What Is a Flow Restrictor and How Does It Work?

Posted by
John Woodard on March 11, 2024

Flow restrictors are perhaps best known for their use in showerheads, but they are also used in a variety of applications throughout your home. From filtration systems to water heaters to irrigation systems, water flow restrictors play a valuable role in preserving the health and efficiency of home and industrial water systems. In this article, you can find information on what flow restrictors are, how they work, their uses in RO systems, and answers to common questions about the applications and functionality of different types of flow controllers.

What is a flow restrictor?

flow restrictor, also known as a flow regulator or flow controller, is a device that regulates the flow rate of water through an outlet. It allows water to flow at a constant rate in and out of systems that operate better with consistent volumes of water, such as water filters, irrigation systems, and tankless water heaters. Flow restrictors allow appliances and filtrations to run optimally, preventing damage caused by excess flow. Water in reverse osmosis systems, water softeners, and other filtration systems need specific amounts of time in contact with the filter media or they will not treat the water efficiently or effectively. Flow restrictors keep the ideal water flow for these systems to maintain long lifespans at high efficiency.

How does a flow restrictor work?

Water flow regulators work by opening and closing a two-port valve to maintain a consistent flow rate of water in a water outlet. As the supply line’s volume increases, flow restrictors decrease the area that the water can flow through. They work regardless of fluctuations in water pressure and can be used in many household applications. In low-pressure supply lines, flow restrictors provide comfort of use, and they save energy in high-pressure supply lines. By managing water flow, a flow restrictor prevents built-up pressure from damaging pipes, appliances, and fixtures. Not only do flow regulators save you money by decreasing damage to appliances, but they also save water in lines with high pressure. In applications where a large volume of water is used, such as showers and water filtration systems, a flow restrictor can save you money on your water bill each month.

Do showerheads need flow restrictors?

To meet plumbing codes, shower heads require flow restrictors to keep flow rate below a certain threshold. The Energy Policy Act in 1992 set the national standard for showerhead flow rates at 2.5 gpm. WaterSense rated showerheads, on the other hand, provide a maximum flow rate of 2.0 gpm. In addition to meeting the criteria for maximum flow rate, WaterSense showerheads must meet qualifications related to their spray force, spray coverage, and performance across various pressures.

Flow restrictors can be applied to all bathroom and kitchen faucets, and they must be used for reverse osmosis filtration systems. The average flow rate for faucets is between 1 gallon per minute (GPM) and 2.2 GPM. In states with more environmental regulation, such as California at 1.8 maximum GPM, the maximum flow rate may be lower. A shower head, on the other hand, must have a flow rate below 2.5 GPM. By installing a flow restrictor into your shower head or kitchen or bathroom fixtures, you can save thousands of gallons each year.

Can I remove the flow restrictor from my showerhead?

While you can remove a flow restrictor from a showerhead, doing say may cause your shower to violate plumbing codes. The national standard for maximum showerhead flow rate is 2.5 gpm. Removing the flow restrictor from your showerhead may cause it to surpass this limit, not only causing you to waste water, but also making your shower not up to code.

How do RO flow restrictors work?

RO flow restrictors help maintain a reverse osmosis system by limiting the amount of wastewater the system produces. The flow restrictor is connected to the outlet tubing of the RO membrane, where it prevents pressure inside the membrane from dropping too low. Without a flow restrictor, the pressure in the RO membrane drops too low, and the system fails to function properly. Additionally, the flow restrictor helps prevent the RO system from producing excess wastewater. If the water is allowed to flow through the system to the reject outlet unrestricted, greater volumes of wastewater are produced and pressure applied to the membrane drops. Other factors, such as low incoming water pressure, can impact the operation of an RO system. If this is the case for you, a reverse osmosis booster pump will solve your problem. In other cases, the flow restrictor may need to be replaced.

Learn more: What is an RO booster pump?

Where does an RO flow restrictor go?

Flow restrictors in an RO system insert into the connection between the RO membrane and the drain tubing, also known as the waste pipe. All RO systems come with a flow restrictor because they are needed for the unit to operate smoothly. A flow restrictor should be replaced when you replace your RO membrane, about once every 2 years. This prevents scale buildup from wreaking havoc on your restrictor and, consequently, your RO system.

removing membrane from RO system

Signs of a faulty flow restrictor

Flow restrictors can go bad, and they will show signs when they do. Any of the following symptoms may be signs that your flow restrictor has stopped working correctly:

  • Too much water being drained from your RO membrane
  • Too little water being drained from your RO membrane
  • Poor filtered water quality

If the flow restrictor prevents water from passing through, it will back up water into the RO membrane and prevent it from filtering effectively. If the restrictor allows too much water through, the pressure inside the membrane will drop, and a large amount of water will be wasted.

How do I size a flow restrictor for my reverse osmosis system?

A flow restrictor for a reverse osmosis system should be about 3 to 4 times the production rate of the reverse osmosis membrane. RO flow restrictors are often measured in milliliters per minute (ml/min). To convert from ml/min to gallons per day (GPD), multiply the ml/min of the restrictor by 0.38. The product of those two numbers should be about 3 to 4 times higher than the GPD of your RO system. For example, this 650 ml/min flow restrictor can output about 247 (650 * 0.38) GPD. This means the restrictor is suitable for RO systems that produce between 62 (247/4) and 83 (247/3) GPD.

 RO flow restrictor chart

Where are water flow restrictors used?

Flow restrictors allow for precise flow control that improve system performance, save energy, and provide comfort of use in the following applications:

Do flow restrictors reduce water pressure?

No, flow restrictors do not change pressure, but they do take pressure into account. Flow rate is dependent on three factors: the size of the outlet, the upstream pressure, and the downstream pressure. The pressure differential between both sides of an outlet determines how high the flow rate of a fixture is. To compensate for variations in pressure, flow restrictors decrease the volume that water flows through when pressure increases. Likewise, the opening will widen when pressure decreases. This allows the flow rate to remain constant while the pressure fluctuates.

Are water flow restrictors and pressure regulators the same?

No, water flow restrictors and pressure regulators are not the same, but they can achieve similar results. Flow restrictors limit the amount of water flowing downstream, while pressure regulators affect the pressure of upstream water by changing the flow of downstream water. Water pressure regulators are not designed to control flow rate. However, they affect flow rate by managing the pressure within a water line. Pressure regulators operate at a setpoint, a specific pressure that water within a system must not exceed. When the pressure exceeds this setpoint, the regulator opens to allow pressure to escape until it reaches the setpoint. Unlike flow restrictors, pressure regulators cannot maintain a consistent flow of water, so they cannot serve as an alternative to flow restrictors.

Learn more: What is a water pressure regulator and how does it work?

Flow restrictors, on the other hand, are designed to maintain a consistent flow rate of water. In many cases, flow controllers are used in conjunction with a pressure regulator. When used in unison, these two devices allow water to flow at a constant rate without building up pressure in a supply line. Some systems, such as reverse osmosis filtration systems, require flow restrictors to operate efficiently. Both pressure and flow rate are critical in allowing a reverse osmosis membrane to filter out the most contaminants while wasting the least amount of water possible.

Learn more: What is an RO system and how does it work?



If you have any additional questions, please do not hesitate to contact us.


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