How to Install an Under-Sink Reverse Osmosis System (2026) — Step-by-Step Guide

Please note: Product information is based on our research at time of writing and may change. This guide is general. Always follow the installation instructions that come with your system, and check local plumbing code if you’re unsure.

Introduction

An under-sink reverse osmosis (RO) system filters your drinking water through several stages and a membrane, storing the result in a small tank and delivering it from a dedicated faucet. Installing one is a bigger job than swapping a filter cartridge. You’ll tap into the cold supply, mount a faucet on the sink or countertop, and connect a drain line.

This guide covers tank-based systems, which are what most under-sink RO kits are. Tankless electric systems skip the tank steps but need a nearby outlet and have their own instructions. To choose a system first, see our best under-sink water filters review, or the whole-house vs. under-sink comparison.

Tools and Materials Needed

• Your RO system — including the feed adapter, faucet, drain saddle, tubing and storage tank
• Variable-speed drill with a 1/2 inch bit — APEC lists 1-1/4 inch for an air-gap faucet
• A diamond core bit for granite, or a titanium bit for steel — per iSpring’s instructions
• Adjustable wrench and Phillips screwdriver
• Teflon tape
• Utility knife
• Bucket and towels
• A standard tire pressure gauge — to check the tank

Before You Start: Four Checks

1. Your countertop or sink material. iSpring’s manual says to use a diamond core bit for granite and a titanium bit for steel, warns against a hammer drill on natural stone, glass and ceramic, and cautions that porcelain chips easily. For natural stone it recommends hiring an experienced professional. APEC advises marking stainless steel with a center punch so the bit doesn’t walk.

2. Your shutoff valve. You’ll need a cold water shutoff under the sink that turns. If it’s stuck or corroded, see our guide to removing a stuck or corroded shutoff valve, or call a plumber.

3. Your drain. The system’s drain line connects to the sink drain through a saddle clamp, so you need an accessible section of drain pipe. If yours is inaccessible or unusual, ask a plumber.

4. Your water. APEC’s manual says not to use the system with water that is microbiologically unsafe or of unknown quality without adequate disinfection before or after it.

The manuals we read differ on the exact order and details, so follow your own system’s manual. The steps below are the ones iSpring and APEC have in common.

Step 1 — Shut Off the Water and Release the Pressure

Turn off the cold water shutoff under the sink, then open the kitchen faucet to release pressure and make sure the water has stopped, as iSpring’s manual describes. If the shutoff won’t stop the water, APEC says the main supply to the house must be shut off.

Step 2 — Fit the Feed Adapter

The feed water adapter connects to the cold shutoff valve. iSpring notes a bushing that converts a 3/8 inch compression connection to 1/2 inch NPT, so check your shutoff valve’s thread against the adapter before you start.

Step 3 — Drill and Mount the Faucet

Mark the location, then drill the faucet hole in the sink deck or countertop with the bit your material calls for. A standard RO faucet takes a 1/2 inch hole, and APEC lists 1-1/4 inch for an air-gap faucet. Go slowly, especially on stone, porcelain or stainless steel. Then mount the faucet as your kit describes.

Step 4 — Install the Drain Saddle

The saddle clamps onto the drain pipe. iSpring’s steps are to drill a 1/4 inch hole in the drain pipe, paste the sticky pad around the hole, insert the tubing, and tighten the back plate with its two screws. APEC’s instructions place the saddle above the trap and warn that misaligned holes will block the drain water and can damage the membrane, so line the saddle hole up carefully.

Step 5 — Mount the System and Connect the Tubing

Mount the filter housings or manifold under the sink as your manual shows, then connect the tubing between the feed adapter, filters, faucet, drain saddle and tank. Follow the color codes or port labels in your manual, and push each tube in fully.

Step 6 — Prepare and Connect the Tank

iSpring’s manual has you wrap the tank fitting with 10 to 15 turns of Teflon tape, clockwise, and tighten by hand without overtightening. Check the tank’s air pressure with a tire gauge before connecting it. iSpring’s manual gives 7 to 10 psi, and APEC’s tanks come pre-charged at 5 psi (4 gallon) and 7 psi (14 gallon), so check your own tank’s specification.

Step 7 — Start Up and Flush

Turn the cold water back on slowly. iSpring’s manual has you flush the first three stages into a bucket until the water runs clear, then open the RO faucet and let it trickle for at least 15 minutes. The tank then fills. iSpring lists 1.5 to 2.5 hours and APEC lists 2 to 3 hours.

Step 8 — Discard the First Tank and Check for Leaks

Both iSpring and APEC say not to use the first tank of water. Drain it fully through the RO faucet. Check every connection for leaks as it fills, and iSpring recommends checking daily for the first two weeks.

Troubleshooting

Leak at a tubing connection. Turn off the supply, take the tube out and check that it’s cut squarely and pushed in fully. Refit it and test again.

Water at the drain saddle. Turn off the water and check that the saddle pad is seated flat around the hole and the back plate screws are snug, without overtightening.

Slow or no water from the RO faucet. The tank may still be filling, so give it the hours your manual states. Then check that the supply valve is open, no tubing is kinked, and the tank’s air pressure matches its specification.

Drain problems after installing. APEC warns that a misaligned saddle hole blocks the drain water and can damage the membrane. Shut off the supply and recheck the alignment.

A Note on Plumbing Code and Air Gaps

Plumbing codes address how drinking water treatment units discharge to the drain. The Uniform Plumbing Code language we found calls for discharge through an air gap or an approved air gap device, and adoption differs from place to place. Both iSpring and APEC leave code compliance to the installer, and APEC’s manual says the installation needs to comply with state and local laws. Some RO faucets are air-gap faucets, which need a larger hole. If you’re unsure what your area requires, ask your local building department or a licensed plumber. For how an air gap works in general, see our dishwasher air gap vs. high loop comparison.

Tankless and Electric Systems

Tankless RO systems, such as Waterdrop’s, work differently. The manual we read describes a powered unit with a power cord and no storage tank to pressurize, so it needs an outlet nearby and skips the tank steps. It also flushes itself automatically. Follow the system’s own manual for those parts of the job.

When to Call a Licensed Plumber

• Your countertop is natural stone such as granite or quartz
• The cold shutoff is corroded or won’t turn, or there’s no accessible cold supply
• The drain can’t take a saddle, or you’re unsure what your local code requires
• A leak continues after you’ve re-seated the connections

Frequently Asked Questions

Q: Can I install an RO system myself?

Many homeowners do. It needs a drill, hand tools and patience, and the drilling and drain connection are the two steps to take slowly. Hire a professional for natural stone.

Q: Do I need an air-gap faucet?

It depends on your local code. Some manufacturers sell both types, and an air-gap faucet needs a larger hole (APEC lists 1-1/4 inch). Ask your local building department if you’re unsure.

Q: Can I drink the first tank?

No. Both iSpring and APEC say not to use the first tank of water. Drain it fully first.

Q: How long until I have RO water?

The tank takes roughly two to three hours to fill, with iSpring listing 1.5 to 2.5 hours and APEC 2 to 3. Then discard that first tank.

Q: Is a whole-house filter the same thing?

No. An under-sink RO system treats drinking water at one faucet. See our whole-house filter review for the other approach.

Conclusion

Installing an RO system is a step up from most under-sink jobs, but a careful homeowner can do it: shut off and depressurize, fit the feed adapter, drill and mount the faucet, connect the drain, plumb the system and tank, then flush and discard the first tank. Take extra care on the two steps that can go wrong, and bring in a plumber for stone countertops or unclear code questions. To choose a system, see our under-sink filter review.