Almost every service call on a housing-and-cartridge filter comes down to four parts: the cartridge, the o-ring, the sump and the head. The cartridge is consumable and planned for; the o-ring is the part that leaks; the sump is the part that cracks; the head is the part that stays, which is why identifying it correctly matters more than anything else you buy.
The head, the sump and the o-ring
A cartridge filter is three pieces. The head is the top casting with the inlet and outlet ports, often with a pressure relief button and a mounting bracket; it stays on the wall for the life of the system. The sump is the removable bowl that holds the cartridge and takes the pressure, and it is the piece that eventually crazes and splits, usually from sunlight, heat or overtightening. Between them sits a single large o-ring that does the sealing. That o-ring is the cheapest part in the assembly and the cause of most drips. Replace it every time the sump comes off, seat it clean and dry in its groove, and give it a smear of silicone grease rated for potable water rather than petroleum jelly, which swells the rubber.
Getting the size right before you order
Cartridge sizes are near-universal but not forgiving. The two common lengths are ten inch and twenty inch, and the two common diameters are a standard two and a half inch and the wide format commonly called big blue at about four and a half inches. A ten inch standard cartridge will physically drop into a twenty inch sump and simply let water bypass around it, so measure the old one rather than trusting a memory. Port threads are the other trap: heads come with different inlet sizes, and a three quarter inch head on a one inch main is a bottleneck you install once and regret every shower. Note the cartridge end style too, since some are flat gasketed and some have a knife edge that must seat on the sump post.
The parts people forget to order
A housing wrench is specific to the sump diameter and brand, and the moment you need one is the moment the sump is stuck. Mounting brackets are sold separately from many housings, and mounting a filter by its plumbing rather than a bracket puts the weight of a full sump on the pipe. A bypass valve or a pair of ball valves either side of the housing turns a cartridge change from a whole-house shutdown into a five minute job. Pressure gauges before and after the housing are the cheapest diagnostic in the utility room, because a growing difference between them tells you the cartridge is loading up long before anyone in the house notices the shower has weakened.
What to keep on the shelf
Keep one spare o-ring, one spare of each cartridge in the train, and the wrench, together, near the unit. Write the install date on the sump with a marker at every change; the tape and the marker are worth more than any filter change indicator. If your system feeds a reverse osmosis unit or a UV lamp, treat the pre-filter cartridges as safety parts rather than comfort parts, because a loaded sediment cartridge starves the equipment behind it. Our replacement water filters guide covers cadence by cartridge type. Keep the original housing model number photographed on your phone, since faded labels are the usual reason a simple part order turns into a guessing exercise.
Questions people ask about water filter parts
Can I reuse the o-ring when I change a cartridge?
You can, but it is the usual cause of a slow drip. Inspect it every change, keep a spare, and replace it as soon as it looks flattened or nicked.
Will any ten inch cartridge fit my housing?
Only if the diameter and end style match too. A standard two and a half inch cartridge will not seal in a wide big blue sump, and water will simply bypass it.
What lubricant is safe on filter housing seals?
Use a silicone grease rated for potable water. Petroleum based products swell and degrade the rubber and can taint the taste of the water.
Why did my housing crack?
Usually sunlight, hot water backfeed, or overtightening. Mount housings out of direct sun, on a bracket, and hand tighten with the wrench only as far as the seal needs.