Why is My Reverse Osmosis Tank Not Filling Up?

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Reverse osmosis tank not filling with water? In this guide, we’ve answered the question: “Why is my reverse osmosis tank not filling up with water?”

Read on to learn the 4 most common reasons why the water tank in your reverse osmosis system might not be filling up.

📌 Key Takeaways:

  • On average, an RO pressure tank should take 2-4 hours to fill completely.
  • Some of the reasons why your reverse osmosis tank might be filling slowly or not at all are a pressure imbalance in the tank, clogged RO membrane/filters, low incoming water pressure, and excessive water usage.
  • You can resolve these issues by repressurizing the RO tank, changing the membrane/filters, installing a booster pump upstream of the RO system and using less water/upgrading to a higher-capacity system.

🤔 Why Is My Reverse Osmosis Tank Not Filling Up? 4 Causes & Solutions

Here are some of the problems that might be affecting water flow into your RO storage tank, and how to resolve them:

1) Tank Pressure Is Too Low/Too High

The main culprit behind a water flow issue in an RO system is a problem with the water storage tank pressure.

When empty, your reverse osmosis tank should have a pressure of 8 pounds per square inch (PSI). A pressure imbalance either way can cause water filling issues:

  • If the pressure in the tank is too high, the tank shut-off valve will close. In normal circumstances, this would prevent the tank from overfilling and leaking back into the RO system. However, if the tank pressure is already too high before it starts filling, the shut-off valve will block water from entering the tank even if it’s empty.
  • If the pressure inside the tank is too low, there won’t be enough pressure to send water out of the tank, through your cold water pipe, and out of the reverse osmosis faucet. You might think that the tank isn’t filling, but actually, the tank is full – you simply can’t access the water.

You can check the pressure of your RO storage tank yourself. Just switch off the water supply valve upstream of the reverse osmosis system, then drain the tank until it’s empty of all water. Use a pressure gauge to measure the water pressure.

Checking RO tank pressure with pressure gauge

✅ How To Resolve:

What can you do if the pressure in your reverse osmosis tank is incorrect?

  • If the pressure is too high, you can relieve some of the pressure by draining the water (if you haven’t already) then gently pressing the Schroeder valve (which is a bit like a bicycle tire valve) on the tank. This will release excess air. You might want to place a bucket under the tank in case any leftover water leaks out too.
  • If the pressure is too low, use a bicycle pump to increase it. Again, make sure the water is drained out of the tank first. Continue to add air into the tank, stopping periodically to take a pressure reading, until the pressure reaches 8 PSI. Check your user manual to confirm that this is the correct tank pressure for your reverse osmosis system.

2) Clogged RO Membrane Or Filters

Clogged filters or a clogged reverse osmosis membrane are two other very common causes of a reverse osmosis storage tank that won’t fill.

Clogged filters/RO membranes restrict water flow into the RO tank because they reduce the rate at which the water can be filtered.

Contaminants build up gradually in the RO membrane and filter stages, blocking their pores and limiting the volume of water that can filter through. This is a normal outcome of using the filters in your RO system for an extended period, although your RO filters may become clogged at a faster rate due to iron fouling or hard water scaling, or simply if your water’s TDS concentration is high.

✅ How To Resolve:

The easiest way to resolve a water flow issue caused by clogged filters/membranes in your reverse osmosis system is to replace the old filters as and when needed.

Check your user manual to see exactly when your filters should be changed. For under-sink reverse osmosis systems, the general lifespans for filters are:

  • Sediment pre-filter: 6-9 months
  • Carbon pre-filter: 9-12 months
  • Semipermeable membrane: 24 months
  • Carbon post-filter: 12 months

To change your filters, turn off the water supply valve and open the RO faucet to relieve pressure. Remove the old filters and click or twist the new filters in place. Switch the water supply valve back on, then wait up to 3 hours for the storage tank to fill.

Open your reverse osmosis faucet and check for water. If you get a steady flow of water from the RO system, the issue should have been fixed.

3) Low Incoming Water Pressure

Even if your reverse osmosis system and tank are working properly, low pressure from your incoming water supply line might be the reason why the pressure tank can’t fill.

