An under-sink reverse osmosis unit is a small job with a long list of details, and most problems homeowners run into come from a step that was skipped or done out of order. Knowing the sequence lets you check each stage as it happens and ask about anything that does not fit.
Before the crew arrives
Ask the installer to confirm in writing the faucet location, the drain connection method, where the storage tank will sit, and whether the unit needs electrical power. Measure the cabinet under the sink, including its depth and the clearance behind the pipes, and clear it out so the crew has room to work. Ask whether a booster pump or an extra filter stage is part of the plan, since low incoming pressure is a common reason for that addition.
Step one: the site check
A careful visit starts with a look at the cold water supply line, the shutoff valve that feeds the sink, the drain line, and the space around both. The installer should check water pressure at the sink and tell you the result. If the cold line is old, corroded, or hard to reach, the crew should say so before cutting into it, and ask whether a new shutoff valve is needed.
Step two: shut off and mount
The cold supply to the sink is closed, the faucet is opened to relieve pressure, and the unit and tank are mounted according to the manufacturer's instructions. Watch for a tank set on an unstable surface or a filter housing hung where it cannot be reached for a cartridge change. Ask that the shutoff valves be labeled so the next person knows which valve controls the unit.
Step three: drain and feed connections
The drain should connect by the method in the plan, such as a drain saddle or an air gap fitting, and the crew should explain which one they used and why. The feed tubing should run without kinks or sharp bends. Every compression fitting should be checked by hand after it is tightened, then watched with a dry paper towel for a drip. Local plumbing rules apply to drain connections, so ask whether the connection meets the requirements for your town.
Step four: the dedicated faucet
Most under-sink units feed a separate faucet, which needs its own mounting hole in the sink or countertop. If the sink has no spare hole, the crew should tell you the options before drilling, since the choice affects appearance and how the faucet mounts. Check that the faucet handle operates smoothly and that the hoses do not twist when the faucet is used.
Step five: flush, fill, and check
Follow the manufacturer's flush procedure, which usually clears air and loose carbon fines from the new filters and drains the tank before first use. Let the tank fill as the manual describes, then run the faucet and watch the connections for a while. Ask the crew to show you the leak check they performed. Taste the water only after the flush is complete, because the flush is what makes the first glass worth drinking.
What to leave with you
Before the crew leaves, ask for the manual, a written list of the filters installed with their replacement intervals, the location of the shutoff valves, the warranty paperwork, and a service contact. A spare set of filters on hand helps when the first change comes due. If anything is missing, ask for it before the crew leaves, since chasing paperwork later takes far longer than getting it on the spot.