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How to Test RO’s Shut-Off Function:

The RO system should shut off automatically when the tank is filled. When the RO fails to shut off after tank is filled, the waste water will keep running down the drain, depleting the pre-filters and membrane.

See Fig. 16. The Auto-Shut-Off (ASO) valve is located at point C. The Check Valve is located at point E. These two valves control the RO’s shut off function.

If one of these valves fail (valve worn out, clogged, or defective), the system cannot shut off, and the waste water fill keep running non-stop.

Do test #1 and #2 below to determine if the RO can shut off, and if the valves are OK.


Test#1: Can the RO system shut off?

–    Draw 2-3 glasses of water from spigot. RO will start making water to fill tank.

–     Turn OFF the tank’s valve to mimic “tank full”.

–    If your RO feeds multiple output points (icemaker, bathroom, etc), shut OFF those lines.

–    Wait for 3- 5 minutes, then check to see if the waste water stops running.

–    Check waste water by either “listening” or actually taking out the drain line to look at it.

–    If waste water stops running –>  The RO is shutting off properly. Both the ASO valve and the Check Valve are working fine.  Stop testing.

–    If waste water continues to run  –>  Then either the Check Valve and/or the ASO valve is defective.  Proceed to Test #2.


Test#2:  Test Check Valve and ASO valve:

–    Make sure there is some water in the tank (tank not empty).

–     Remove the Black drain line from the drain saddle (so you can check waste flow drainage).

–    Turn OFF the Cold feed water supply.

–     Turn ON the tank valve.

–    Check the Black drain line to see if there is any water draining out from this line.

–    If water does drain out from the black line –> Then this water is coming from the storage tank. This means the Check Valve is broken, it is allowing the water in the tank to back flow out into the drain line.


Solution:  Replace Check Valve  ( see Fig.16 point E )

–    If no water drains out from the black line (no waste water running)  –> That means the Check Valve is OK.  The RO’s non-shut off is caused by a defective ASO valve, not caused
by the Check Valve.


Solution:  Replace ASO valve  ( located at Fig.16 point C )