The check valve is one of the least visible parts of a reverse osmosis system and one of the most common causes of the two faults owners find hardest to diagnose: a system that runs to drain continuously, and a storage tank that never fills. It is a small one way valve on the membrane's permeate outlet.
What it is there to stop
Water stored in the tank is under pressure from the air charge behind the bladder, and without a check valve that pressure would push permeate backwards through the membrane housing and out to the drain line. The check valve allows flow only from the membrane towards the tank. It also keeps the automatic shutoff valve able to sense tank pressure correctly, since that valve depends on a pressure differential that a leaking check valve destroys.
The symptoms when it fails
The classic one is continuous flow to drain with the tank never filling, because stored water is escaping backwards as fast as it is being made. A partly failing valve gives a system that fills very slowly and wastes far more water than it should. Confusingly, a failed automatic shutoff valve produces a similar picture, and the two are separated by isolating the tank: close the tank valve and if the drain flow stops when the tank is isolated, the check valve is the candidate.
Where it is and how it is replaced
It is usually a small barbed or push fit insert in the permeate line right at the membrane housing outlet, sometimes integrated into the housing elbow itself, and it can be easy to miss. It has a flow direction and fitting it backwards produces a system that makes no water at all, so note the arrow before removing the old one. Depressurize before touching anything. It is an inexpensive part and is worth replacing as a matter of course during a membrane change.
Check the others while you are in there
Three components produce overlapping symptoms and it is worth clearing all of them at once. The automatic shutoff valve stops production at tank pressure and fails in a similar way. The flow restrictor on the drain line sets the pressure across the membrane, and a missing or wrong one wastes water or fouls the membrane. And the storage tank air charge, checked with the tank empty, causes weak delivery that gets misread as a production fault. Our reverse osmosis guide covers the whole sequence.
Questions people ask about reverse osmosis check valve
What does the check valve do?
It allows permeate to flow only towards the storage tank, stopping pressurized tank water pushing back through the membrane to drain.
How do I tell it from a failed shutoff valve?
Isolate the tank. If the continuous drain flow stops when the tank valve is closed, the check valve is the likely cause.
Where is it fitted?
In the permeate line at the membrane housing outlet, sometimes built into the housing elbow, and easy to overlook.
Can it be fitted backwards?
Yes, and the system then makes no water at all. Note the flow arrow before removing the old valve.