An under-sink reverse osmosis unit is a common home install, and most of the trouble comes from doing the steps out of order or skipping the leak checks. This guide lays out the sequence a careful installer follows, so you can watch for the right things whether you do the work yourself or hire it out.
Before the first fitting
Confirm that the kit contents match the model number on the manufacturer's sheet. Clear the cabinet, and find the cold supply shutoff under the sink and the drain point where the waste line runs. Check that there is room for the membrane housing, the storage tank, and the faucet line to reach without kinking. Read the manufacturer's instructions for the tank pressure and the order of the filter stages, because those details differ by model.
Step 1 and Step 2: cut the supply and set the faucet
Shut off the cold supply at the valve under the sink, then open the faucet to drain the line and drop the pressure. Mount the faucet, drilling a new hole only if the sink has no spare opening. Confirm the faucet is seated and tight before you move on.
Step 3 and Step 4: drain and feed connections
Fit the drain saddle clamp on the waste line above the trap, following the manual for the hole size and position. Then run the feed tubing from the cold supply through a tee or adapter to the unit. Compression and push-fit connections should be snug, not forced. Plastic tubing should not be bent sharply or pinched behind the cabinet.
Step 5 and Step 6: stages and storage tank
Connect the filter stages in the order the manual shows, and set the storage tank to the pressure the manufacturer specifies. Do not guess at the pressure. An incorrect setting is a common reason a unit runs slowly or fills poorly.
Step 7: open slowly and check for leaks
Open the supply slowly and let the system fill. Before you flush anything, inspect every fitting, the drain saddle, the tank connection, and the faucet base. A small weep at a compression fitting is easier to fix now than after the cabinet floor is wet.
Step 8 and Step 9: flush and confirm the drain
Run the flush the manual describes and discard the initial water. Check that the drain flows freely and that the tank refills after a draw. A unit that never refills, or a drain that gurgles badly, needs attention before you drink from it.
What to watch for in the installer's work
Ask whether the installer checks the feed water before connecting, and whether the shutoff is confirmed closed before cutting tubing. Ask for the flush step to be shown to you. Good installers leave you with the model number, the filter change schedule, and a written note of what was installed, and they tell you where the shutoff is. If you are not comfortable with any of these steps, ask whether a licensed plumber handles that part of the job.