Reverse Osmosis Troubleshooting – All Common RO System Problems
Written by: Gene Fitzgerald // Last Updated:
Please note: This page may contain affiliate links. If you buy a product or service through such a link we earn a commission at no additional cost to you. Learn more about our product review process or check our FTC affiliate disclosure.
Reverse osmosis systems usually last for years without causing trouble.
But even if yours doesn’t, the good news is that most problems are really simple to diagnose and repair, especially with the help of our RO Troubleshooting Guide below.
With it you hopefully have the right tool at hand to face and solve just about any issue without the need to call a professional and spend any money.
RO System Drains Constantly (Drain Line Keeps Running)
The functioning of a reverse osmosis system is based on pressure. Simply put: Feed water flows into the unit, gets purified and is then stored in the storage tank for later use. When the storage tank is full it triggers an automatic shut-off (ASO) valve to close. The valve stops more water from entering the system. A second valve, the check valve, prevents reject water from running down the drain line.
If tank pressure is too low or the shut-off valve or check valve is broken, water might flow continuously down the drain. This means that your RO system not only wastes a lot of water, the noise can also become quite annoying after a while.
First, use a pressure gauge to measure the pressure in the storage tank when empty. The tank should read about 6-8 psi. In case the pressure is too low you need to repressurize.
How to Drain & Repressurize an RO Tank
Shut off the water supply to the system.
Close the storage tank valve.
Disconnect the tank from the system and take it outside. Open the tank valve. In the beginning, water will pour out quickly, but after some time the flow will decrease to a trickle.
Since a lot of water is still inside the tank, you have to pump air into the pressure valve with the help of a bicycle pump or compressor. With each pump more water will pour out.
Once the tank is empty, take a pressure gauge to measure the pressure inside. For most tanks you should aim for somewhere between 6 to 8 psi (optimal value might state on tank label). Very carefully repressurize if need be using the pump or compressor.
Now you can reconnect the tank to your reverse osmosis system. Open the tank and feed water valve and allow the system to refill.
The problem reoccurred a couple of days later? This means that the bladder inside the tank is ruptured and cannot hold the air. All you can do is replace the whole tank.
Tank pressure is within the desired range? Then it’s likely that either the shut-off valve or the check valve is defect and needs to be replaced…
Here is how you can test if both ASO and check valve are functioning correctly:
Let the storage tank fill completely. You know when the tank is full by trying to lift it up.
Draw 2 to 3 glasses of water from the RO faucet to reduce the pressure inside the storage tank. This will cause your system to start processing more water to refill.
Close the tank valve to mimic a full tank.
Wait for 5 minutes.
Check if water stops flowing down the drain line by either listening closely or by pulling the drain line out of the drain saddle. If so, both valves are working just fine. If, however, water does continue to flow down the drain, either the automatic shut-off valve or the check valve is broken. In that case continue with test #2.
Let the tank refill.
This time keep the tank valve open and turn off the feed water valve.
Check if water is flowing down the drain by either listening closely or by pulling the drain line out of the drain saddle. If so, the water is coming straight out of the storage tank which means that the check valve is broken and needs replacement. If no water is flowing the ASO valve is broken and has to be replaced.
Other Possible Causes
Other possible causes for a system to constantly drain are:
Feed water pressure is too low (usually below 40 psi). Increase the water pressure to the required level, e.g. by applying a booster pump.
Your system wasn’t installed properly so that the membrane causes the problem. Carefully reconnect all pieces making sure that they are in their correct position. Need help with how to install a reverse osmosis system?
The RO membrane is in bad condition. Replace it.
A worn-out flow restrictor inside the drain line is causing the problem. Replace the restrictor.
No matter how hard you try, sometimes a unit is broken beyond repair. If you need a replacement, you can find reviews of the best RO systems here.
RO Storage Tank Not Filling Up
If a storage tank does not fill there is usually an underlying pressure issue. Either tank pressure is too high or feed water pressure too low.
An empty storage tank should read roughly 6 to 8 psi. You can measure this with a standard bicycle gauge for example. Reduce pressure if need be.
If the feed water pressure is less than 40 psi you need to either increase the pressure in your entire home or use a pump specifically designed for reverse osmosis systems.
That wasn’t it? Also consider:
Clogged filters or bad RO membrane – Clogging usually occurs when a membrane has to process exceptionally hard water. You have two options: You can either replace membranes more frequently which will add to the costs. Or you can install an additional pre-treatment system – think water softener – which will remove all hardness minerals from the water before it enters the filter system. If one or more clogged filters are causing the problem, again, replacement is necessary.
A dripping faucet indicates that one or more system parts are loosely fitted. To stop the leaking, thoroughly tighten all connections. Push the tubing further into their ports, valves and the drain saddle.
