A reverse osmosis system contains four valves doing four different jobs, and they are easy to confuse because three are small plastic items buried in tubing. Between them they cause most of the faults that get blamed on the membrane, so being able to name them and test each one is worth the ten minutes it takes.
The feed valve and the tank valve
The feed valve is the tap that supplies the system, usually a saddle or a tee off the cold line under the sink, and closing it is the first step of any service. The tank valve sits on the storage tank's outlet, on the single line that both fills and empties it, and it isolates the tank. Leaving the tank valve closed after servicing is the single most common cause of no water at the faucet, and it is the first thing to check when a system stops delivering.
The automatic shutoff valve
This senses the pressure in the storage tank and stops production when the tank is full, by cutting off the feed to the membrane. It works on a differential between tank pressure and feed pressure, so it depends on the check valve being sound. When it fails, the system runs to drain permanently and the water bill notices. It is a small plastic block with four tube connections, sitting in the tubing rather than mounted on anything, which is why it is easy to overlook.
The check valve on the permeate line
A one way valve at the membrane housing outlet that stops pressurized tank water pushing backwards through the membrane and out to the drain. When it fails, the system runs continuously and the tank never fills, which looks identical to a failed shutoff valve. Separate the two by closing the tank valve: if the drain flow stops with the tank isolated, the check valve is the candidate. It has a flow direction, and fitting it backwards produces a system that makes no water at all.
The flow restrictor, which is not a valve
Worth mentioning because it sits in the same tubing and gets confused with them. The restrictor limits the reject stream to maintain pressure across the membrane, and it is a fixed orifice rather than something that opens and closes. A missing or wrong one either sends most of the water to drain or fouls the membrane by not flushing it enough. Check it whenever the waste ratio looks wrong. Our reverse osmosis installation guide covers the commissioning sequence.
Questions people ask about ro valve
My system suddenly gives no water, where do I look?
The tank valve first. Leaving it closed after servicing is the most common cause of no water at the faucet.
What does the automatic shutoff valve do?
It senses tank pressure and stops production when the tank is full. A failed one leaves the system running to drain permanently.
How do I tell a failed check valve from a failed shutoff valve?
Close the tank valve. If the continuous drain flow stops with the tank isolated, the check valve is the likely cause.
Is the flow restrictor a valve?
No, it is a fixed orifice limiting the reject stream to hold pressure across the membrane. A wrong one wastes water or fouls the membrane.