How to Install a Reverse Osmosis Water Filtration System

Aaron Stickley is a licensed plumber with 15 years of experience in commercial, new residential plumbing, and residential service and repair. He started his own residential service and repair plumbing business. Aaron's articles about plumbing on The Spruce span four years.

Richard Epstein is a licensed master plumber with over 40 years experience in residential and commercial plumbing. He specializes in estimating as well as design and engineering for plumbing systems, and he works for one of New York's largest union plumbing construction companies.

• Working Time: 2 - 4 hrs
• Total Time: 2 - 4 hrs
• Yield: Reverse osmosis water filtration system

• Skill Level: Intermediate
• Estimated Cost: $150 to $1000

Installing a reverse osmosis water filtration system offers several advantages over other types of water filtration since it includes four to five different stages of filtration that remove the most harmful contaminants. In a reverse osmosis system, a semipermeable membrane removes ions, molecules, and larger particles from drinking water, including harmful heavy metals such as lead.

Compared to the other point-of-use filtration systems, reverse osmosis tends to be more effective than simpler carbon black (charcoal) filtration systems, and considerably less expensive than ultraviolet disinfectant systems, which are the very best at killing pathogens. Best of all, reverse osmosis filter systems are very easy to clean and maintain and are fully approved by the EPA.

Below, we've detailed how to properly install a reverse osmosis water filtration system.

Tip

Where water supplies are adequately treated with chlorine, a reverse osmosis system can be the best choice — although they are very inefficient, using five gallons of water for every gallon of permeate produced.

Whole-Home Vs. Point-of-Use Reverse Osmosis Filters

While there are reverse osmosis systems you can choose from to treat all the water in the home, it is more typical for an under-counter unit to be installed beneath the kitchen sink where most of the water used for drinking and cooking is provided.

These point-of-use RO filters include four or five separate filtration canisters, each of which filters a different group of contaminants, plus a storage tank that holds the purified water. A countertop spigot, separate from the house faucet, delivers the purified water when needed.

Although the physics of how reverse osmosis filtration works is complicated, installation is fairly straightforward, requiring only basic plumbing skills.

Before You Begin

Before purchasing a reverse osmosis (RO) system, measure the space beneath your sink. Because a reverse osmosis system filters the water so much, the water moves slowly through it. For this reason, it includes a tank to store filtered water for immediate use.

However, this tank occupies a lot of space. If your sink cabinet is filled with garbage disposal or other items, an RO unit with its tank and multiple filters may be hard to fit.

Remote-Installation of RO Systems

If you don't have enough space beneath your sink, point-of-use RO systems can also be installed in a remote location such as a basement, garage, or utility room, then be run up to the sink location.

If you choose remote installation, the filter system must be installed on the cold water line past the water softener if your home is equipped with one. Keep in mind that remote locations will require more work to install. Plus, you may need to buy additional tubing to connect the components.

What You'll Need

Equipment / Tools

• Tape measure
• Screwdrivers
• Level
• Adjustable wrench
• Hacksaw
• Channel-lock pliers

Materials

• Reverse osmosis filter kit for under-sink installation
• Adapter tee (if needed)
• Plumber's pipe-seal tape

Instructions

How to Install a Reverse Osmosis System

Plan the Installation

Measure the space beneath the sink to ensure you have enough room for a reverse osmosis system. Also, make sure you have an idea of what cold water source can be used. Most units come with a tee connection that taps into the 1/2-inch cold water supply line going to the faucet. Other RO units use a 3/8-inch flex line that comes down from the faucet. In this case, an adapter tee above the shut-off valve can be used to provide a source of water for the filter system.

Prepare the RO System

Once you purchase your reverse osmosis system, unbox the storage tank and filter components and double-check to make sure that they'll fit under the kitchen sink in the desired locations.

Tip

When performing any plumbing tasks, it's wise to test-fit components before cutting and gluing. This will let you know if adjustments to the drain pipes or water supply pipes will be necessary, which may require assistance from a plumber.

Install the Sink Spigot

Install the filter's sink-top spigot first so you can run the lines under the countertop before it gets too crowded beneath the sink. To do so, follow these steps:

1. Mount the RO spigot to the kitchen sink deck or countertop with the hardware provided. Many sinks will have an existing knock-out opening that can be used for the spigot. Sometimes the knockout intended for a sink sprayer can be used for the filter's spigot. In the worst-case scenario, you can bore a new hole in the sink deck or countertop to hold the filter's ​spigot.
2. Feed the water line down through the opening and connect the air gap (red tube) to the faucet before locking the spigot down onto the housing.

