How to install an under sink reverse osmosis system

Reverse osmosis systems dispense from their own small faucet rather than the main kitchen faucet, because the treated water needs to bypass the cold supply line entirely and come straight from the storage tank. Many kitchen sinks have a spare hole already, capped for a soap dispenser or sprayer that was never installed, and that spare hole is the easiest path. Where no spare hole exists, one has to be drilled through the sink deck or countertop, which on stone counters is a job for whoever installed the counter rather than a do it yourself step.

Some plumbing codes require an air gap faucet rather than a standard one, specifically because reverse osmosis produces a drain connection that a non air gap faucet does not physically separate from the supply. An air gap faucet has a visibly taller base than a standard RO faucet, and it makes a gurgling sound whenever the system drains. That gurgle is the air gap working correctly, not a defect, and it surprises almost everyone the first time they hear it.

A conventional tank system runs at roughly twenty percent recovery, meaning about four gallons go to drain as concentrate for every gallon of treated water produced, and that concentrate needs a way out of the plumbing that does not risk it siphoning back into either the supply or the drinking water side.

A drain saddle valve clamps onto the vertical tailpiece of the sink drain, below the trap, and is drilled through with the fitting's own bit so the connection is sized correctly and sealed against the pipe wall.

Placement above the trap risks trap siphoning; placement too close to the disposal outlet risks backflow of disposal waste into the RO drain line. Most manufacturer instructions specify a location on the vertical tailpiece a short distance above the trap and clear of the disposal connection, and following that placement rather than whatever spot is most convenient avoids both problems.

The feed water comes from a tee installed on the cold supply line under the sink, typically using a saddle valve or a dedicated feed water adapter that clamps onto the supply line or replaces the angle stop entirely. A dedicated feed adapter that replaces the angle stop is the more reliable long-term connection; a saddle valve that pierces the pipe wall is faster to install but is also the connection most likely to develop a slow weep over years, so it is worth checking periodically even after a clean initial install.

Reverse osmosis systems use color coded quarter inch push to connect tubing between the feed valve, the prefilter stage, the membrane housing, the storage tank, and the faucet, with each color typically standardized by the manufacturer for feed, product and drain lines. Push to connect fittings need a clean, square cut on the tubing end and a firm push until the collar seats; a tubing end cut at an angle or not pushed fully in is the single most common cause of a slow weep discovered days after the install rather than immediately.

The storage tank has its own shutoff valve, usually a small ball valve at the top of the tank, which lets the tank be isolated from the rest of the system for a filter change or a pressure check without draining the whole system down. See a standard four gallon storage tank for the reference shape most systems ship with, and how to set the tank's air charge is its own procedure, covered separately.