If the leaking originates from the bottom of the faucet stem there is no way around replacing the piece.
Water is coming out of the hole of the air gap faucet above the sink? This commonly known issue is called an “air gap leak“. It’s the result of a blocked drain line which usually happens when debris has accumulated in the drain.
Now, the purpose of an air gap is to assure that water cannot flow backwards into the RO unit. During normal operation, water runs from the storage tank to the faucet and falls through a small pocket of air, the air gap.
When the drain line starts to clog up it creates backpressure which causes water to flow out of the gap and all over your countertop.
All you have to do to fix this is to clear off the drain line so water can flow freely. Take a pipe cleaner or wire brush to clean out the one end of the drain line that is connected to the drain saddle. Also clean the saddle itself, as stuff tends to grow in there.
What’s more, drain saddles tend to shift so the holes aren’t properly lined up anymore, which may also restrict the water flow. Make sure that drain saddle and drain line hole are aligned.
How to prevent an air gap leak in future?
Completely eliminating the chance of an occasional leak is almost impossible. However, you can take the necessary precautions to reduce their frequency quite significantly:
First of all, avoid flushing large chunks of food and other stuff down the drain.
Furthermore, regularly treat your drain line with natural cleaner to dissolve any blockades.
Leaking Filter/Membrane Housing
A misplaced or worn-out rubber O-ring is most likely responsible for causing a filter/membrane housing to leak. Here is what you should do:
Close the feed valve so no more water can flow into the system.
Close the storage tank valve.
Unscrew the leaking filter housing.
Inspect the O-ring(s). Replace if cracked or generally in a bad condition.
Make sure that O-rings are placed correctly and sit tight.
Screw the filter housing back on and hand tighten. Use the housing wrench to tighten an additional quarter turn or so.
Open up the tank and feed water valve.
If the filter housing is still leaking it may be damaged and needs replacement. You can check for this by switching it with one of the other housings.
It’s also quite common for the membrane housing cap to come loose over time which displaces the O-ring. Periodical retightening is an easy fix.
Slow/No Water Flow From Faucet & Low Pressure
You had your reverse osmosis system for a number of years and like every other morning you are about to enjoy a refreshing glass of purified drinking water. For some reason, however, the glass only fills half full and the water flow goes down to a trickle.
Little or no flow from the faucet means that you either have low volume of water or low pressure.
The root of the problem is most likely a malfunctioning storage tank. Inside the tank is an air bladder that, as more water runs into the tank, increases pressure. If the bladder leaks it cannot build up enough force. As a result, you are not getting water delivered to your faucet in the same amount you are used to.
What can you do about this?
First, lift the tank to see if it’s full. A full tank weighs somewhere around 28 lbs as opposed to an empty one weighing almost nothing. If the tank feels heavy the defect is probably in its inside.
Next, you want to measure the pressure of the tank when empty. An empty tank should read somewhere between 6 to 8 psi. If the pressure in your tank is lower then you need to repressurize it (learn more above).
The problem reoccurred a couple of days later? This means that the bladder inside the tank is ruptured and you need to replace the whole tank.
Other Possible Causes
Slow faucet water flow can also be an indicator for overall low feed water pressure (below 40 psi). A booster or permeate pump is your best option here. Other reasons that could be causing the issue are:
Clogged filters or bad RO membrane – Clogging usually occurs when a membrane has to process exceptionally hard water. You have two options: You can either replace membranes more frequently which will add to the costs. Or you can install an additional pre-treatment system – think water softener – which will remove all hardness minerals from the water before it enters the filter system. If one or more clogged filters are causing the drop in flow, again, replacement is necessary.
RO Membrane not properly seated
Clogged flow restrictor
Bent tubing – Bent tubing may result in pressure loss which in turn causes low water flow. Check the tubing and straighten all lines that are bent.
Empty storage tank
Feed water valve or tank valve closed
RO Water Tastes Bad
Bad water taste and/or odor usually originates from a biofilm that has accumulated in one or more of the filtration stages. This can happen after months or years of use and might also lead to cloudiness.
Therefore, what you should do is replace any clogged filters or a fouled membrane immediately. And from now on keep in mind to replace said components in a more timely manner (filters at least every 6 to 12 months).
Remember: If you let germs grow in your reverse osmosis system they can pose a serious health threat.
Alternatively, you can invest in a unit that features a modular filter design. This allows you to dispose of the entire filter/membrane housings with every replacement which has proven to be the best way to prevent the accumulation of harmful pathogens. The downside is that this type of system is more costly to purchase and to maintain.
