An under sink reverse osmosis installation is within reach of a competent homeowner, and the parts that go wrong are predictable: the drain connection, the faucet hole and the commissioning flush. Work in the order below, dry fit everything before committing, and leave the system running to drain for as long as the instructions say before anyone fills a glass from it.
Plan the cabinet before you cut anything
Lay the parts out and check three clearances. The filter stack needs room below it equal to a cartridge length so elements can be pulled down and changed without dismantling the manifold. The pressure tank needs a stable place to stand upright, and it is heavy when full. The tubing runs need gentle bends rather than kinks. Then decide the faucet position: a spare knockout in the sink deck is ideal, a hole through a stainless or composite worktop is a job for a step drill and patience, and a hole through stone is a specialist's work.
Feed valve, drain saddle and faucet
Turn the cold supply off and take the feed from the cold shut off with the adapter or saddle supplied, then confirm it is dry before going further. Fit the drain connection above the trap and on the vertical or horizontal tailpiece as the instructions specify, never below the trap, so the drain seal keeps sewer gas out of the reject line. Angle the drain saddle so the connection enters above any standing water. Mount the faucet, tighten it against the deck without over compressing the seal, and connect its tubing last so nothing is under strain.
Tank, tubing and the leak check
Check the tank's air charge against the specification before connecting it, because a tank shipped with the wrong pressure or one that has lost charge in storage is the most common cause of a system that never delivers properly. Connect the tubing with the fittings supplied, pushing each line fully home and pulling back gently to confirm it is seated. Open the feed slowly, then check every joint by hand and with a paper towel, twice: once immediately and once an hour later, since a slow weep at a push fitting often shows only after pressure has settled.
The commissioning flush is not optional
New membranes and carbon cartridges carry manufacturing residues and fines, and the first tank of water is not drinking water. Fill and drain the tank the number of times the manufacturer states, usually more than once, with the faucet open and the tank valve open so the whole volume turns over. Only then take a sample of the taste. Once it is in service, note the date, set the prefilter interval, and check the treated water against the feed with a meter every few months so a failing membrane shows as a number rather than a surprise.
Questions people ask about installing a reverse osmosis system
Does the drain connection have to be above the trap?
Yes. Connecting below the trap bypasses the water seal and lets sewer gas into the reject line and back towards the system.
How many times should a new system be flushed?
As many times as the manufacturer states, usually at least two full tank fills, to clear manufacturing residues and carbon fines from the cartridges and membrane.
Can I install one without drilling the worktop?
Often yes, if the sink deck has a spare knockout for a soap dispenser or sprayer. Otherwise a countertop system avoids the hole entirely.
What if the tank fills very slowly after installation?
Check the tank air charge and the feed valve first. Low line pressure, a partly closed feed and a waterlogged tank all mimic a faulty membrane on a new system.