Your RO System Runs to Drain (or Purges) Continuously: Causes and Fixes

Customer question

"My RO system keeps running water to the drain and won't stop. Is it broken?"

Answer

Probably not. A reverse osmosis system is designed to send some water to the drain while it is producing. That drain flow is how the membrane stays clean, so a gurgle or trickle to the drain is normal, especially right after you draw water and the system starts refilling the tank.

The one test that tells you normal from fault: does the sound or flow stop once the storage tank is full? If it stops when the tank fills, that is normal operation and there is nothing to fix. If it never stops, you have a fault to track down.

When it truly never stops, work the causes cheapest and easiest first: check the storage-tank air pressure first (most common, and free), then the check valve and the automatic shutoff, and finally the drain-line flow restrictor.

Steps

Work these in order. Stop as soon as the drain flow stops when the tank is full.

Run the normal-vs-fault test first. Let the system fill until the storage tank is full, then listen and watch the drain line. If the drain flow stops once the tank is full, this is normal operation and you are done. If the drain flow never stops, continue to step 2.
Check the storage-tank air pressure (do this first). If the tank cannot hold its air charge, the system never reaches the pressure that tells it to shut off, so it keeps trying to fill and runs continuously.
- Depressurize the system and drain the tank fully before reading anything (see Safety below).
- With the tank empty, read the air valve with a low-PSI gauge. As general guidance, an empty RO storage tank typically targets roughly 6 to 8 PSI.
- If it is low, add air with a hand pump to about 7 PSI. Do not exceed about 10 PSI.
- If water comes out of the air valve, the internal bladder is ruptured and the tank must be replaced.
- Confirm the manufacturer pressure spec for your specific tank model [CONFIRM] before adjusting.
Check the check valve. The check valve, located near the membrane housing, stops backflow and lets the system shut down once the tank is full. A stuck check valve can cause continuous drain flow. Clean or replace it. If you are not familiar with this part, stop and contact support.
Check the automatic shutoff (ASO) valve or solenoid. On systems with an automatic shutoff, the system keeps running if the shutoff fails to close when the tank is full. Note that a flat (unpressurized) tank can keep the ASO from ever triggering, so always confirm tank air pressure in step 2 first. If tank pressure is correct and it still never stops, suspect the ASO valve or solenoid.
Check the drain-line flow restrictor. The flow restrictor on the drain line sets how much water goes to drain. If it is clogged, missing, or the wrong part, drain flow can run continuously or the drain ratio can be off. Inspect it and replace it with the correct restrictor for your system.

Safety / before you start

BEFORE YOU OPEN ANYTHING - READ FIRST

Depressurize the system before opening any fitting, valve, or the tank. Turn off the feed water and the tank ball valve, then open the RO faucet to relieve pressure.

Do not overinflate the storage tank. Add air slowly with a hand pump and do not exceed about 10 PSI. Too much air pressure can damage the tank and hurt performance.

Keep the work area dry.

Well water or commercial/whole-house RO: do not guess. These systems are sized and configured differently. Route whole-house and commercial RO questions to Crystal Quest engineering.

When to contact support

Reach out to our support team if:

The system still never stops to drain after you have corrected the tank air pressure.
Water comes out of the air valve (ruptured bladder, tank replacement needed).
You are not familiar with the check valve, the ASO valve/solenoid, or the flow restrictor and are unsure how to inspect or replace them.
You have a whole-house or commercial RO system. Do not guess. We will route you to engineering.