How to replace the filters of an under-sink reverse osmosis system

An under-sink reverse osmosis (RO) system is several parts in a row, and each part has its own timetable. Replacing them is a routine job that many owners do themselves, and it goes smoothly when you know what each stage is, work in a sensible order and finish by checking for leaks. This guide covers the usual stages, a safe routine, what the storage tank is for and what to look at afterwards. It is general information. Your system's manual comes first, and if you are unsure about any step, ask a licensed plumber.

The usual stages in plain words

Systems differ, and some add extra stages, so compare this list with your own manual. Water normally travels through these parts in order:

1. Sediment pre-filter. The first cartridge in the line. It is made for solid particles such as grit, and it sits first so that the stages after it are not left to handle them.
2. Carbon filter or filters. One or two cartridges made with activated carbon, either loose granules or a pressed block. The maker's description says what each one is made for.
3. The membrane. A thin sheet with extremely small openings, usually rolled into a cylinder and sealed inside its own housing. This is the part that gives reverse osmosis its name. It works slowly, which is why most systems add a tank or a pump.
4. Post-filter. A final cartridge between the tank and the faucet, often carbon as well.

What the storage tank is for

A reverse osmosis membrane produces water at a trickle. The storage tank collects that water between uses, so when you open the faucet you get a steady stream instead of a trickle. Systems without a tank usually rely on a pump instead, and if yours has one, follow the manual's instructions for switching it off before you work on the system. A tank has its own valve, and many manuals tell you to close it before changing parts. Tanks and complete systems are listed under RO Tanks & Systems.

What to replace, and when

The instructions for your system decide the schedule. Many makers give both a time and a volume, and you change a part at whichever limit you reach first. Each part has its own schedule, and in general the pre-filters come due more often than the membrane, but the exact intervals belong to your maker, not to this page. When several parts fall due together, the usual habit is to work in the order the water flows, from the first stage to the last, unless your manual says otherwise.

Before you order, find the model name or number of your system and the stage each part belongs to. Complete sets for the cartridge stages are under RO Filter Sets, and the membrane has its own section, RO Membranes. Check the size and connection against your manual before you choose, because the product pages describe the parts and your manual describes your system.

A safe routine, step by step

1. Read the manual first. Gather the new parts, the housing wrench that came with the system, towels, a shallow tray or bucket, and a marker.
2. Turn off the water supply to the system, using the feed valve your manual describes. If the manual says to close the tank valve, close it too.
3. Release the pressure the way the manual describes, usually by opening the system's own faucet until the flow stops. Housings still hold water after that, so keep the tray under them.
4. Note the order of the housings. Take a photo and put a strip of tape with a stage number on each one, because housings can look alike.
5. Open one housing at a time with the wrench, holding it upright. Take out the old part, look at the O-ring (the round seal) and the inside of the housing, and clean or replace them as the instructions say.
6. Fit the new part exactly as the instructions show. Check that the O-ring sits in its groove, and tighten as the manual says. Do not force a housing: over-tightening can damage seals and plastic.
7. Turn the supply back on slowly and watch each housing and joint while the system fills.
8. Flush the new parts as the instructions say. Many tell you to run the system, and to discard the first water that comes out, for a stated time or volume. Membranes in particular come with their own instructions. Keep a new membrane sealed in its wrapping until you fit it, and handle it with clean hands.
9. Write the date on each housing or on a label, and put the next due dates in your calendar.

Some manuals add a sanitising step when certain parts are changed. If yours does, follow it exactly and use only what the maker names. The same goes for lubricant on O-rings: use only what the instructions allow.

Check fittings and tubing afterwards

Changing parts opens the housings and pressurises everything again, which tests every joint. When the system has refilled, dry the outside of each housing with a paper towel, then look and feel around every housing, every push-in fitting, every connection to the tank and the drain connection. A small drip is still a leak. If you find one, turn off the supply, release the pressure and look at the joint: is the O-ring seated, is the housing tightened as the manual says, is the tubing pushed in fully? Check again after an hour, again at the end of the day and again the next morning, ideally with a dry paper towel under each joint so a slow drip shows up.

Look at tubing and fittings every time you change a filter. Spare lengths and connectors are under Tubing & Connectors, and Installing & maintaining gathers the small parts and tools that make the job easier. If a joint keeps leaking after you have followed the manual, ask a licensed plumber, and remember that local plumbing rules can apply to some work.

After the change

Set a reminder for the next due date. The terms you will meet on a pre-filter's description, such as rating, construction and size, are explained in Sediment cartridges explained. If your question is about your water rather than your system, contact your local water utility (most publish an annual water quality report), a certified testing laboratory or a health professional. A filter page cannot answer that for you.