How to Install a Reverse Osmosis Flow Restrictor

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Installing the correct flow restrictor for your reverse osmosis system is vital for its proper functioning.

Selecting a restrictor that is the right size and type, plus installing it correctly, means you will have less wastewater and great quality permeate water.

So, how do you install an RO flow restrictor? It depends on which type you have. Read on!

Key Takeaways

Here is how to install a reverse osmosis flow restrictor:

  • Inline-style:
    1. Shut off water supply and storage tank valve.
    2. Open RO faucet to drain remaining water.
    3. Disconnect or cut drain line.
    4. Connect flow restrictor in the right direction.
    5. Open water supply valve and storage tank valve.
  • Insert-style:
    1. Disconnect the tubing from RO membrane.
    2. Insert the flow restrictor.
    3. Reconnect the tubing.

How to Install a Reverse Osmosis Flow Restrictor

There are two methods for installing a reverse osmosis flow restrictor, depending on which kind of restrictor you use, inline or insert style.

Where to Install an RO Flow Restrictor

The inline style RO flow restrictor is simply a pipe fitting that sits in between the wastewater line of your reverse osmosis system (you’ll possibly need to cut into the tubing), while the insert style sits inside the line at the very beginning.



The following instructions assume that you do not have a flow restrictor installed on your reverse osmosis system yet.

  1. Shut Off Water Supply and Storage Tank Valve: Turn off the water supply to your reverse osmosis system and close the storage tank valve to prevent any water from flowing out during the installation process.
  2. Open RO Faucet to Drain Remaining Water: Next, open the faucet of your reverse osmosis system. This will allow any remaining water in the system to drain out.
  3. Disconnect or Cut Drain Line: With the water drained, you can now cut the wastewater line. This is the line that runs from the outlet of the RO membrane housing to the drain.
  4. Connect Flow Restrictor in the Right Direction: Install the flow restrictor. Make sure it’s facing the correct direction – the direction of the water flow. Connect it to the cut drain line.
  5. Open Water Supply Valve and Storage Tank Valve: With the flow restrictor properly installed, you can now turn on the water supply and open the storage tank valve. Check for leaks and ensure the system is operating correctly.


The process for an insert-style flow restrictor differs slightly. This particular type of flow restrictor should be inserted into the fitting located between the reject outlet of the membrane housing and the tubing that connects to the drain.

  1. Disconnect the Tubing: Start by disconnecting the tubing from the membrane housing reject outlet.
  2. Insert the Flow Restrictor: Next, insert your insert-style flow restrictor into the fitting. Ensure it’s properly positioned and secure.
  3. Reconnect the Tubing: Once the flow restrictor is in place, reconnect the tubing to the membrane housing reject outlet.

Remember, installing the flow restrictor in the correct direction of the water flow for both types is crucial. If installed backward, the flow restrictor won’t function properly and could potentially damage your reverse osmosis system.

plumber installing reverse osmosis system under sink

What Is a Flow Restrictor and How Does It Work?

A flow restrictor is a device that manages the water flow rate through an outlet, ensuring a consistent volume for systems like water filters and tankless heaters. It optimizes appliance functioning and prevents damage from excessive flow. Flow restrictors are essential for efficient water treatment in reverse osmosis systems and water softeners.

Flow restrictors operate by adjusting a two-port valve to maintain regular water flow, irrespective of changes in water pressure. Controlling water flow prevents potential damage to pipes and appliances, thereby saving money. They also conserve water in high-pressure lines, which can reduce your water bill.

Flow restrictors apply to all kitchen and bathroom faucets and are mandatory for reverse osmosis filtration systems. The average flow rate for faucets is between 1-2.2 gallons per minute, with lower rates in environmentally regulated states like California. Shower heads should have a flow rate below 2.5 gpm. Installing flow restrictors can save thousands of gallons of water annually.

In reverse osmosis systems, flow restrictors limit wastewater production by maintaining sufficient pressure within the system. They are attached to the RO membrane’s outlet tubing, preventing the pressure from falling too low, which could impair the system’s function. They also stop the system from producing excessive wastewater. However, if the incoming water pressure is too low, a booster pump may be needed, or the flow restrictor might require replacement.


Inline Flow Restrictors

A common type of reverse osmosis flow restrictor is the inline type, which is also known as encapsulated capillary style flow restrictor.

Essentially, the flow restrictor sits inline, so on the wastewater line somewhere between the RO membrane housing outlet and the drain.

Insert Flow Restrictors

Insert style flow restrictors should be inserted in the fitting between the membrane housing reject outlet and the tubing that leads to the drain.

These are also known as capillary style flow restrictors.

Sizing RO Flow Restrictors

To maintain optimal speed and performance, it is recommended to choose a flow restrictor for your reverse osmosis system that is 3 to 4 times the production rate of the RO membrane. Flow restrictors are typically rated in milliliters per minute (ml/min), which can be converted to gallons per day (gpd) by multiplying by 0.38. This resulting value should be about 3 to 4 times greater than the gpd of your RO membrane.

There are different flow restrictor sizes available, such as 200, 250, 300, 360, 400, 420, 450, 500, 525, 600, 750, 800, 1000, and 1200 mL/min. To ensure compatibility, match the flow rate of the flow restrictor with your RO membrane. Calculate this by dividing your RO membrane’s GPD rating by 0.38 to get the flow rate in mL/min, and then multiply this value by 3 or 4 to determine the appropriate flow restrictor size.

If you don’t want to do the math, you can simply use this chart!

Membrane Rating in gpd Membrane Rating in mL/min Smallest Flow Restrictor Size Largest Flow Restrictor Size
15 39.5 118.4 157.9
18 47.4 142.1 189.5
25 65.8 197.4 263.2
36 94.7 284.2 378.9
50 131.6 394.7 526.3
75 197.4 592.1 789.5
100 263.2 789.5 1052.6
150 394.7 1184.2 1578.9

When to Change a Reverse Osmosis Flow Restrictor

A flow restrictor in a reverse osmosis system can malfunction, and there are certain signs to watch out for when this happens. These include excessive or insufficient water being drained from your RO membrane or a noticeable decline in the quality of filtered water.

If the flow restrictor obstructs water flow, it can cause a backlog into the RO membrane, inhibiting its filtering ability. On the other hand, if too much water is allowed through, the membrane’s internal pressure may fall, leading to water wastage.

To maintain the optimal performance of your RO system, it’s suggested by some manufacturers that you replace the flow restrictor each time you replace the RO membrane. This helps prevent scale buildup, which can block the capillaries. Also, if you’re switching to a different flow-rate membrane element, ensure the flow restrictor is also changed to the corresponding size.

If you have any questions about reverse osmosis flow restrictor installation please don’t hesitate to leave a comment below!

About the Author Gene Fitzgerald

Gene Fitzgerald is one of the founders of BOS and currently head of content creation. She has 8+ years of experience as a water treatment specialist under her belt making her our senior scientist. Outside of BOS, Gene loves reading books on philosophy & social issues, making music, and hiking.
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