Check valve

A one-way valve called a check valve in the "permeate" line as it leaves the RO membrane housing. This is usually a small, inexpensive part, but it is essential to the function of the unit. Without a properly working check valve, the unit will not shut off properly when the tank is full, a great deal of water will be wasted, and eventually the membrane will be ruined.

The function of the check valve is to isolate the membrane from back pressure from the storage tank and to provide a solid wall for the shutoff valve to push against.

Check valves can be of two types: the very small valve that is mounted inside the elbow fitting that takes the product water from the membrane, or a more substantial (and more expensive) inline valve that is installed in the line between the membrane's permeate port and one of the tank ports on the auto shutoff valve.