A common concern with a Reverse Osmosis drinking water system is water continuously running to drain. Since the Reverse Osmosis drinking water system (RO) flushes it's waste water while it is filling the tank, water running to drain during production is normal but after 3 or 4 hours of zero water usage there should be zero water running to the drain. There is an ASO or Automatic Shut Off valve which in conjunction with the check valve shuts off the water when the tank is full. This valve requires a minimum of 40 psi of pressure to shut off the flow. In rare cases where the membrane becomes extremely fouled this can also cause water to continuously flow to drain so if you haven't changed your membrane for several years now is the time before proceeding.
A couple of tests need to be completed to check the system and see if the ASO or the check valve is defective:
1) First, starting with a full tank, draw 2-3 glasses of water from the faucet, the RO will start making water to top off the tank.
2) Turn OFF the ball valve from the tank by turning the handle 90 degrees, which simulates a "tank full" condition. If your RO feeds a fridge, turn off that line too.
3) Wait for 3- 5 minutes, then check to see if the drain water stops running by listening or actually removing the drain line to observe, if the water has shut off both the ASO and Check valve are working properly.
4) If water continues to run to the drain then either the Check Valve and/or the ASO valve is defective.
5) Be sure that the tank is full or close to it and remove the drain line from the drain saddle so you can check the flow.
6) Turn OFF the Cold feed water supply and open the tank valve.
7) Check the drain line, if water flows then this water is coming from the storage tank which means the Check Valve is broken. If no water drains out from the black line this means the ASO is defective and needs to be replaced.