If your home’s water pressure is too low, there won’t be enough force to send water quickly through the RO filters and membrane. As a result, the system will run less efficiently and more water will be wasted, causing the tank to fill very slowly – if at all.

Most RO systems operate with an ideal water pressure of 60 PSI. At the very least, your water pressure should be 40 PSI – any lower and you may experience the problems outlined above.

Adjusting water pressure regulator in a home

✅ How To Resolve:

Consult your user manual to see what your water pressure should be for your reverse osmosis system, then use a pressure gauge to measure the pressure in the water supply line leading to the system.

If your water pressure is too low, you’ll need to buy and install a booster pump upstream of the RO system.

Confused about a low pressure reading? If your POE water pressure seems fine but there’s a significant pressure drop before your RO system, check that the water supply valve upstream of the unit is fully open. If not, it’ll restrict water flow to the system, reducing your water pressure. This is an easy fix: just open the valve fully to allow water to flow freely through.

4) Excessive Water Usage

Using too much water – more than your RO system is designed to handle – will give the impression that your reverse osmosis pressure tank isn’t filling.

Tank-based reverse osmosis systems work by slowly filtering water and sending it into the purified water tank. If you use water faster than the rate at which the tank can be filled, it’ll seem like the tank isn’t filling.

This won’t be a problem for most people. In normal circumstances, you’ll probably only turn your faucet on occasionally to fill a glass – or, at most, a jug – of water.

However, if you have a reason to use large volumes of water in one go from your kitchen faucet, the rate of water leaving the tank could exceed the volume of water refilling the tank, causing it to run dry.

✅ How To Resolve:

There are two simple ways to resolve a filling issue with your RO storage tank that’s caused by excessive water usage:

  • Use less water, or space out your water usage to avoid overwhelming the system
  • Upgrade to a larger system with a bigger tank and a faster flow rate, which can handle your water demands

The second option is the costlier one, so when possible, it’s best to assess your current water usage habits and see if you can reduce or alter your intake.

If you need a lot of cold water for non-drinking purposes, make sure to take it from your regular faucet, not the RO faucet. That means you can save your RO filtered water for drinking water only.

📥 What is a Reverse Osmosis Tank?

Now you know the answer to your question, you might want a quick refresher on what a reverse osmosis tank is, and the purpose that it serves.

A reverse osmosis tank is a pressurized tank that stores water that has been purified by a reverse osmosis system. The tank sits downstream of the RO filtration cartridges and collects water until it’s full, giving you access to purified water whenever you switch on your faucet.

The purpose of a reverse osmosis storage tank is to make sure you can enjoy RO water on demand, without having to wait around every time you open the RO faucet. RO purification is a slow process due to the resistance of the multiple filtration stages. By collecting water in a pressurized tank in advance, a tank-based RO system enables you to access the water immediately whenever needed.

You can find reverse osmosis tanks in various sizes and capacities, depending on the intended install location for the system. The larger the system and the more fixtures it’s intended to serve, the bigger the tank. Most RO systems are under-sink units, so they have relatively small 2-4-gallon water tanks.

Reverse osmosis tank

🔎 How Should a Reverse Osmosis Tank Work?

To understand why a reverse osmosis tank might not fill up as it should, it’s helpful to know how an RO tank should work under proper operating conditions.

An RO tank contains pressurized air, which enables it to deliver water to your faucet, eliminating the need for a separate booster pump.

There are two chambers in an RO tank: an air chamber and a water chamber. As water flows into the tank, it causes the air to become compressed. This air pressure is what ultimately forces water out of the tank and through your pipes when you switch on a faucet.

The fuller the tank, the higher the air pressure. The tank will stop filling automatically once it reaches full capacity.

Then, when you turn on a faucet and use some of the water from the tank, the water level will drop and the RO system will continue to filter water until it refills.

🚰 How Long Does a Reverse Osmosis Storage Tank Take to Fill?

In normal working circumstances, with the right water pressure and a properly functioning reverse osmosis membrane, a reverse osmosis tank should fill to capacity within 2-4 hours.

