Installing an under-sink reverse osmosis system is a half day job for a competent homeowner and mostly a matter of doing five connections properly. Nothing needs soldering. What catches people out is the order of work, the drain saddle position, and the flushing at the end that most instruction sheets treat as an afterthought.
Before you touch anything
Lay out the box and check every part is there, because discovering a missing fitting halfway through with the water off is a bad afternoon. Clear the cabinet completely and put a towel down. Check incoming pressure, since RO performance depends on it and a marginal supply needs a booster or permeate pump planned in from the start rather than retrofitted. Decide where the manifold mounts, leaving enough clearance below the cartridges to drop them out, and where the tank stands upright. Confirm you have a faucet hole or a plan for making one. Then shut the cold supply at the angle stop, open the tap above to relieve pressure, and have a bowl ready for the water still in the line.
The feed, the drain and the faucet
Fit the feed valve to the cold angle stop, cold side only, and check it seats and seals before moving on. Next the drain: the saddle clamps onto the sink tailpiece above the trap, on the opposite side of the disposal inlet, and you drill through the tailpiece with the supplied bit and align the saddle hole precisely with it. Position it so the drain tubing runs downhill all the way with no dips, since a sagging drain line is behind most gurgling complaints later. Then mount the faucet in its deck hole, with the base seal above and the washers and nut below, snug enough not to rotate. Cut every tube square, push it fully into each fitting until it stops, and add the locking clips where the kit supplies them.
Tank, manifold and first startup
Mount the manifold on the cabinet wall with the cartridges hanging where you can reach them, and fit the cartridges in the order printed on the housings. Stand the tank upright and connect it through its own valve so it can be isolated. Check the tank air charge with the tank empty of water before connecting it, since this is far easier now than later. Connect feed, membrane, tank and faucet in the order the manual specifies, then double check every push fitting is home. Open the feed valve slowly, open the faucet, and let air push out while you watch every joint with a dry paper towel. Leave the cabinet dry and check again after an hour and again the next morning.
The commissioning flush, and the record you should keep
A new system must be flushed before you drink from it. Carbon fines make the first water gray, and the membrane and tank both carry preservative from manufacture. Let the tank fill, then drain it completely through the faucet, and repeat as many times as the manual specifies. Discard that water. Once the system is running properly, take a TDS reading of the feed water and one of the treated water on the same day and write both on a label stuck to the manifold, along with the install date. That pair of numbers is your baseline: comparing against it in a year is the simplest honest test of whether the membrane is still doing its job, and our under-sink RO comparison explains what a rising treated figure means.
Questions people ask about how to install reverse osmosis system
Do I need a plumber to install a reverse osmosis system?
Not usually. It is push fit tubing with no soldering, though drilling a stone countertop for the faucet and any code-required air gap are worth handing to a professional.
Where does the RO drain line connect?
To a saddle clamped on the sink tailpiece above the trap, away from the disposal inlet, with the tubing running downhill for its whole length to avoid gurgling.
How long before I can drink the water?
After the commissioning flush. Fill and fully drain the tank as many times as the manual specifies, to clear carbon fines and the preservative in a new membrane.
Can I install the system somewhere other than under the sink?
Yes. Mounting it in a basement or utility space below and running tubing up to the faucet is common where the cabinet is too full or too shallow.