When to change your water filter (and what happens if you don’t)

Why filters don’t last forever

A water filter doesn’t make the unwanted stuff vanish. It catches it and holds onto it. A sediment cartridge traps sand, rust and grit in its layers. A carbon cartridge grabs the chlorine and chemical tastes and keeps them. Either way, the cartridge is a container, and containers fill up.

Think of a vacuum cleaner bag. When it’s new, it picks up everything. As it fills, the suction fades, and eventually it stops collecting altogether. A filter cartridge follows the same arc. It just happens invisibly, sealed inside a housing under your sink. That’s why a simple schedule beats waiting for something to look wrong.

The schedule, filter by filter

Sediment and carbon cartridges — the workhorses inside most undersink, countertop and whole-house systems — are typically due every three to six months. Closer to three if your water carries a lot of dirt or your household gets through plenty of water; closer to six on clean municipal supply with lighter use. The number that outranks any rule of thumb is your own cartridge's stated limit: a time limit and a litre limit, whichever comes first.

Membranes last much longer. The membrane is the very fine final barrier in a reverse osmosis or ultrafiltration system, the stage that stops what the other cartridges can’t. It typically runs 12 to 24 months, depending on your water quality. It lasts that long partly because the cartridges in front of it take the punishment first. Which is a good reason not to skip the small changes: fresh pre-filters protect the stage you replace least often.

UV lamps work on a roughly yearly cycle. The lamp in a UV steriliser is rated for 8,000 hours (about 11 months of continuous use), so change it at least annually. Here’s the catch: an ageing lamp usually still glows, but glowing is not the same as disinfecting. It weakens gradually, so change it by the calendar, not by eye.

Shower filter cartridges follow the same rhythm as the small cartridges: every three to six months, depending on how much water runs through them. A family showering daily should plan for the shorter end.

How a cartridge tells you it’s done

The clearest sign is flow. When your filtered tap slows from a steady stream to a dribble, the likeliest reason is a sediment cartridge that has done its job and is now full, like that vacuum bag. It isn’t broken; it’s finished. (Slow flow can have other causes, such as low supply pressure or a kinked pipe, so if a fresh cartridge doesn’t fix it, look further.)

The second sign is taste. If the chlorine taste or smell you bought the filter to remove starts creeping back, the carbon has reached its capacity and water is passing through with less and less happening to it.

One honest caveat: carbon can quietly reach capacity without an obvious change in taste, and a UV lamp gives no signal at all. So treat these signs as early warnings, not as your system. The date on the calendar is the reliable one.

What actually happens past the due date

There is no alarm bell on the due date. This is about drift rather than a cliff edge. But the drift is real, it runs in one direction, and it deserves respect in proportion to the job: if a cartridge exists for taste, a late change costs you taste; if it exists for safety (a membrane or UV stage on water you don't otherwise trust), running past the limit means running unprotected without knowing it.

Carbon works by adsorption: chlorine and chemical molecules stick to its surface, a bit like cars filling parking bays. Once every bay is taken, water flows through as normal but nothing more is being removed. Your tap still runs, the housing looks the same, and the filtering has quietly stopped. There is no warning light.

There’s a second, less obvious issue. A long-overdue cartridge sits damp and packed with the organic material it has collected over the months — a comfortable environment where bacteria can multiply. That’s not a reason to panic if you’re a few weeks late; it is the reason a cartridge should be swapped rather than left in place indefinitely.

And an overdue sediment cartridge slowly clogs, dropping your water pressure and making every stage after it work harder, including that membrane you’d rather replace every two years, not sooner.

Or let us do the remembering

Six months is exactly the sort of interval human memory is bad at, which is why SUPERPURE runs a free replacement-reminder programme. Register your filter by scanning the QR code on your SUPERPURE box or starting from the Support page on this site, and we’ll simply remind you when your cartridges fall due. No cost, no obligation, one less thing to hold in your head. One honest limit: a reminder works off the calendar, not the cartridge. It cannot measure how much water you have actually pushed through, so if your household's use is heavy, change ahead of the email.

If you’d rather manage it yourself, that works too: write the install date on the housing with a marker and set a recurring reminder on your phone. Whichever route you choose, the filter you remember to change is the one that keeps doing its job.