my apec roes-50 system is making a constant hissing sound from the drain line, is that normal?

Diagnosis

The constant hissing sound emanating from the drain line of your APEC ROES-50 indicates that the system is stuck in a continuous production cycle. In a healthy RO system, water flows to the drain only while the storage tank is filling. Once the tank reaches full capacity, the internal pressure triggers the Auto Shut-Off (ASO) valve to clamp down on the feed water, stopping the flow. If you hear a non-stop hiss, the ASO valve is failing to close, meaning your system is wasting gallons of water per hour and potentially over-pressurizing the storage tank. This failure typically occurs due to one of three reasons: a lack of sufficient incoming water pressure to trigger the valve, a mechanical failure (debris or a torn diaphragm) inside the ASO valve, or a faulty check valve located at the outlet of the membrane housing. If the check valve fails, water leaks back from the membrane into the system, tricking the ASO valve into thinking the tank isn't full, which keeps the drain line open indefinitely. You can confirm this by closing the blue storage tank valve; if the hissing continues for more than 15-20 minutes after the tank is closed, the ASO valve or check valve is definitely compromised.

How to Fix It

To resolve the continuous drain flow, follow these steps to isolate and replace the failing components:

1. Shut off the main cold water supply valve and open the RO faucet to bleed off all remaining pressure in the lines.
2. Locate the ASO valve, which is the small square plastic component with four tubing connections located near the pre-filter housings.
3. Disconnect the 1/4" LLDPE tubing from the ASO valve by pushing in the locking clips and pulling the lines.
4. Inspect the tubing for any kinks or sediment buildup that might be preventing pressure from reaching the valve.
5. Replace the ASO valve with a new genuine APEC replacement part. Ensure the tubing is pushed firmly into the quick-connect fittings until you feel a "click" or a seat, as a partial seal can cause pressure leaks that prevent the valve from shutting off.
6. If the hissing persists after replacing the ASO, you must replace the membrane check valve. This is the small elbow fitting located where the permeate (clean water) line exits the membrane housing. Remove the old elbow and install a new one, ensuring the directional arrow points toward the storage tank and faucet.
7. Slowly turn the water supply back on and monitor the drain line. The hissing should stop once the storage tank reaches approximately 30-40 PSI.

If That Didn't Work

• Verify Incoming Water Pressure: Use a standard pressure gauge on your cold water line. The ROES-50 requires a minimum of 40 PSI to function correctly. If your home pressure is below 40 PSI, the ASO valve will never receive enough force to snap shut. In this case, you must install an inline permeate booster pump to artificially increase the pressure to the membrane.
• Check for Storage Tank Air Bladder Failure: If the tank's internal bladder has ruptured, the tank cannot hold the backpressure required to trigger the ASO valve. Press the air valve (Schrader valve) on the bottom of the tank. If water squirts out of the air valve, the tank is compromised and must be replaced immediately.
• Inspect the Membrane for Fouling: If the RO membrane is severely scaled or clogged, it may not produce water fast enough to build the necessary pressure to close the ASO valve. If your membrane is more than 24 months old, replace it with a new TF RO membrane.
• Clear the Flow Restrictor: Check the drain line flow restrictor (the small plastic plug in the waste line). If it is completely blocked, it can cause erratic pressure swings that prevent the ASO valve from seating properly. Replace the flow restrictor if it shows signs of mineral buildup.

Heads Up

• Avoid Over-Pressurizing: Never leave a failing ASO valve running for days; this can lead to the storage tank overfilling, which may cause the tank to leak or the safety relief valve to trigger.
• Sanitization Cycle: Whenever you open the system to replace the ASO or check valve, it is a best practice to sanitize the housings using a mild bleach solution to prevent bacterial growth in the stagnant areas of the plumbing.
• Tubing Cuts: When replacing valves, always use a sharp utility knife or specialized tubing cutter to make perfectly square cuts. Angled cuts in the 1/4" tubing often lead to slow leaks at the ASO valve connections.
• Filter Schedule: To prevent debris from clogging the ASO valve, replace your three pre-filters every 6 months. Sediment buildup in the first stage is the leading cause of pressure drops that cause ASO failure.