Why Is My RO Tank Not Filling With Water?

Inspect the water supply valve, ensuring it is in a fully open position. If it is clogged or only partially open, the RO unit might not build up sufficient water pressure to operate. Inspect the unit’s supply lines. A kinked line will restrict water flow and prevent proper operation.

System Flow Rate: The type of RO system you have will impact how quickly water will fill your water storage tank.  This means that the 100 GPD system will replenish the water storage tank twice as fast a 50 GPD system. If you suspect your system is not producing at capacity, Below we provide steps to determine how much water your RO system is actually producing in a 24-hour period.

What’s a Reverse Osmosis Tank?

A reverse osmosis tank is a pressure tank that collects water being purified by the reverse osmosis membrane. Since RO filtration is a slow process, it takes more time to push water molecules across the semipermeable membrane. So, the storage tank accumulates the purified water for immediate access.

The RO tank has an inner lining made of a material called butyl. This material prevents the stored purified water from getting into contact with the tank’s steel casing. Inside the tank, there’s pressurized air to pump water to your faucets. Reverse osmosis tanks are designed to fit the cabinet below your sink.

Reverse osmosis tanks come in various capacities, depending on the maximum volume of water they can hold. However, the capacity indicated by the manufacturer is usually higher than the tank’s actual capacity.

For example, a tank labeled 4 gallons doesn’t mean it holds the same volume of water. It can hold around 3 gallons of water, and the remaining volume occupied by air and metal bladder.

How a Reverse Osmosis Tank Works?

As mentioned, reverse osmosis tanks use pressurized air to deliver purified water into your home’s faucets on demand. You don’t have to install a booster pump to facilitate water flow out of the system.

In every reverse osmosis tank, you’ll find two chambers; a water chamber and an air chamber, both separated by a bladder. However, the two chambers’ position usually changes depending on the size of the RO storage tank.

For smaller tanks with capacities of 1-10 gallons, the water chamber sits atop the air chamber. As RO tanks get bigger and bigger, the pressurized air chamber sits atop the water chamber. That helps to increase the pressure inside the tank through gravity.

So, how does the reverse osmosis tank work? When water from the RO system gets into the storage tank, it starts to compress air in the air chamber. As a result, the pressure inside the chamber will increase to propel water from the tank to your faucets.

An RO tank doesn’t require an electric pump to work. However, if the tank didn’t have a pressurized air chamber, water would fill up the tank but wouldn’t move to the faucets. In that case, you would have to install a delivery pump to pump water out of the tank.

A properly installed reverse osmosis (RO) unit will give you water almost as pure as distillation. It performs this task by passing filtered water across a semipermeable membrane that excludes anything larger than a water molecule. Wastewater is routed away from the filter into the drain. The purified water is stored in a pressurized holding tank until needed. If your unit stops working, 

If your Reverse Osmosis water tank isn’t filling, and you’re wondering how long it normally takes to fill an RO tank, the short answer is it typically takes 2 to 4 hours to fill a standard reverse osmosis holding tank (2.8 gallons or 10.6 L).

A properly installed reverse osmosis (RO) unit will give you water almost as pure as distillation. It performs this task by passing filtered water across a semipermeable membrane that excludes anything larger than a water molecule. Wastewater is routed away from the filter into the drain. The purified water is stored in a pressurized holding tank until needed. If your unit stops working, try the steps below.

Reverse Osmosis Tank Not Filling Up?

Check the Water Supply

Check your pressure. The RO unit needs about 40 pounds of pressure to operate correctly. Many older water wells were set up to operate below this pressure. If you have a well or low water pressure, you might need to install a booster pump for the unit to operate efficiently.

Inspect the water-supply valve, ensuring it is in the fully-open position. If it is clogged or only partially open, the RO unit might not build up sufficient water pressure to operate.

Inspect the unit’s supply lines. A kinked line will restrict water flow and prevent proper operation.

Inspect the Storage Tank

Open the unit’s tap and allow the water to run until the tank is empty. Use a pressure gauge to check the air pressure in the tank. The pressure in an empty tank should be eight pounds. If the pressure in the tank is too high, water will be unable to enter from the filters. If the pressure is too low, the water inside the tank will not flow to the faucet.

Inspect the water-supply valve, ensuring it is in the fully-open position. If it is clogged or only partially open, the RO unit might not build up sufficient water pressure to operate.

Inspect the unit’s supply lines. A kinked line will restrict water flow and prevent proper operation.

Replace the Filters

Close the water-supply valve and open the RO faucet. Allow the tank to drain. Unscrew both the pre- and post-filter housings. Replace the old filters and screw both units back in place. These filters should be replaced at regular intervals to prevent damage to the delicate membrane of the RO filter.

After replacing the two filters, close the RO faucet and open the water supply valve. After three hours, check the faucet. If you have gone through all the above steps and still fail to get water out of the faucet, you will need to replace the RO membrane.

Inspect the water supply valve, ensuring it is in the fully open position. If it is clogged or only partially open, the RO unit might not build up sufficient water pressure to operate.

Inspect the unit’s supply lines. A kinked line will restrict water flow and prevent proper operation.

Replace the RO Filter

Close the water-supply valve. Open the faucet and drain the storage tank. Close the faucet after the tank has drained. Unscrew the filter housing and remove the RO membrane. Wash the inside of the filter housing with warm, soapy water. Rinse it thoroughly. Put on a pair of clean rubber gloves to handle the new RO membrane. Bacteria from your hands can contaminate the filter membrane. Insert the new RO membrane into the housing and replace the housing on the filtration unit.

Open the water-supply valve. After eight hours, open the faucet and allow the tank to drain completely. This step is necessary to wash out the preservatives that were inside the new membrane. Close the faucet. Once the tank has filled again, the unit will be ready to use.

How to Determine Water Flow Rate of Your Reverse Osmosis System


Step 1: Make sure the incoming water line to the RO system is turned “ON” and turn the ball valve on top of the reverse osmosis storage tank to the “OFF” position (generally a 1/4 of a turn).

Step 2: On your reverse osmosis faucet, flip the handle to the “up” position, so the faucet is now locked into a continually open/flow position. At this time any water in the lines will drain.

Step 3: After there is no more water in the system lines, wait about 5 minutes to make sure all water has drained. (NOTE: If you get no flow from the reverse osmosis faucet, the system is not producing water).

This flow rate represents the flow rate the system is producing water and the rate that the reverse osmosis storage tank would be filling if the valve on the reverse osmosis pressure tank was in the “open” position.

Step 4: Once you have a continual drip or slow flow from the faucet, using a measuring cup, measure how much water drips/flows from the reverse osmosis faucet into the measuring cup for 60 seconds.

Step 5: Now it’s time to do the math!

Take the number of ounces your RO system produced in one minute.
Multiple this number by 1440, which is the number of minutes in a day.
Divide this number by 128 which is the number of ounces in a gallon.
This number is the amount of water your RO system is producing over a 24 hour period.


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