How Do Reverse Osmosis Units Know That the Storage Tank is Full?


Modern undersink reverse osmosis units use a simple but effective shutoff device to turn off water production when the unit’s storage tank is full. The shutoff system monitors the pressure in the storage tank and shuts off water coming into the RO membrane when tank pressure reaches approximately 2/3 of the pressure of the incoming tap water.  Thus, a “full” tank in a standard RO unit is defined by the pressure of the water entering the RO unit.  If incoming pressure is 60 psi, a full tank holds about 40 psi; if incoming pressure is 50, a “full” tank holds about 30 psi. Shutoff is done at around 2/3 of incoming pressure because as the tank fills increasing back pressure makes production less efficient. It isn’t practical to fill the tank past 2/3 full.

The Payne brand shutoff pictured above is installed as follows:

1. After water leaves the RO unit’s prefilter, it enters the “In” port of the shutoff valve, lower right in the picture. It then makes a horseshoe turn and exits the “Out” port, lower left in the picture, through which it flows to the inlet side of the RO membrane.

2. When the “permeate” water (the product water of the RO unit) leaves the other end of the membrane housing, it flows to one of the “tank” ports on the other side of the shutoff valve. It doesn’t matter which port it enters, since the “tank” ports are interchangeable and water flows either way on the permeate side of the valve. Water then makes a horseshoe turn inside the top side of the valve and leaves through the other tank port. From there it flows to the storage tank.

3. The two halves of the valve are separated by a piston, which keeps the permeate water on one side and the incoming tap water on the other. As long as the pressure on the tank side is less than 2/3 the pressure on the tap water side, the piston remains open and the unit continues to produce water. As the RO produces water and slowly fills the storage tank, however, pressure on the tank side of the piston eventually becomes strong enough to force the piston toward the tap water side and shut off the incoming tap water, stopping production. The RO unit stays off until enough water is removed from the storage tank to drop the pressure on the tank side of the piston, allowing tap water pressure to push the piston toward the tank side and start RO production again.

The Flowmatic shutoff valve above works exactly like the Payne valve, although the flow pattern is straight through rather than horseshoe style. In other words. water enters lower right and flows straight ahead through the valve and out the other side.

It is important to know that in order for the shutoff system to work, a check valve (one-way valve) must me installed in the permeate tube between the membrane and the shutoff valve. Without the check valve to hold the back pressure from the tank, the shutoff valve cannot function.


The Payne shutoff valve is clipped to the membrane housing of the unit above. The tubes on the right side carry tap water to the membrane. On the left, or “tank” side, permeate water leaves the membrane, passes through the cigar-shaped check valve, and enters one of the shutoff valve’s “tank” ports. It leaves via the other tank port and flows to the tee, which sends it to the storage tank.

Note that the inline check valve will not be found on most RO units, since RO manufacturers usually prefer a tiny, inexpensive check valve that”s contained in the elbow fitting where the permeate water leaves the membrane housing.

Troubleshooting your shutoff system

If you hear water running to drain more than you think it should or have some other reason to suspect that your RO unit isn’t shutting off properly, here’s an easy test you can do to check its performance.

Run a few glassfuls of water from the RO unit to start the unit producing water, then turn off the RO faucet and remove the drain line from the drain saddle connected to the undersink drain pipe.  Drop the end of the drain line into a bottle or pan  so that the drain water trickles into the container. Next, turn off the valve at the top of the RO storage tank. Water should stop flowing from the drain line within a couple of minutes. When it stops, leave the valve off, empty the container, put the tube back into the container, and come back in 10 minutes.  If there is no water in the container, the shutoff system is working perfectly.  The unit is shutting off and holding its shutoff.

If the drain fails to shut off, you need to find the reason.  The main suspects are the shutoff valve or the check valve, not necessarily in that order.

If the drain shuts off initially, but comes back on during the 10 minutes, continue to watch it.  If it comes on, runs briefly, then shuts off,  and repeats this pattern over and over, you need a new check valve.

More about reverse osmosis shutoff valves and other reverse osmosis part