RO Tank Full But No Water? Here Is What to Do!

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A confusing issue that you may run into with your reverse osmosis system is a storage tank that feels full but won’t dispense water when you open the faucet.

There are a few reasons that could be causing such a problem with your RO tank, so it makes sense to run through them one at a time and determine the source of the issue.

Key Takeaways

An issue that pops up from time to time is an RO tank that feels full but won’t dispense water when you open the faucet. There are three primary causes of this issue.

  1. A closed storage tank valve prevents flow from the tank – simply open the valve.
  2. Low pressure inside the tank – repressurize tank when empty.
  3. A ruptured internal tank bladder – replace tank.
  4. (Kinked tubing and/or a clogged post-filter can also be the source of the problem, but those are less common – straighten tubing and replace post-filter.)

RO Tank Full But No Water? Here Is What to Do!

Before you begin with the troubleshooting, make sure your RO tank is actually full. Lift it to get an idea if any water is in the tank at all. A typical 4-gallon tank will weigh somewhere in the neighborhood of 30 pounds, while an empty tank will weigh almost nothing.

If the tank feels like it has water inside, continue with the troubleshooting process outlined below.

The three most common reasons for a full RO tank not to dispense water are:

  1. A closed storage tank valve
  2. Low pressure inside the tank
  3. A ruptured tank bladder

Under Sink Reverse Osmosis System

While these are the most common reasons for a full tank not to dispense water, other issues could be causing problems.

A kink in the tubing running between the storage tank and the RO faucet could be stopping the flow. Additionally, a clogged post-filter could be stopping the flow after the tank but before it can reach the RO faucet. These are less common issues than the three listed above, so unless you spot an obvious kink or suspect the post-filter is clogged, run through the other three possibilities first.

1. Storage Tank Valve Closed

This one is fairly obvious, but you may have accidentally closed the storage valve at some point and forgotten to open it again. This can easily happen while changing the reverse osmosis membrane or filters or while cleaning/sanitizing the tank.

To check if the tank valve is closed, turn the valve to the opposite position that it’s currently at and retry the RO faucet. If it starts working – voila – problem solved.

2. Low Reverse Osmosis Storage Tank Pressure

RO storage tanks use air pressure to operate effectively with the other components of the system. The storage tank is pressurized to avoid the need for a booster pump to push water out of it.

Inside the pressurized tank are both an air chamber and a water chamber separated by a bladder. When water fills the water chamber, the air chamber compresses and increases in pressure.

A typical storage tank should have an internal pressure – meaning when empty – between 6 and 8 psi depending on the manufacturer. Air can leak out of the tank over time, causing the internal pressure to drop which will slow or even stop water from exiting the tank.

To check if you have an issue with low pressure you’ll first need to empty the tank. This is done by disconnecting it from the rest of the system, opening the tank valve, and allowing the water inside to flow out. When no more water flows from the valve, connect a bicycle pump or air compressor to the pressure valve to push out whatever water is left.

At this point, you can check the tank’s internal pressure using a tire pressure gauge on the pressure valve. If it’s low, then you’ve located the source of your problem and can repressurize it using your bicycle pump or air compressor.

Go slow and careful while doing this to avoid damaging the internal bladder, especially if you’re using an air compressor.

3. Ruptured Tank Bladder

The last major possibility for a full RO tank that won’t dispense water is a ruptured tank bladder. This can happen given enough time and will require a new storage tank to replace the ruptured one.

Generally, what will happen with a ruptured bladder is the tank will send about one glass to the RO faucet at a time followed by a slow trickle. This is because the pressure inside the tank is too low to push significant water up through the tubing and faucet.

Another way to check if the internal bladder is ruptured is by connecting a tire pressure gauge to the tank’s pressure valve. If water spurts out, you can be sure the bladder is ruptured.

If you’re still unsure if you’re dealing with a ruptured bladder you can test it by pressurizing the empty tank to 6 – 8 psi. Then reconnect the tank to the rest of the RO system and allow it to fill with water. This can take between several hours for a standard-sized tank, so be patient. If the water flow from the tank decreases significantly after waiting a few hours/days, you can once again be sure you’ve got a ruptured bladder.

As mentioned previously, there is nothing you can do to repair a ruptured bladder, the entire tank must be replaced.

4. Kinked Tubing

Bent or kinked tubing can also cause a blockage between the storage tank and the RO faucet. Typically, if the tubing is kinked it won’t completely block the water flow but rather slow it down to a trickle. If that sounds like what you’re dealing with, then a quick examination of the tubing can’t hurt.

If you find any kinks in your tubing, either straighten them out or replace the old tubing with new tubing.

plumber installing reverse osmosis system under sink

5. A Clogged Post-Filter

If you’ve gotten this far in the troubleshooting process, and still haven’t found the source of your problem, then you are likely dealing with a clogged post-filter. Be sure to check that the tank valve is open, the tank is properly pressurized, the bladder is intact, and there are no kinks in the tubing before assuming you have a clogged post-filter.

To confirm your suspicions about the post-filter, you will need to disconnect the post filter tubing and see if water flows from the disconnected tubing section. To do this first close the tank valve as well as the feed water supply to the system. Then remove the post-filter and re-route the tubing to a drain or bucket. Turn back on the tank valve to see if any water flows from it. If it does, you have a clogged post-filter that needs replacing.

How Reverse Osmosis Pressure Tanks Work

To give you an idea of how RO storage tanks work you’ll need to learn a bit about how they use pressure to operate with the rest of the reverse osmosis system. Besides, if the storage tank wasn’t pressurized it would require a pump to get water up to the faucet.

So, RO storage tanks are also called hydropneumatic tanks. Hydropneumatic tanks contain both an air and water chamber divided by an internal bladder. The air chamber compresses when water fills the water chamber which increases the tank’s internal pressure.

When the water inside the tank reaches a certain level, the pressure in the tank triggers the RO system’s automatic shut-off valve (ASO) – which stops the flow of water into the entire system. This valve prevents the excessive production of wastewater, as without it, the system would continuously produce water and simply dump it down the drain.

When you pour a glass of filtered water from the faucet, the tank empties, which causes the internal pressure to drop triggering the ASO valve to open up and allow more filtered water to flow into the RO system.

Interestingly, the size of the tank determines the internal design of the air and water chambers. In smaller tanks under 10 gallons, the air bladder sits below the water chamber. In larger tanks, the air bladder sits above.

If you have any thoughts about the question, RO tank full but no water, 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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