APEC ROES-50 drain line is constantly running water, even when the tank is full. is this normal?

Diagnosis

A constant stream of water flowing into the drain line on an APEC ROES-50 system is not normal behavior once the storage tank has reached full capacity. In a properly functioning system, the Automatic Shut-Off Valve (ASOV) located in the RO head assembly is designed to stop the flow of feed water into the membrane once the back-pressure from the full storage tank reaches a specific threshold. When water continues to run to the drain, it indicates that the ASOV is failing to "seat" or close, allowing water to bypass the membrane and exit through the waste line indefinitely. This failure is typically caused by one of three things: a ruptured internal diaphragm within the ASOV, a buildup of mineral scale or sediment preventing the valve from closing fully, or a malfunctioning check valve (the one-way valve located at the exit of the membrane housing). If the check valve fails, water from the storage tank leaks back through the membrane and out the drain, which tricks the ASOV into thinking the tank isn't full, keeping the valve open in a perpetual loop. You can confirm this by closing the tank ball valve; if the drain water stops immediately, the issue is likely the check valve. If the drain continues to run even with the tank closed, the ASOV itself is the culprit.

How to Fix It

To resolve a constant drain flow, follow these precise steps to isolate and fix the valve assembly:

1. Verify Tank Pressure: Close the blue ball valve on top of the storage tank. Open the RO faucet and let the water run until it stops. If the drain line is still running water while the tank is isolated, the ASOV is definitely stuck or broken.
2. The "Impact" Method: While the system is pressurized, use the handle of a screwdriver to lightly tap the plastic body of the Automatic Shut-Off Valve (the small component where the feed water line enters the RO head). Sometimes a small piece of debris or calcium scale prevents the diaphragm from seating; a physical jar can often snap it shut.
3. Check Valve Inspection: Locate the elbow fitting where the pure water exits the membrane housing before entering the ASOV. If you hear a clicking sound or see water flowing back toward the membrane when the tank is closed, the internal check valve seat has failed. You must replace the membrane housing outlet fitting or the entire membrane element if the check valve is integrated.
4. ASOV Replacement: If tapping fails and the check valve is functional, the ASOV diaphragm has likely perished. Shut off the main feed water supply and open the RO faucet to bleed pressure. Use a wrench to unscrew the ASOV from the RO head. Replace it with a genuine APEC replacement valve. Ensure the O-rings are lubricated with a small amount of food-grade silicone grease to prevent leaks during installation.
5. System Reset: Once the part is replaced, open the feed water valve and allow the tank to fill completely. Monitor the drain line for 30 minutes after the tank is full to ensure the flow has completely ceased.

If That Didn't Work

• Replace the RO Membrane: If the membrane is severely fouled or has developed a tear, it may not create enough osmotic pressure to trigger the ASOV. If your membrane is older than 24 months, replace the ROES-50 membrane element entirely.
• Check Feed Water Pressure: Use a pressure gauge to ensure your incoming water pressure is between 40 and 80 PSI. If the pressure is too low (below 30 PSI), the ASOV may not have enough force to snap shut, or the system may never reach the pressure required to shut off the flow.
• Inspect the Pre-Filters: Check the three pre-filter canisters. If the sediment or carbon filters are completely clogged, the pressure drop across the filters can cause erratic behavior in the ASOV, preventing it from closing properly.
• Verify Tank Air Pressure: With the tank empty of water, use a tire gauge to check the air valve at the bottom of the tank. It should be between 7 and 10 PSI. If the air pressure is too low, the tank won't provide the necessary back-pressure to trigger the ASOV.

Heads Up

• Avoid Over-tightening: When replacing the ASOV or fittings, do not over-tighten the plastic nuts. This can crack the housing or deform the O-rings, leading to slow leaks that are harder to diagnose than a running drain.
• Monitor Waste Ratios: A healthy ROES-50 system should produce significantly more waste water than pure water during the filling process, but it must stop completely once the tank is full. If you notice the drain running during the filling process is excessive, check for kinks in the waste line.
• Sanitization: Whenever you open the RO head assembly to replace the ASOV, it is a best practice to sanitize the system using a diluted bleach solution or a commercial RO sanitizer to prevent biofilm buildup in the new valve.
• Check for Leaks: After any repair, wipe all connections with a dry paper towel. Even a microscopic drip at the ASOV connection can lead to water damage over time.