There are a few factors that affect an RO water tank fill time, including:

  • The tank size. The larger the tank, the more water will be required to fill it to capacity, and the longer it’ll take.
  • The filter/reverse osmosis membrane age. Older filters that are clogged with contaminants will filter water more slowly, reducing the speed of the tank filling.
  • Your water pressure. The higher the pressure in your incoming water line, the quicker water will flow through the RO system, and the faster the tank will refill.
  • System age & efficiency. Many older RO units simply aren’t designed to filter water as efficiently. Newer, more efficient systems will filter water – and therefore fill the RO tank – at a faster rate.
  • Membrane pore size – Different RO membranes have different pore size. The smaller the membrane pore size, the more resistance during filtration, and the longer the process will take. This affects the speed of the tank filling.
  • System component size. The size of the components in an RO system may affect the tank refill speed. Some systems are higher-capacity and can filter larger volumes of water, with faster tank fill times.

If you’re unsure how quickly your reverse osmosis storage tank should be filling, check you user manual or contact the manufacturer for this information.

Reverse osmosis tank water line

📑 Final Word

It’s not ideal if your reverse osmosis water tank isn’t filling – but luckily, this problem is usually easy to resolve.

Follow the guidance in this article to fix a water flow issue in your RO tank. If none of these possible issues apply to your situation, it could be that the tank is broken and is unable to hold pressurized air. In that case, the cheapest solution is usually to replace the tank.

Got more questions about reverse osmosis tanks? Check out the FAQs below.

❔ FAQ:

Why is my RO tank not refilling?

The most common reasons why your RO storage tank isn’t refilling are a pressure imbalance in the tank, clogged RO membrane/filters, low water pressure, and excessive water usage.

Should reverse osmosis tank be full?

Yes, your reverse osmosis storage tank should be full, or close to full, throughout the day. A reverse osmosis tank works to refill itself when the water pressure in the tank drops (which happens when you use some of the water). The pressurized air chamber in the tank sends water from the tank to your faucet, and as the tank empties, the RO system continues to filter water to make up for the pressure loss.

Why is my reverse osmosis not working after changing filters?

If your RO system isn’t working after changing the filters, you might have installed a filter incorrectly, or there could be an air lock inside the system. Another possible cause is that you forgot to turn your water supply valve back on, so water isn’t entering the system.

Why is my reverse osmosis system not producing water?

Reverse osmosis systems might not produce water if there’s a water flow issue into the storage tank. Check to see if the filters or RO membrane need replacing, measure your tank pressure with a pressure gauge to ensure it’s adequate, and adjust the pressure if necessary.

Should the RO tank have water in it?

Yes, the RO pressurized holding tank should always contain water. If the tank contains no water, it’s likely that there’s an issue with the flow of water through the system that needs to be addressed.

  • Brian Campbell
    President & CEO, CWS, CWR

    Brian Campbell, a WQA Certified Water Specialist (CWS) and Certified Water Treatment Representative (CWR) with 5+ years of experience, helps homeowners navigate the world of water treatment. After honing his skills at Hach Company, he founded his business to empower homeowners with the knowledge and tools to achieve safe, healthy water. Brian's tested countless devices, from simple pitchers to complex systems, helping his readers find the perfect fit for their unique needs.

2 thoughts on “Why is My Reverse Osmosis Tank Not Filling Up?”

  1. Avatar for Brian Campbell

    After replacing all filters and membrane, I have full pressure at the faucet whether the tank valve is open or closed. The tank is very light so appears to be empty. Could the new membrane be faulty? Everything was working before and I usually change everything after 1 year. There was no appreciable debris or discoloration of the filters.
    I live in an apartment and there are only 2 of us so water usage is not very high. I installed the system due to a water softener being installed in the building.
    Any suggestions would be appreciated.

    1. Avatar for Brian Campbell

      Hey Stuart, thanks for reaching out. This could indeed indicate a faulty membrane. Additionally, there could be an issue with the tank bladder which may need to be recharged or replaced. Let us know how it goes.

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