First: which taps are affected?
This is the step people skip, and it separates a filter problem from a plumbing one immediately.
Taste the water at several fixtures — the filtered tap, a kitchen cold tap, a bathroom tap, and an outside hose bib if you have one.
Every tap tastes the same, including filtered. Either the filter is not treating at all, or water is getting around it.
Only some taps are affected. Those fixtures are plumbed upstream of the filter, or on a branch that bypasses it. That is an installation issue, not a media one — and no amount of cartridge replacement will change it.
The filtered tap is fine but the taste appears in the shower. Perfectly normal for a point-of-use system. A tap-mounted or under-sink filter treats one fixture; it was never treating the whole house.
Worth knowing
Taste and smell are not measurements. They tell you something changed, not how much chlorine is present — and sensitivity varies enormously between people. If the actual concentration matters to you, a test is the only way to know it.
Exhausted carbon, and why it surprises people
Activated carbon works by adsorption: chlorine molecules bind to an enormous internal surface area within the media. That surface fills up. Once it is full, water passes through untreated.
The important part is that this is invisible. A sediment filter shows you it is spent — it turns brown and flow drops. Carbon does neither. It looks identical on the last day as on the first, and flow is unchanged.
So the failure presents as a taste appearing suddenly, with no other symptom, in a system that appears to be working normally.
Capacity is measured in gallons, not months, which is why manufacturers' intervals are only a starting point. A large household on hard water exhausts a cartridge considerably faster than the label suggests. Two people and six people running the same system are not on the same schedule.
The practical response is to date the cartridge when you fit it and shorten the interval if the taste returns early — not to assume the system has failed.
Bypass, which is more common than it should be
Most whole-house filter installations include a bypass loop so the filter can be serviced without shutting the house's water off. That loop has valves.
If the bypass is open — left that way after a cartridge change, or opened deliberately for irrigation and forgotten — water goes around the filter entirely. The system looks completely normal. The housing is there, the cartridge is in it, nothing is leaking.
This is worth checking first on any system where the taste appeared shortly after servicing, because that is exactly when it happens.
Internal bypass is subtler: a cartridge seated poorly, a missing or perished O-ring, or a cartridge slightly undersized for the housing lets water pass around the media rather than through it. The clue is that the taste is reduced rather than absent — some water is being treated, some is not.
When changing a cartridge, check the O-ring and seat it properly. That is the difference between filtering and appearing to.
Flow, and contact time
Carbon needs contact time. Water has to be in the media long enough for adsorption to occur, and that time is a function of how fast it is moving through.
Every system has a rated service flow. Exceed it and treatment falls off — not because anything is broken, but because the water is not in the carbon long enough.
That produces an intermittent, confusing symptom: fine first thing in the morning, chlorine taste when the dishwasher, a shower and the irrigation are all running. Same filter, same cartridge, different flow rate.
It usually means the housing is undersized for the household rather than that anything has failed. A larger housing, or a second in parallel, is the fix. Replacing the cartridge more often will not help, because the cartridge was not the problem.
Chlorine, chloramine, and matching the media
This is the cause people almost never suspect, and it explains systems that never worked properly rather than ones that stopped.
Many utilities disinfect with chloramine — chlorine combined with ammonia — instead of free chlorine, because it is more stable through the distribution system.
Standard activated carbon removes free chlorine efficiently. It is poor at chloramine, which needs far longer contact time or a catalytic carbon designed for it.
So a perfectly good filter, correctly installed, with fresh media, can still leave a distinct taste — because it is the wrong media for what is in the water. And if a utility changes its disinfection method, a system that worked for years can stop working with nothing having changed at your end.
Your water provider publishes an annual water quality report stating which disinfectant is used. That single fact determines whether your media is right, and it is worth checking before buying anything else.
Order of treatment
Where a house has several pieces of equipment, sequence matters — and getting it wrong shortens the life of everything downstream.
Sediment filtration goes first, so grit does not reach anything else. Carbon goes next. Softening goes last.
That order protects the softener resin from chlorine, which degrades it over time, and stops sediment blinding the carbon prematurely.
On a well the sequence is different again, because iron has to be handled before the softener — see well water plumbing. And if the complaint is scale rather than taste, that is a hardness question; carbon will not touch it.