Replacing reverse osmosis filters is a job most households can do at the kitchen sink in under an hour. An under sink unit has three kinds of element on different schedules: sediment and carbon prefilters that change often, a membrane that lasts years, and a carbon post filter that polishes the taste on the way to the faucet.
What changes and how often
The sediment prefilter goes first in the line and is replaced most frequently, because it takes the grit that would otherwise damage everything behind it. The carbon prefilter follows and does something critical: it removes chlorine, which attacks and destroys the thin film membrane if allowed through, so letting this cartridge run past its life is what kills membranes early. The membrane itself lasts far longer, typically several years, and is judged on performance rather than the calendar. The carbon post filter sits between the storage tank and the faucet and is changed roughly annually to keep the stored water tasting fresh. Manufacturer product pages list the rated life of each element for their system.
The change, in order
Close the feed valve on the cold supply, close the valve on the storage tank, then open the RO faucet and leave it open to relieve pressure. Put a shallow tray under the unit because the housings hold water. Undo each housing with the supplied wrench, tip out the old cartridge, wipe the sump clean, inspect and lightly grease the O ring with silicone, and fit the new cartridge the correct way up. Work one housing at a time so nothing gets mixed up. For the membrane, pull the tubing from the cap, unscrew or unclip the cap, pull the old membrane out with pliers and push the new one fully home so the seals seat inside the vessel.
Sanitizing, restarting and flushing
Because the system is open, this is the moment to sanitize it if the unit has been apart for long or the water has ever tested poorly. Follow the manufacturer procedure rather than improvising a dose, and never leave chlorine sitting against a membrane, which is why sanitizing is done with the membrane out and the housings empty. To restart, open the feed valve slowly, leave the tank valve closed and let the system run to drain through the open faucet for several minutes to flush carbon fines, then open the tank valve and let it fill. Discard the first full tank, and the second if the water still tastes of carbon, before drinking.
Knowing when the membrane is finished
Prefilters run on a schedule, but the membrane is judged on results. The reliable check is a dissolved solids meter reading of the feed water and of the product water: a healthy membrane rejects the large majority of what comes in, and a rejection rate that has fallen markedly since the system was new means the membrane is done. Slow tank refills, water that tastes flat again, or a unit that runs to drain constantly all point the same way, though a failed automatic shutoff valve or a low tank air pressure can mimic them. Our reverse osmosis replacement filters guide covers matching elements to your housing sizes.
Questions people ask about replacing reverse osmosis filters
In what order should RO filters be replaced?
Work through the line: sediment prefilter, then carbon prefilter, then the membrane if it is due, then the post filter. Keeping the order avoids fitting the wrong cartridge in the wrong housing.
Why does the carbon prefilter matter so much?
It removes chlorine, which damages a thin film membrane. Running that cartridge past its life is the most common reason a membrane fails years earlier than it should.
Do I need to sanitize when I change filters?
Not every time, but it is sensible if the system has been open a while or has ever tested poorly. Follow the manufacturer procedure and never leave a sanitizer in contact with the membrane.
Should I throw away the first tank of water?
Yes. New carbon sheds fine particles and the first tank often tastes of it. Drain a full tank, and a second if any taste remains, before using the water.