How Do I Install an Under-Sink RO System?

How to Install an Under-Sink RO System

Installing an under-sink reverse osmosis (RO) system gives you high-quality drinking water right at the sink. This guide walks you through the full installation from start to finish - with the little details that prevent leaks, noise, and “why is this not working?” moments.

Important Notice

This guide is provided for educational purposes to explain common installation steps and best practices. Plumbing setups and code requirements vary by location. For the safest and most reliable installation, Crystal Quest recommends using a qualified, licensed plumber and following all local plumbing codes. Improper installation can cause water damage.

System Requirements (Quick Check)

Before you start, make sure your setup falls within typical RO operating conditions:

Cold potable water only
Feed pressure: ideally within 30-60 PSI (do not exceed 60 PSI)
Operating temperature: 40–85°F
Tank pre-charge: about 7 PSI when the tank is empty

Tip: If your home’s water pressure is high, install a pressure regulator. Higher pressure can stress fittings and increase leak risk.

Tools and Materials Needed

Most installs use:

Safety glasses
Adjustable wrench / crescent wrench
Phillips + flathead screwdriver
Variable-speed drill + bits (1/8" and 1/2" are commonly used)
Tube cutter (recommended—clean square cuts matter)
PTFE/Teflon tape
Flashlight/work light
Small air gauge + hand pump (for checking tank pressure)

Package Contents Checklist

Before starting, confirm you have:

RO unit (prefilters + membrane assembly)
Storage tank + tank shutoff/ball valve
RO faucet + mounting hardware
Feed connection hardware (commonly a T-valve or saddle valve, depending on kit/version)
Drain saddle clamp assembly
Tubing + fittings
Mounting bracket + screws

Also inspect everything for shipping damage before installing.

Step-by-Step Installation

Step 1: Prep the Work Area

Turn off the cold water shutoff valve under the sink.
Open the cold side of your kitchen faucet to relieve pressure.
Clear out the cabinet completely.
Place towels or a tray under the shutoff valve and plumbing connections.

Space planning (makes service easier later)

The RO unit usually mounts on the left or right cabinet wall
Keep it closer to the front so filter changes are easy
The storage tank usually sits toward the back
If wall mounting, keep the RO unit at least ~2" above the cabinet floor
Tank can be vertical or horizontal depending on space

Step 2: Install the RO Faucet

Your RO faucet is where the purified water comes out. The cleanest install is usually at the sink deck or countertop near your main faucet.

Option A: Use an Existing Hole (Sprayer / Soap Dispenser)

Remove the sprayer/soap dispenser if present.
Clean the surface around the hole.
Insert the faucet stem down through the hole.
From below, install the rubber washer, metal washer, and mounting nut (order may vary by faucet).
Tighten firmly, but don’t overtighten—you can crack surfaces or distort the gasket.

If you disconnect an old sprayer, make sure the sprayer line is properly capped/plugged so it doesn’t leak.

Option B: Drill a New Hole (If Needed)

Choose a location with good clearance above and below.
For stainless steel: use a step bit or hole saw and drill slowly (cutting oil helps).
For stone countertops: professional drilling is strongly recommended to avoid cracking.
Deburr/smooth sharp edges before mounting the faucet.

Hole size note: Faucet hole sizes vary by faucet style (air-gap vs non–air-gap, manufacturer design). Always drill/cut the hole size specified by the faucet hardware/instructions included with your faucet.

Step 3: Connect to the Cold Water Supply (Feed Water)

This step provides source water to the RO system. Most installs use one of two feed methods:

Option A: T-Valve (Most Common on Flex Lines)

Confirm cold water is OFF.
Disconnect the cold-water supply line from the shutoff valve.
Apply PTFE/Teflon tape where appropriate (threaded connections).
Install the T-valve adapter inline (between the shutoff and the faucet supply line).
Reconnect the faucet supply line.
Connect the RO feed tubing to the T-valve outlet (push-to-connect or compression, depending on your fitting).
Turn the water back on slowly and check for leaks.
Leave the RO feed valve OFF for now until you’re ready for startup.

Option B: Saddle Valve (Some Configurations)

Saddle valves are sometimes used on rigid piping depending on kit/version and plumbing setup.

Best practices:

Choose an accessible location (side of vertical pipe is best).
If installing on a horizontal run, tap the side/top (not bottom) to reduce sediment issues.
Keep the saddle valve closed until startup.

