Reverse osmosis plumbing is three connections and a lot of tubing. Feed water in, concentrate out to the drain, treated water up to a faucet. Every callback and every soaked cabinet traces back to one of those three, or to a push fit joint that was never pushed fully home, so it is worth doing each one deliberately.
The feed connection
Reverse osmosis runs on cold water only, because hot water damages the membrane and the carbon cartridges. The standard method is an adapter tee at the cold angle stop under the sink, which gives the system its own quarter turn shutoff so you can isolate it without shutting the sink. Do not tee off a hot line and do not tee downstream of a fixture. On a well, note the pressure the system needs: membranes are rated at a feed pressure, and a well running at the low end of its pressure switch range can leave the system producing very little.
The drain connection
Concentrate has to leave continuously while the system produces, and it has to leave without anything siphoning back. The usual fitting is a saddle clamped around the sink tailpiece above the trap, drilled and sealed to the tubing port, positioned high on the pipe and above the trap weir. A purpose made standpipe or an air gap faucet is the alternative and is required by some local codes. Keep the connection away from a dishwasher branch, where turbulence causes noise, and never route the drain line into the disposal body below the inlet.
The faucet and any extra runs
The dedicated faucet needs a hole in the sink deck or counter, which on stainless means a step bit and on stone means a diamond core bit, water cooling and a decision you cannot reverse. Sit the faucet where a jug fits under it and where the shank can be tightened from below. From there people commonly add a run to the refrigerator ice maker, which needs a tee, a long tubing route and a check that the system can supply both. A permeate pump is the other common addition, since it reduces backpressure on the membrane.
Fittings, tubing and leak testing
Most systems use push fit fittings and semi rigid tubing. Cut the tube square with a proper tube cutter, never with side cutters, because an angled or crushed end will not seal on the collet. Push the tube fully home, tug it back to confirm the collet has gripped, and use the locking clip if one is supplied. When everything is made up, open the feed slowly, let the system fill, then dry every joint with paper and come back to it twice: once after twenty minutes and once the next morning. A weep you cannot see today becomes a cabinet floor you replace next year.
Questions people ask about reverse osmosis plumbing
Can reverse osmosis be plumbed into a hot water line?
No. Hot water damages the membrane and the carbon cartridges, so the feed must come from a cold supply, normally through an adapter tee at the cold angle stop with its own shutoff.
Is a drain saddle or an air gap required?
It depends on local plumbing code. Some jurisdictions accept a saddle clamped above the trap, others require an air gap faucet or a purpose made standpipe fitting, so check before buying.
Why does my push fit joint keep weeping?
Almost always a tube cut on an angle or crushed by the wrong tool, or a tube that was never pushed fully into the collet. Recut square with a tube cutter and reseat it fully.
Does a whole house reverse osmosis system plumb the same way?
No. Whole house systems need a storage tank, a repressurization pump and a much larger drain provision, so the plumbing is closer to a mechanical room installation than an under sink job.