Reverse osmosis system installation under the kitchen sink

Reverse Osmosis Company is an independent referral service, not an installer or plumber. When you send the form, your details go to an independent local water treatment pro who can contact you and prepare a quote.

What is being installed

The EPA’s WaterSense program calls an under-sink RO system a point-of-use system: one connected to a single fixture, such as under the kitchen sink. Inside it, pressure forces water through a semi-permeable membrane. The water that passes through is the treated water, called permeate. The water that does not pass carries away what the membrane held back. The EPA calls it concentrate or brine, and it goes down the drain.

Around the membrane sit the filters. Pre-filters come before it and, on many systems, a post filter after it. The EPA describes RO systems as typically a three, four or five stage filtration process, and the five-stage layout drawn across the top of this page is one arrangement. Systems differ: some use a storage tank and some do not. The parts you will see in the cabinet after an install are these:

• The filter housings and membrane, mounted to the cabinet wall or standing on the cabinet floor.
• A cold water connection, a branch off the cold supply line that feeds the system.
• A drain connection, which carries the concentrate into the sink drain.
• The RO faucet, a small separate tap on the sink or countertop for drinking water.
• A storage tank, on systems that use one, which holds treated water ready at the faucet.
• Tubing between all of them, and to a refrigerator or ice maker if you ask for one.

What a pro checks before the system goes in

Much of what makes a system pleasant to live with is settled before a single fitting is tightened. A reverse osmosis installer looks at the house as well as the box. Expect questions on most of these:

Your water

City water or a private well? If you are on city water, your utility publishes a yearly water quality report, and it is worth having it to hand when you talk to a pro. If you are on a well, the EPA advises testing every year for total coliform bacteria, nitrates, total dissolved solids and pH, using only laboratories certified to do drinking water testing. A recent test result tells the pro what the system needs to deal with. Without one, the first step may be a test. See reverse osmosis for well water.

What you want the system to do

The Texas Commission on Environmental Quality puts the first question plainly: what contaminant do you want to remove or reduce? Better taste, a contaminant on a test, or simply no more bottled water each lead to a different conversation. Say which one is yours at the start.

The sink and the cabinet

Room under the sink matters: the housings, any tank, the garbage disposal if there is one, and room left to change filters later without taking everything apart. The countertop matters too. The RO faucet needs a hole: some sinks have a spare one, and some countertops need one drilled, which is a job for someone with the right bit for that material.

The drain and the water pressure

Because the system sends its concentrate down the drain, the pro looks at where the drain connection can go and how it ties into the sink’s drain line. Because RO works by pressure forcing water through the membrane, the pro may also ask about the water pressure at the house.

Extra outlets

If you want RO water at the refrigerator’s water dispenser or ice maker, say so before the quote. It changes the tubing run and sometimes the parts, and it is easier to plan than to add.

Choosing the system to install

Many people arrive at an installation with a system already in mind, and many leave the choice to the pro. Either way, three things are worth checking before anything goes under the sink.

Ask about	Why it matters
Certification	NSF/ANSI 58 is the standard for reverse osmosis drinking water treatment systems. Ask which contaminants the system is certified to reduce, and check that yours is on the list.
Water to the drain	The EPA says a typical point-of-use RO system sends five gallons or more of reject water to the drain for every gallon it treats, and some up to 10. A WaterSense labeled unit sends 2.3 gallons or less.
Filters and membrane	Ask which filters and which membrane it takes, whether they are easy to buy, and what the maker’s change schedule is.
The quote	Ask what is included in writing: the system, the faucet, any countertop drilling, any extra outlets, and the first filter set.

The TCEQ’s advice applies in any state: select a unit certified by NSF, Underwriter’s Laboratory or the Water Quality Association to remove the contaminants you are concerned about, and make sure the certification covers those contaminants. The Water Quality Association (wqa.org) publishes information on home treatment systems.

Permits for an RO drain connection

Many cities require a plumbing permit for a new RO drain connection. The rules are local, so there is no single answer for the whole country. Ask the pro, when they contact you, whether your city requires one and who pulls it. If you rent, check with the owner before any drilling or plumbing work.

Installing it yourself, or hiring a pro

Much of page one for this search is videos of people installing a system themselves, and plenty of handy homeowners do. The tasks are within reach for someone comfortable with plumbing: shut off the cold supply, tap into it, mount the housings, make the drain connection, fit the faucet, flush the system as its maker says.

A reverse osmosis plumber or installer earns the fee in the places a video glosses over:

• Reading your water report or well test, and matching the system to it.
• Drilling a stone or quartz countertop for the faucet without cracking it.
• Making a drain connection that does not leak.
• Knowing your city’s permit rules.
• Running a line to the refrigerator neatly.
• Being someone local you can ask about later filter changes and repairs.

Replacing an old RO system

If a system is already under your sink, the job may be a replacement rather than a new install. The connections are there; the question is whether to reuse them. Tell the pro the age of the old system if you know it, whether it leaks, and whether you can still buy its filters. Sometimes the right answer is a repair or a filter and membrane change rather than a new system, and a straight quote says so.

How to get ready for the visit

1. Clear the cabinet under the sink so the pro can see the supply line, the drain and the space.
2. Find the cold water shut-off valve under the sink, and the house’s main shut-off.
3. Have your utility’s water quality report or your well test results to hand.
4. Decide where you want the RO faucet, and whether you want a line to the fridge.
5. Write down any questions about filters, certification or water use, so you do not forget them.

After the installation

Before the pro leaves, ask to be shown three things: where the system’s own shut-off is, which housing holds which filter, and the maker’s schedule for changing each one. A small label inside the cabinet door with the date of each change saves guesswork later. Follow the maker’s instructions for flushing a new system before you drink from it.

Questions to ask before you say yes

• Which contaminants is this system certified to reduce, and under which standard?
• Is it WaterSense labeled? How much water goes to the drain per gallon treated?
• Does my water need testing first?
• Where will the housings, the faucet and the drain connection go?
• Does my city need a permit, and who pulls it?
• What is the maker’s filter and membrane schedule, and what do replacements involve?
• What does the written quote include?

Those questions work with any installer. If you want one to contact you, send the form with your ZIP code and choose ‘New RO install’ or ‘Replace an old RO system’. To understand the machine first, read how reverse osmosis works. If you want treated water at every tap, read about whole-house reverse osmosis.

Questions people ask