Replace your RO filters by turning off the water supply, unscrewing the old cartridges, and installing new ones in the same order
A reverse osmosis (RO) system filters water through a series of cartridges that trap sediment, chlorine, and other contaminants. Each cartridge has a lifespan—usually 6 to 12 months depending on your water quality and how much water you use. When flow slows or water tastes off, the filters need replacing. The job takes 15 to 30 minutes and requires no special tools beyond what came with your system.
Most RO systems sit under the sink in a housing with three to five cartridge slots stacked vertically. You'll replace them one at a time, starting from the top. The order matters because each stage does different work: the first cartridge catches sediment, the second removes chlorine, and the third (and sometimes fourth) polish the water further. The final stage is the RO membrane itself, which you may not replace as often as the pre-filters.
Key Takeaways
- Turn off the water supply at the shutoff valve under your sink before you start, then open the faucet to release pressure.
- Replace cartridges in order from top to bottom, and always install new ones in the same slots—do not swap their positions.
- Flush the system for 5 to 10 minutes after installation to clear air and loose carbon particles from the new filters.
- Mark the replacement date on each new cartridge with a permanent marker so you know when to replace it again.
- If water still tastes bad or flows slowly after replacement, the RO membrane may need replacing—a job best left to a professional.
What you need before you start
Gather your replacement cartridges before you begin. You need to know which model RO system you own—check the manual or look for a label on the housing itself. Cartridge types are not universal; a filter for one brand will not fit another. Order the exact cartridges your system uses, or buy a replacement kit that includes all the pre-filters and membrane together.
You will also need a bucket or large bowl to catch water that drains from the housing when you open it. Have a clean towel nearby. Some systems come with a wrench or cartridge removal tool; if yours did, keep it handy. If you do not have the tool, you can usually unscrew cartridges by hand, though they may be tight.
Turn off the water and release pressure
Locate the shutoff valve under your sink—it is usually a small knob on the cold water line that feeds your RO system. Turn it clockwise until it stops. Do not force it; it should turn smoothly.
Next, open the RO faucet (the small tap on your counter or sink that dispenses filtered water) and let water run for 10 to 15 seconds. This releases the pressure built up inside the housing. You will hear air hiss out. When the water stops, the system is depressurized and safe to open.
Remove the old cartridges one at a time
Place your bucket under the housing to catch any water that spills. Most RO housings have a large plastic cap or head at the top that holds the cartridges. Unscrew this cap by turning it counterclockwise. It may be tight—use your wrench or removal tool if needed, or wrap a towel around it for grip.
Once the cap is off, you will see the cartridges stacked inside. The top cartridge (usually white or blue) is the sediment pre-filter. Grip it firmly and pull straight up. It should slide out. If it is stuck, twist it gently side to side as you pull. Set it aside.
Repeat this for each cartridge, working from top to bottom. Note the order as you remove them—you will install new ones in the same sequence. The bottom cartridge is usually the RO membrane, which looks different (often a white cylinder with a plastic cap). If your manual says the membrane lasts longer than the pre-filters, you may not replace it this time; check the cartridge dates or your manual to be sure.
Install the new cartridges in the correct order
Before you insert a new cartridge, rinse the inside of the housing with clean water to remove any loose sediment or debris. Wipe it dry with a clean towel.
Take the first new cartridge (the sediment pre-filter) and insert it into the top slot. Push it straight up into the housing until it seats firmly—you should feel it click or lock into place. Do not force it; it should slide in smoothly. Repeat for each cartridge in order, always working from top to bottom and always installing them in the same slots they came from.
Once all cartridges are installed, screw the housing cap back on by turning it clockwise. Tighten it firmly but do not over-tighten; you want it snug enough that it will not leak, but not so tight that you cannot remove it next time.
Flush the system and check for leaks
Turn the shutoff valve back on by turning it counterclockwise. Water will begin to flow into the housing again. Open the RO faucet and let water run for 5 to 10 minutes. This flushes out air bubbles and loose carbon particles from the new filters. The water may look cloudy or taste slightly of carbon at first—this is normal and will clear.
While the water is running, check under the sink for leaks around the housing cap and cartridge connections. If you see drips, turn off the water, wait a moment, and tighten the cap a bit more. Turn the water back on and check again. Small drips at the cap often stop once the system pressurizes fully.
After 10 minutes, turn off the faucet. The system is now ready to use. The water should taste fresh and flow at normal speed. If it still tastes bad or flows slowly, the RO membrane may be failing—call a plumber or the system manufacturer for help.
Mark the replacement date on your new filters
Use a permanent marker to write the installation date on each new cartridge. Write it on the plastic housing or the cartridge itself. This straightforward step saves you from guessing later about when you last replaced them.
Most pre-filters last 6 to 12 months; RO membranes often last 2 to 3 years. If you use a lot of water or have poor water quality, filters may need replacing sooner. If you use very little water, they may last longer. Checking the date each time you look under the sink keeps you from forgetting.
Frequently Asked Questions
What if water leaks from the housing cap after I install new cartridges?
Turn off the water, wait 30 seconds, and tighten the cap another quarter turn clockwise. Turn the water back on and watch for leaks. If it still drips, turn off the water again and remove the cap. Check that the rubber O-ring (a small rubber seal) inside the cap is clean and seated properly. Wipe it dry, reseat it, and try again. If leaking continues, the O-ring may be damaged and need replacing.
Can I replace just one cartridge, or do I have to replace them all at once?
You can replace cartridges individually as they wear out. The pre-filters usually need replacing first, often before the RO membrane. Check your manual for the expected lifespan of each stage. If one cartridge is clogged but others are fine, replacing just that one is normal and saves money.
The water still tastes bad after I replaced the filters—what's wrong?
First, make sure you flushed the system for at least 10 minutes after installation. If the taste persists, the RO membrane itself may be failing and need replacement. This is a more involved job; contact the system manufacturer or a plumber. Do not drink the water if it tastes salty or off—the membrane may not be filtering properly.
How do I know which cartridges to order?
Check your RO system manual or look for a model number on the housing label. Search that model number online or call the manufacturer to confirm the exact cartridge part numbers. Buying the wrong size will not fit. Many systems also sell replacement kits that include all pre-filters and the membrane together, which is simpler than ordering them separately.
Do I need to turn off the water supply every time I replace a filter?
Yes. Turning off the water and releasing pressure prevents water from spraying out when you open the housing. It also protects the system from damage and makes the job cleaner and safer.