How to Install Reverse Osmosis

If you're looking to install an under-counter Reverse Osmosis drinking water filtration system, we take you through the process step by step so there will be no surprises. A DIY reverse osmosis system installation can save you a bundle.

RO PuROTwist 4000

Install Reverse Osmosis Under Sink

Before you get started, make sure you have these items ready:

  • Reverse Osmosis System
  • RO storage tank
  • Faucet and Installation Kit

Tools Needed to Install a Reverse Osmosis Drinking Water System:

- Wrenches Sizes 7/16”, 9/16”, ½” & 5/8”
- Phillips Screw Driver- Drill With 3/8” Chuck
- Drill Bits Sizes ¼” or 1 1/8” For Air Gap faucet

DIY Reverse Osmosis Install Instructions


Step 1. Installation Location

Before you get started, clean out under the sink and make sure there is ample space to install the RO system. Locate the “cold” water shut off valve and sink drainpipe.

Step 2. Close the Cold Water Valve

Shut off the “cold” water supply under the sink or the location where the system will be installed. If the existing “cold” water valve is inoperable, the water supply to the house must be shut off. Once the water is shut off, relieve the line pressure by turning on the cold water faucet. 

Step 3. Connect To Cold Water Line

There are several options when connecting the reverse osmosis unit to the cold-water source. They are: 

A. Saddle valve (Standard) – Assemble saddle valve clamp on the “cold” water line. Turn the pipe clamp adjustment plate to fit the contour of the pipe. (Small radius for 3/8” pipe, larger radius for 7/16” through 5/8” pipe). Tighten bolt so saddle valve is firmly attached to feed water pipe (be careful not to over tighten).

B.  Ez adapter. (Optional) : Use some Teflon tape to prevent leaks. Assemble 90 degree needle valve into the feed adapter.

For "Flex line" installation: Disconnect the flex line at the sink and install the feed adapter. Reconnect the flex line to the adapter.

For "Solid line" installation: Disconnect the line at the sink cut off approximately ¾” off the line. Install the feed adapter and reconnect line to the feed adapter. 

Step 4. Drain Line Connection: NOTE: If the drain line pipe is corroded, we strongly suggest replacing it.

At a point approximately six inches above the trap, drill a 5/16” diameter bole through one wall of the pipe. Next, attach the drain clamp; making sure that the hole in the clamp is aligned with the hole in the pipe. Use a punch or drill bit to align the holes while tightening the clamp. Be careful not to over tighten the clamp.

Step 5. Install Faucet:

The faucet must be positioned with aesthetics, function and convenience in mind. An ample flat area is required for the faucet base, so the base nut can be properly tightened. Conditions may exists which eliminate the need to drill a hole in the sink.

5a. If a hole previously installed in the sink, covered by a chrome plate cover, then simply remove the cover and mount the faucet.

5b. If a spray hose that may not be functioning or needed. Remove the spray hose and plug the outlet under the main faucet. If the sprayer uses a diverter at the base of the spout remove it, as the sprayer diverter may pop up and shut the water off to the main faucet.

5c. If space is not available on the upper sink area, the faucet can be located in the counter top close to the edge of the sink. Be careful to watch for obstructions below the counter such as drawers, cabinet walls, support braces etc.. If the counter top is ceramic tile the method for drilling the faucet hole is the same as for drilling a porcelain sink.

5d. The drilling process although not complicated, requires a certain amount of caution and preparation. Porcelain enameled sinks can be chipped if care is not exercised when drilling the hole for the faucet assembly. There are several ways of drilling the holes in to porcelain sinks without chipping; we have found these two methods work very well.

D.1 Using a carbide grinding wheel, grind away the porcelain where the ¼” diameter hole is to be drilled. Drill a ¼” diameter hole through the metal. This method results in a very clean and smooth hole.

D.2 Using a heavy duty variable speed drill and a carbide tip drill bit, carefully drill a ¼” diameter hole through the porcelain and metal sink.

5e. For stainless steel sinks, drill a ¼” diameter hole. Lightly file the edge of the hole to make sure it is smooth and free of any burrs. Caution: Do not allow metal chips to remain on the porcelain surface of the sink for any length of time, the metal chips will stain the sink and be very difficult to remove.

Step 6. Faucet Installation.

Once the hole has been drilled in the sink, the faucet stem may be inserted in the hole. Be sure the faucet body, faucet base and the rubber faucet base washer are in place above the sink.

Install the star lock washer and nut on the faucet stem under the sink and tighten firmly while aligning faucet in the desired direction. Once the faucet is installed, attach the ¼” tubing on to the bottom of the faucet stem and tighten.

Note: Some states require the use of an air gap faucet. To assure compliance check you local plumbing code. Locate the drain connection away from the garbage disposal to prevent potential contamination and system fouling.

