Installing reverse osmosis under a kitchen sink is within reach of a careful homeowner with a drill and a couple of hours, and the parts that go wrong are always the same three: the drain connection, the faucet hole and the tubing pushed home only halfway. Get those right and the rest is connecting color-coded lines in the order the manufacturer numbered them.
Before you cut anything
Lay the whole kit out and check the cabinet will take it. You need room for the storage tank standing upright, room to swing the filter housings down for cartridge changes, and access to the cold supply stop. Confirm your incoming pressure is within the unit's stated range, because reverse osmosis is driven by pressure and a low-pressure supply produces very little water and wastes more of it. If your water is hard or heavily chlorinated, sort out softening or carbon prefiltration first, since hardness scales the membrane and chlorine destroys it. Vendors publish these feed water requirements on the product pages for their under-sink systems.
Feed, faucet and drain
Shut off the cold stop, relieve the pressure, and fit the feed adapter between the stop and the faucet supply line. Next the faucet: most sinks have a spare hole under a blank or the soap dispenser, and if not you are drilling stainless or, far more carefully, granite or composite. Then the drain. A saddle clamp on the sink tailpiece above the trap is the common method, drilled once and clamped square, and it must go above the trap and away from the disposal inlet so waste cannot siphon back. Some local codes require an air gap faucet instead of a plain saddle, so check before you buy the kit.
Tubing, tank and the first flush
Push-fit connectors are reliable if you insert the tubing fully, right past the O-ring until it stops, then tug it back to confirm it has locked. Cut tubing square with a proper cutter, not scissors, and avoid kinks and tight radii. Fit the tank valve with thread tape, connect the tank line, and open the feed slowly. Then do the flush the instructions insist on: the first tankful or two carry manufacturing residues and carbon fines, and running them to waste is not optional. Sanitize the system if the parts have been sitting around, and leave a paper towel under every joint overnight so a slow weep shows itself.
Living with it afterwards
Set reminders for prefilter changes, because the carbon prefilter is what keeps chlorine off the membrane and a neglected one costs you the most expensive part. Membranes last for years if the prefilters are honored, and a falling flow rate or a rising taste of dissolved solids is the sign they are done. Check the tank's air charge with the tank empty if delivery gets sluggish, since a flat bladder tank feels exactly like a failing membrane from the faucet. If you are considering the same technology for the whole house rather than the sink, our whole house reverse osmosis system page explains why that is a much larger project.
Questions people ask about installing reverse osmosis
Do I need a plumber to install reverse osmosis?
Not usually for an under-sink unit, if you are comfortable fitting a feed adapter and a drain saddle. Drilling a faucet hole in granite or quartz is the part worth paying for.
Where does the drain line connect?
To the sink tailpiece above the trap, away from the disposal inlet, using a saddle clamp or an air gap faucet where local code requires one.
Why is my new system producing so little water?
Usually low feed pressure, a closed feed valve, a kinked drain line, or a storage tank whose air charge has been lost. Check pressure and the tank before blaming the membrane.
Do I have to flush a new RO system?
Yes. The first tankful or two carry carbon fines and manufacturing residues and must be run to waste before you drink from it, exactly as the instructions specify.