Prepare and Position the Tank

Before putting the RO tank into position under the sink, install the tank connector. Apply plumber's pipe seal tape onto the threads of the tank nipple, then screw the spigot connector onto the tank, making sure to avoid cross-threading. The connector only needs to be hand-tight and should thread on easily. Now, position the tank in place beneath the sink. Where feasible, position it directly below the sink-top spigot.

Mount the Filter Assembly

To install the filters, follow these steps:

1. Remove the filter assembly cover and mark the locations for the hanger washers on the wall below the sink, using a level to ensure that the unit is straight. This assembly needs to be a minimum of 15-1/2 inches from the floor to allow enough room to remove the filters for maintenance.
2. Screw the hanger washers onto the wall at the marked location, then set the assembly on the washers.

Attach the Water Line

The green 1/4-inch water line from the filter assembly now must be connected to the water supply line. Start by turning off the water supply. How you make this first tubing connection can vary depending on your situation. Most filters come with a 1/2-inch adapter that will work fine if the sink faucet uses a 1/2-inch flex line. If the sink faucet uses flex tubes of a different size, however, you'll need to use an adapter tee to make the transition to the water supply tube running to the filter unit. The green supply tube is connected to the adapter tee with a nut, ferrule, and insert.

Connect the Other Tubing

• A tube running from the water supply to the first canister on the filter unit (the prefilter).
• A tube from the last canister filter (the RO canister) to the sink drain.
• A tube to the storage tank.
• Finally, there's a tube running from the storage tank to the spigot on the countertop.
• In some 5-stage systems, the storage tank water also goes through a "polishing" filter on its way to the sink-top spigot.

Tip

Keep in mind—tubing configurations can vary, so always consult the manufacturer's instructions on the proper tubing connections.

For this particular reverse osmosis system, the yellow tubing runs to the storage tank, the blue tubing inserts into the RO assembly, and the black tube fits into the drain adapter. Before connecting the yellow tubing, follow the manufacturer’s instructions regarding sanitizing the system.

Connect the Drain

For a whirlpool reverse osmosis system, a drain tee is included. To attach the system to the drain, follow these steps:

1. Position the filter's tee fitting up against the existing drain pipe, then mark and cut the pipe, using a hacksaw. When making the cuts, make sure that the tee will fit tightly.
2. Connect the filter's drain tee fitting onto the drain pipe using the provided slip washers and nuts—the connections are made in the same way that a drain trap is assembled.
3. Push the filter's black drain tube into the adapter tee.

Perform a Pressure Test

Pressurize and test the RO system by following these steps:

1. Turn the water supply on and open the cold water on the sink to get any extra air out of the pipes. The pressure will start to build up in the system; you will have to wait for about two hours to get full pressure.
2. Once the system is pressurized, check all of the fittings and tighten any that are loose or leaking.
3. Before using the water for drinking, drain the system by leaving the spigot on and allowing it to purge completely for 24 hours. The water will be a slow trickle after the tank is empty. Leave it on for a full 24 hours, and continue to test for leaks throughout this time.

A reverse osmosis filtration system can easily be installed by DIYers who are familiar with basic plumbing skills. However, in some scenarios, the drain or water supply line may need to be relocated, which may require a professional plumber.

According to HomeAdvisor, installing a reverse osmosis water filter averages $2,200. However, a point-of-use reverse osmosis filtration system can cost as low as $150. On the other hand, a whole-home RO system can top $4,800.

Because water that has been treated with a reverse osmosis filtration system has such a low TDS (total dissolved solids), it becomes more open to dissolving anything it contacts. This low-TDS water can slowly dissolve pipes and fixtures as it passes through them. In copper plumbing materials, copper can leach into the water posing a health hazard and deteriorating the pipes over time. To avoid this, users often pair RO systems with plumbing materials less prone to leaching, such as PEX.

The Spruce uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.

1. https://www.epa.gov/system/files/documents/2022-12/WS-Products-Specification-RO-Systems-factsheet.pdf
2. https://www.epa.gov/watersense/point-use-reverse-osmosis-systems
3. Learn how much it costs to Install a Reverse Osmosis System. (n.d.). Retrieved March 1, 2024, from https://www.homeadvisor.com/cost/environmental-safety/reverse-osmosis-system/