Bacteria grows in stagnant water over time, which can be the source of bad taste and smell. If you haven’t used your RO system for a while, it’s a good idea to flush the entire unit, the storage tank in particular, once or twice before you start using the water.
Noisy Air Gap Faucet or Drain
When you put your system into use for the first time or you just recently replaced one of the filter cartridges, you might hear strange noise coming from the drain line or air gap faucet.
This is nothing to worry about. It’s caused by air being pushed out of the system.
However, the issue should not persist for more than a couple of days. If the noise doesn’t subside and it’s something that is bothering you, make sure that all tubing is set straight. The noise can also be caused by a restriction in the drain tube or saddle. If that doesn’t help, it’s time to check the entire system for gaps and correct any errors.
Problems with Permeate and Drain Flow Rate
Permeate flow rate refers to the rate of water passing through the RO membrane.
To determine if you have a problem with your permeate flow rate, you’ll need to perform a little testing and calculation. You’ll need to establish both the permeate flow rate and the drain flow rate, and then compare them to the ideal numbers to get an idea of what is going on.
To find the permeate flow rate:
First up, you’ll need to shut off the tank valve and open the RO faucet. This will send the water directly from the RO membrane to the faucet. Wait until the water is flowing consistently, and then collect 60 seconds’ worth of water flow in a measuring cup.
Related to the permeate flow rate is the drain flow rate, which refers to the rate of flow through the drain line and out the drain saddle.
Reverse osmosis systems require flow restriction at the drain line to keep the pressure high enough for the RO membrane to work correctly. This is the job of the flow restrictor.
To find the drain flow rate:
First, disconnect the drain line from the drain saddle, and then position the line inside a measuring cup. Run the faucet for another 60 seconds and take note of the water quantity collected.
Now that you have both of these flow rate figures, it’s time to compare them to the chart below:
If both results are lower than the optimal range, then remove the pre-filters from their housings and re-run the test. If the results stay the same, it’s likely that you have a low-pressure issue, and may need to install a pressure regulator.
If the drain flow rate is within the acceptable range, but the permeate flow rate is very low, then this is an indication that you have an issue with your RO membrane. The membrane may be calcified or clogged and is no longer functioning properly. You’ll likely need to replace the RO membrane, and you may need to install additional pre-filters to reduce the level of TDS (total dissolved solids) in your water.
If the drain flow rate is high, but the permeate flow rate is very low, this indicates a problem with your flow restrictor. If the flow restrictor has failed or is not working properly the system won’t have the necessary pressure for the RO membrane to work properly. In this case, you’ll likely need to replace the flow restrictor.
Components of a Reverse Osmosis Filter System
Reverse osmosis systems are relatively complex systems, utilizing multiple pre and post-filters in addition to the primary RO membrane among other components. Let’s take a look:
Feed water valve: The feed water valve is the connection between your sink’s cold water supply and the RO system’s first pre-filter.
Pre-filter(s): The pre-filter or pre-filters are the filtration stages before feed water reaches the RO membrane. Typically, there will be a sediment pre-filter to remove silt and sand, and a carbon pre-filter to remove chlorine/chloramine, both of which can prematurely age the membrane.
RO membrane: The RO membrane is the primary filtration stage, and removes a wide array of contaminant particles down to .0001 microns in size.
Storage tank: Because RO systems filter slowly, a storage tank is needed to keep a usable amount of filtered water on hand at all times. Water passes from the RO membrane to the storage tank before it passes through the final post-filter.
Post-filter: A final post-filter is used after the water has passed through all of the other filtration stages. This stage is intended to ‘polish’ the taste and remove any residual contaminants left in the water.
Check valve: The check valve blocks the water flow backward towards the RO membrane. This prevents potential membrane damage.
Flow restrictor: A flow restrictor restricts the flow of wastewater from the RO membrane housing outlet. This is needed to maintain adequate water pressure in the system.
Drain line: The drain line transports the contaminant-filled wastewater from the RO membrane to the drain saddle on your sink’s drain pipe.
RO faucet: This is simply the faucet that dispenses filtered water at your kitchen sink. It’s separate from your main faucet, so you can still use unfiltered water for cleaning tasks.
If you have any questions or thoughts about reverse osmosis troubleshooting please don’t hesitate to leave a comment below!
About the Author Gene Fitzgerald
Gene Fitzgerald has been with BOS since the very beginning. She is head of content creation and has fully immersed herself into the home water treatment industry only to become an expert herself. Outside of BOS, Gene loves reading books on philosophy & social issues, making music, and hiking.
Learn more about Gene and the rest of the BOS Team.
From August 27th to September 3rd – the water tasted bitter and caused a sick feeling after I drank. Now today it is ok. Could there be something temporary that caused water to taste bitter for a few days?Reply