handle iron, scale or specific contaminants. Choosing between them is a question about your supply rather than about the hardware, and every community water system publishes an annual Consumer Confidence Report, with the EPA maintaining a finder for it. In a tank-based system there is no cartridge at all: instead the tank holds a bed of loose media over a gravel underbed, with a riser tube and distributor at the bottom collecting treated water evenly across the bed.
The parts that only tank systems have
A backwashing filter or softener adds a control valve on top of the tank, which is the brain and the moving part: it sets the service, backwash, rinse and refill cycles, and it holds the bypass that lets you take the unit out of service. Below the valve sits the riser tube and distributor basket, and beside the tank on a softener sits the brine tank with its own float, safety shutoff and injector line. Our whole house water filter guide covers how these assemblies are sized and staged for a house.
Questions people ask about water filter components
What is the sump on a water filter?
It is the bowl that screws onto the cap and holds the cartridge. The cap stays plumbed in; the sump comes off at every change, which is why the O-ring between them matters.
Why does my filter housing keep leaking?
Almost always the O-ring: dirty, dried out, damaged or not fully seated in its groove. Clean it, grease it with food-safe silicone and reseat it before blaming the housing.
What does the pressure relief button do?
It vents trapped pressure so the sump can be unscrewed safely after the inlet is shut. Without one, you open a downstream tap to release pressure instead.
What is the riser tube in a media tank?
It is the central pipe running to a distributor basket at the base of the tank, collecting treated water from the bottom of the bed so flow passes evenly through the media.