A reverse osmosis system has four kinds of consumable and treating them as one service item is how households either overspend or quietly ruin a membrane. Two stages run on a calendar because their exhaustion is invisible. One runs on measurement. One is easy to forget entirely. Getting the distinction right is most of what keeps the system honest.
The prefilters: calendar, not appearance
The sediment prefilter takes the mechanical load and usually shows itself by slowing production. The carbon prefilter protects the membrane from chlorine and shows nothing at all: it has a capacity rather than a visible end point, and when it is exhausted the chlorine goes straight through to the membrane and damages it permanently. Both are changed annually in a typical household, sooner where the feed water is dirty. This is the cheap job that protects the expensive component, and slipping it is the single most costly habit in RO ownership.
The membrane: measurement, not calendar
Do not replace a membrane because a year has passed. Take a dissolved solids reading of the feed water and another of the treated water sampled before the storage tank, and compare them. A membrane doing its job leaves a wide gap; a spent one lets the readings converge. Slow production alone is not evidence, because a blocked prefilter, low line pressure and a storage tank that has lost its air charge all produce exactly that symptom and are far more common.
The polishing and remineralization stages
The final carbon filter after the tank gives the water its finished taste and is changed with the prefilters. If the system has a remineralization cartridge, that is the one people forget: its media is consumed rather than blocked, so it produces no pressure change and no clear taste cliff, it simply stops adding minerals and the water goes flat again. Put it on the same annual schedule as everything else rather than waiting for a symptom that arrives slowly.
Buying and doing it
Buy the annual set as a kit for your specific system rather than piece by piece, and add spare housing o-rings and a tube of food grade silicone grease, since the o-ring is what turns a filter change into a leak. Do the change with the housings open as an opportunity to sanitize, with the membrane and carbon removed first because chlorine based sanitizer destroys the membrane. Then flush the system as the manual states before drinking. Our RO filter guide covers matching cartridges to the common systems.
Questions people ask about replacing ro filters
How often should RO prefilters be replaced?
Annually in a typical household, sooner on dirty feed water. Their exhaustion is invisible, so they run on a calendar rather than appearance.
How do I know when the membrane needs replacing?
Compare a dissolved solids reading of the feed with one from the membrane output before the tank. Converging readings mean it is spent.
My system is slow. Is that the membrane?
Usually not. Check the prefilters, the line pressure and the tank's air charge first, since all three cause slow production.
What is the stage people forget?
The remineralization cartridge. Its media is consumed silently, so the water simply goes flat again with no other warning.