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8 Steps to Reverse Osmosis Water Filtration System Installation

Fact checked by Stephen Conklin

reverse osmosis water filtration system installation

An RO system helps purify water for drinking and cooking, so there is no doubt that most homeowners consider installing it in their homes. But if you were thinking about reverse osmosis water filtration system installation under the sink, how can you do it properly?

Here is what you can do to install an RO system by yourself. First, prepare the necessary tools, then follow the 8 steps in this article.

Table of Contents

What You’ll Need to Install Reverse Osmosis System


Before you start with the reverse osmosis system installation, prepare the needed tools to save yourself from trouble and stress.

  • Screwdrivers
  • Tape measure
  • Adjustable wrench
  • Level
  • Channel-lock pliers
  • Hacksaw
  • Optional: adapter tee
  • Pipe-seal tape
  • RO filter kit for under-sink installation

Step-by-Step Instructions


While the entire process of installation sounds scary, it is straightforward if you have a basic plumbing background.

Step 1: Plan before installation

If you are yet to buy an RO system, consider the space available under the sink for the unit – including its tank and filters. Take the measurements of the space and write them down for reference when buying your reverse osmosis system.

What water source will be used? Most RO systems are with a tee connection tapping into the typical 1/2-inch water supply line to the faucet. The rest are with a 3/8-inch flex line coming down from the faucet, so you need to buy an adapter tee for the shut-off valve so that water can be provided to the filter.

Step 2: Prepare the unit

Take the unit out of the packaging along with its filter, storage tank, and other components. Check that the entire RO system fits into the space where you plan to install it.

You also need to test fit all the parts to figure if you need to adjust any pipes before the installation. In case they need adjustment, use the services of a professional plumber. If you have a water softener, you need to install the RO system past it.

Some reverse osmosis systems can also be installed in another place, such as the utility room or garage other than under the sink, and then running up to the location of the sink.

It might be a good option if you have limited space because the unit won’t take much of it under the sink. Take note, though; this needs additional tubing to install the unit properly.

Step 3: Install the filter’s spigot


After planning and preparing the unit for installation, get to work. Install the filter’s spigot to ensure the lines are going under the countertop. Then, start mounting the filter’s spigot to the countertop or sink deck with the hardware that comes with your reverse osmosis water filtration system.

If your sink does not have a knock-out opening or a sink sprayer for the filter’s spigot, you can bore a hole that will hold it.

Next, start feeding the water supply line down through the knock-out opening before connecting the red tube or air gap into your faucet. Once done, lock the spigot onto the filter’s housing.

Step 4: Put the filter’s tank into place

Before doing this, make sure to install the tank connector properly using seal tape onto the tank nipple’s treads, and then screw the connector onto the filter’s tank. Take note not to screw too tightly. Put the tank under the sink or directly below the sink-top spigot if possible.

Step 5: Mount the RO filter’s assembly

Ensure that the unit will be installed straight using a level and draw markings for the hanger washers on the wall just below the sink. At the marked location, screw the washers and then start with the filter’s assembly on them.

The filter’s assembly should be at least 15 ½ inches from the floor to make room for maintenance.

Step 6: Connect the water supply line and the other tubing


Turn off your water supply.

By now, the 1/4-inch line from the assembly should have been connected properly. The tubing connection is based on your situation. Filters typically come with a ½-inch adapter that is compatible with a ½-inch flex line.

If your sink has a different flex tube size, use an adapter tee that should make a smooth transition to the supply tube and into the filter unit.

[This green supply tube connects to the adapter tee with an insert, a ferrule, and a nut.]

Tubing configurations vary from one manufacturer to another, but generally, they should have a tube that runs to the water supply and then to the filter unit’s first canister.

Another should be coming from the last filter to the drain and another to the storage tank.

Finally, another tube comes from the storage tank and then to the spigot on the sink or countertop. But if you have a 5-stage RO system, there can be a polishing filter going to the sink-top spigot. Nevertheless, check your manufacturer’s instructions to install the tubing connections properly.

For the cleanest RO system installation, use and run the tubing’s additional length to wherever needed, like the back of the cabinets. And if you’re planning to move the reverse osmosis system later, you must leave an additional tubing. But then, you can also consider trimming the tubing for an uncluttered installation.

Step 7: Connect the drain


A drain tee comes with select RO systems. Position the tee’s fitting up against your drainpipe, mark it, and use a hacksaw to cut it. Check that the tee tightly fits when cutting the pipe. Attach the fitting to your drain pipe with the washers and nuts.

Step 8: Perform a water pressure test


Let the extra air out of the pipes by turning the water supply and the cold-water supply on. Wait until the pressure starts building up in the filtration system. Expect about two hours for the full pressure, and then check that your connections are tightened properly and that there are no leaks or loose connections.

Drain the filtration system before drinking the water coming out of it. Leave the spigot on, and allow it to purge for about 24 hours. There will be a slow trickling of water as the tank empties. Leave the RO system running for a day, but always check for any leaks. If everything turns out well, you’re done with the installation!


Drinking clean water is essential to prevent water-borne diseases and ensure that everyone gets to enjoy freshly filtered water. To install reverse osmosis system under the sink might sound complicated and tiresome, you will be just fine with a little plumbing knowledge and the ability to follow the instructions properly. Did you like this guide on reverse osmosis water filtration system installation? Please let us know in the comment section below.

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