Local plumbing codes vary, some areas restrict saddle valves. When in doubt, use a plumber or choose the T-valve method.

Step 4: Install the Drain Saddle (Do This Carefully)

The drain saddle sends RO “reject water” to the drain. Placement matters.

Placement rules that prevent problems

Install the drain saddle above (before) the P-trap
Avoid installing it too close to a garbage disposal outlet (restrictions and clogs are more common there)
If you have a disposal, a common approach is installing on the drain line between the disposal and the trap (not directly on the disposal body)

Installation steps (the reliable way)

Choose the location and clean the pipe surface.
Apply the foam gasket to the saddle half, aligning the gasket hole with the saddle opening.
Clamp the saddle onto the drain pipe evenly (don’t overtighten or cross-thread).
Once clamped, drill through the saddle opening into the drain pipe.
- Drill through one wall of the pipe only (do not drill through both sides).
Connect the drain tubing to the saddle fitting and tighten.

Why clamp first? It ensures the drilled hole aligns perfectly with the gasket, this reduces leaks and avoids misalignment.

Step 5: Set Up the Storage Tank (and Check Air Pressure)

The storage tank holds RO water so it’s available on demand.

Place the tank where it can sit stable (upright or horizontal).
Confirm the tank air pre-charge is around 7 PSI when empty.
Install the tank valve and tank shutoff/ball valve (use thread seal tape if applicable).
Connect the tank tubing to the tank shutoff valve.
Keep the tank valve closed for now until startup.

Step 6: Mount the RO System

Hold the mounting bracket on the cabinet wall and mark screw holes.
Drill pilot holes and mount the bracket securely.
Hang the RO unit on the bracket (or set it on the cabinet floor if wall mounting isn’t possible).
Make sure tubing has room to bend naturally—no kinks.

Step 7: Complete Tubing Connections

Most systems include a connection diagram and often color-coded tubing. Final routing usually includes:

Typical connection points:

Feed water: from your feed valve to the system inlet
Tank line: from the system to the tank shutoff
Faucet line: from post-filter to RO faucet
Drain line: from membrane housing to drain saddle

Best practices for tubing (prevents leaks and low flow):

Cut tubing square (use a tube cutter)
Push tubing fully into quick-connect fittings (seat it completely)
Avoid tight bends and sharp kinks
Keep runs to the tank and faucet reasonably short for best flow
Route tubing so it won’t get pinched by cabinet doors or stored items

Step 8: Startup and Initial Flush

This step removes any manufacturing residue and ensures the system is producing clean-tasting water.

Confirm RO faucet is closed.
Turn on the cold water supply.
Open the RO feed valve slowly.
Check every fitting for leaks.
Open the RO faucet until you get a steady stream, then close it.
Wait about 5 minutes and recheck for leaks.

Flush the first full tank(s)

Let the system produce a full tank of water.
Drain the first tank completely and discard the water.
Let it refill and drain again after several hours.
If any taste remains, drain once more the next day.

RO is slow by design. Depending on conditions, the first full tank can take several hours to fill.

Troubleshooting Common Installation Issues

Leaks at Quick-Connect Fittings

Most commonly: tubing isn’t fully seated or the cut isn’t square.
Fix: turn off water, remove tubing, trim ~1" cleanly, and reinsert fully.

Constant Drain Flow / System Runs Nonstop

Tank may be closed, tank pressure may be off, or a valve/tubing connection may be incorrect.
Check tank valve is open during normal operation and verify tubing routing.

Low Flow at the RO Faucet

Normal while tank is filling.
If it stays low after tank is full: confirm tank valve is open, check for kinked tubing, confirm tank pre-charge is correct.

Reduced Flow at the Main Sink Faucet After Feed Install

Check/clean the faucet aerator as sediment can loosen during installation.

Maintenance Reminders

To keep performance strong:

Replace prefilters/postfilter on schedule (commonly 6–12 months, depending on water quality and usage)
Replace RO membrane as needed (often every 2–3 years, depending on feed water)
Sanitize system as recommended during major service
Check tank air pre-charge yearly (around 7 PSI when empty)
If the system sits unused (vacation), drain the tank and let it refill before drinking

Need Help?

If you’re unsure about drain saddle placement, tank pressure, feed connection choice, or startup behavior, Crystal Quest Technical Support can help you confirm the correct setup for your specific configuration.