What Is a Flow Restrictor in a Reverse Osmosis System?
Written by: Alexandra Uta // Last Updated: Jul 14, 2023
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The flow restrictor is an important component of a reverse osmosis system which unfortunately often gets overlooked.
Many people focus on the functionality of the RO membrane while ignoring the importance of maintaining adequate pressure in the system.
Without a flow restrictor, a reverse osmosis system can technically still work, but its performance will be severely degraded.
So what is a flow restrictor in reverse osmosis exactly and what is its function?
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Key Takeaways
- A reverse osmosis flow restrictor prevents too much water from flowing down the drain.
- It creates backpressure doing so which helps ensure that more water passes through the membrane and gets filtered.
- Using a home reverse osmosis system without a flow restrictor is theoretically possible, but never recommended.
What Is a Flow Restrictor in a Reverse Osmosis System and How Does It Work?
A flow restrictor is a small element which is attached to the wastewater line of a reverse osmosis system. Its purpose, as its name implies, is to restrict the flow rate of water going through the wastewater line and down the drain. It accomplishes that by simply physically limiting how much water can pass through it.
This is important because without a flow restrictor, too much water would flow towards the drain line and bypass the membrane almost entirely – taking the path of least resistance. If this happens, an RO system will still work, but it will not produce as much clean water as it could (nowhere near it) and the purified water will also be of lower quality.
Benefits of Flow Restrictors for Home RO Systems
The main benefit of a flow restrictor for a home reverse osmosis system is that it allows the system to maintain sufficient pressure within the RO membrane for optimal operation. If you don’t use a flow restrictor as we described above, more water will go directly to the drain instead of not passing through the membrane. As a result, you’ll generate a lot more wastewater in the filtration process and not have as much filtered water.
While some amount of wastewater is unavoidable when using reverse osmosis, you can generally do a lot to minimize the amount. Using a flow restrictor is one of the simplest approaches to that. That’s why this part has become an integral component of reverse osmosis systems in general.
Sizing: How to Choose the Right Flow Restrictor for Your RO Membrane
Flow restrictors are typically sized in millimeters per minute, which can be confusing if you’re used to measuring the performance of your RO system in gallons per day. Thankfully, it’s only a matter of a simple multiplication to convert from one value to the other. Multiplying the ml/min rating of a flow restrictor by 0.38 will give you the equivalent rating in gallons/day.
Your flow restrictor should ideally have a flow rate that’s 3-4 times greater than that of the reverse osmosis membrane itself. Therefore, to figure out what size of flow restrictor you need, you simply need to take the flow rate of your membrane (usually stated in gallons/day), divide it by 0.38 to convert to ml/min, and then multiply by 3 or 4 to get the necessary flow restrictor rate.
Most reverse osmosis flow restrictors have their rate printed directly on the unit itself.
If you don’t want to do the math sizing your ideal flow restrictor, you can use our table below:
| 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 |
What If I Under-Size My Flow Restrictor?
If you under-size your flow restrictor, this will increase the wear on your reverse osmosis system. You can expect having to clean or replace the membrane more frequently, as well as some inconsistencies in the TDS rate of your filtered water.
Your membrane will have to deal with a lot more contaminants, which will cause it to wear out more quickly.
What Happens If I Over-Size?
Conversely, if you get a flow restrictor that’s too large, this will decrease the performance of the system as a whole. If you make the restrictor especially large, this will result in a situation where you might as well not have a restrictor at all. More water will go down the drain line, and you will waste a lot more of it in the filtration process.
How Can I Install a Flow Restrictor on My Reverse Osmosis System?
Flow restrictors are simple to install as they usually come with quick connect fittings. Additionally, an arrow painted on the restrictor indicates the direction of the water flow. You simply need to take the restrictor and hook it up to the appropriate section of your reverse osmosis system (more on that below). Usually you would have to cut some tubing to make space for the restrictor.
Where Does the Flow Restrictor Go?
The flow restrictor must always be connected to the drain line of your reverse osmosis system. If your system has color-coded tubing, look for the black line (most of the time). Pay special attention to the direction of the installation of the flow restrictor.
If you have bought a premade reverse osmosis system from the public market, then you probably already have a flow restrictor in place. In that case, simply identify where it’s currently located and swap it out. This will give you the additional benefit of not having to do any calculations about the flow rate of the flow restrictor. You can simply take the value that’s currently present on the restrictor you have installed.
Inline-Style vs Insert-Style Flow Restrictors
Essentially, there are 2 different types of flow restrictors:
Inline-style flow restrictors are simply a pipe fitting that sit inline, in between the wastewater line of your RO unit.
Insert style sits inside the line at the very beginning.
Further Reading:
Do I Need to Change My RO Flow Restrictor? When?
Flow restrictors in reverse osmosis systems wear out over time, just like the reverse osmosis membrane itself. In most cases, a flow restrictor can last for at least a couple of years before it needs a replacement.
For convenience, you can align the replacement of your flow restrictor with the replacement of the reverse osmosis membrane, but you don’t have to. Simply swap out both components when it’s time to change the membrane. Considering the relatively low price of a reverse osmosis flow restrictor, even if you replace it prematurely, you’re not going to waste any significant amount of money.
There are also certain signs you can look for if you want to get an early warning about having to replace your flow restrictor. Pay attention to inconsistencies in the amount of water being drained from the reverse osmosis membrane. If the amount gets too high or too low, this could indicate problems with your reverse osmosis flow restrictor.
And of course, always pay attention to the quality of your filtered water. If it starts declining for no apparent reason, this could be another indicator that you need to replace your reverse osmosis flow restrictor. Especially if the membrane itself doesn’t seem like it needs a replacement just yet.
Can I Use My Reverse Osmosis System without a Flow Restrictor?
Theoretically you can run a reverse osmosis system without any flow restrictor at all. However, this makes practically no sense. Reverse osmosis already wastes a lot of water in its default state. The ratio can be as high as 5 gallons of wastewater for each gallon of purified water.
And that’s when you’re running the system with a flow restrictor in place. Running it without one will multiply this number several times, resulting in huge amounts of wasted water. Even if you are able to repurpose that water for other applications, that’s still very wasteful. And as we mentioned above, the price of a reverse osmosis flow restrictor is generally low enough that installing one shouldn’t even be a consideration under question in the first place.
If you have any questions about the function of the flow restrictor in an RO system please don’t hesitate to leave a comment below!
About the Author Alexandra Uta
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