Air gap faucet installation instructions:

Place the chrome washer and rubber washer on the base of the faucet. Slip the ¼” black line from the system through the hole in the sink. From the topside of the sink, slip the ¼” black line from system onto the barbed fitting located in the faucet base. The 3/8” black line from barb output is to be run as straight down hill as possible to the drain clamp. Avoid low spots or loops. Place faucet into the hole of the sink then from underneath sink, replace parts and tighten the hold down nut. Connect the ¼” blue line to the threaded faucet stem.

Diagram showing the parts of a air gap faucet

Step 7. Unit Location

Place the system and the water storage tank in an area under the sink so they are out of the way. If the system is to be hung on the wall be sure there is enough clearance from the cabinet floor to the bottom of the filter housing sump, leave at least 3 inches. Drill two 1/8” pilot holes that match up to the mounting holes in the systems bracket, mount the system to the cabinet wall.

Step 8. System hook up.

Remove any red caps from the end of the tubing. There may be water present in these lines if the system was wet tested at the factory, so keep a towel handy to wipe up any water.

Note: color of lines may vary from manufacturer to manufacturer – we have attempted to use industry standard colors in describing the system hook up procedures.

8A. Connect the units orange feed water line to the saddle valve or EZ adapter installed on the cold water line. Use the plastic delrin sleeve that are provided in the installation kit and discard any brass ferrules that may have been provided.

8B. Connect the black line from the unit directly to the drain clamp assembly. If an air gap faucet is used see instruction listed under air gap faucet installation instructions.

8C. Connect the green line to the RO water storage tank.

8D. Connect the blue line from the unit to the faucet.

Note: Make sure all inserts, sleeves and ferrules provided in the installation kit are used.


Step 9. Starting Up the System

9A. Turn off the storage tank ball valve, this will ensure no water can enter the tank. Slowly turn on the cold water supply valve to the sink. If you have not already done so, open the valve of the cold-water self-piercing valve (turn counter clockwise to open). Check for any leaks around the valve. If any leaks are detected turn off cold water supply valve and make necessary repairs.

9B. Open the reverse osmosis faucet on the sink. You will hear a gurgling noise. This is normal air being cleared from the system. It will take approximately 10-15 minutes before you actually see water dripping from the reverse osmosis faucet. (Flip the faucet handle up to keep the faucet open during this time.) The initial water dripping from the faucet may be black in color; this is the water flushing carbon fines from the carbon post filters. Allow the water to drip from the faucet for 10-15 minutes then close the faucet

9C. Now open the ball valve on the reverse osmosis storage tank, which will allow the tank to fill. This will take approximately 4-10 hours. During this period of time check all fittings for any leaks. If any leaks are found turn off cold-water line and make the necessary correction. Once the tank is full open the faucet and drain the system completely (until you are getting only a drip from the faucet). Shut the reverse osmosis faucet off and allow the system to re-fill.

9D. It is recommended on new installations that you drain the system 3 times prior to use.

9E. Make a daily check for any leaks during the first week after installation and check for leaks occasionally thereafter.

RO Water Production Capacity Rates

Your new RO drinking water system processes, stores and dispenses water, operating on normal home water pressure between 40 – 65 psi. The inlet water pressure, the water temperature and the amount of TDS (total dissolved solids) affect the efficiency of the reverse osmosis system.

For the charts below, the CTA and CTF Membranes are based on 10 gallons per day reverse osmosis membranes.

A chart showing production rates for low pressure RO systems

Hire a Plumber for RO Installation

Know your limitations. If you're worried you don't have the right tools, background knowledge, or time to install the reverse osmosis system, we recommend hiring a local plumber. You can save a lot of money by purchasing the Reverse Osmosis System online, and then hiring a plumber to do the install. Many of our customers go this route.

Four Recommendations When Hiring a Local Plumber to Install Reverse Osmosis System

Reviews Matter

If you don't already have a plumber whom you know and trust, check out local plumber reviews on the internet. Popular review sites include:

Call Around

Once you've identified a few reputable plumbers in your area, make phone calls and ask questions.

  • Tell them you have already purchased an RO system and are simply looking to have it installed.
  • Ask if they're familiar with RO installations.
  • Ask if you can pay them their hourly rate to install a system.
  • Ask how much time a typical RO installation takes.
  • Ask for references.

Still Have Questions about Installing a Reverse Osmosis System?

If you have additional questions, our team of water experts are always happy to take your calls Monday through Friday, 8 am to 5 pm. That's one of the perks of purchasing from us! When you call in, we will first pull up your order and verify the system is purchased from ESP Water Products. That way, we will know exactly what system you're working with and can best help you. For example, do you know if you want an "air gap" or "non-air-gap" system?

Unfortunately, due to time constraints we cannot provide installation or troubleshooting help for systems not purchased through our website. We recommend you call the company or store where you purchased your system.