I have had an RO unit for many years. Once in a while the drain stops working and my maintenance man said to back flush the little fitting on the drain line. Is that merely a check valve? If not, what is it? Is so, how does back flushing a check valve have any effect? Shouldn't it just not flush backward?

  • Is it a drain or just the outflow pipe?
    – isherwood
    Commented Jan 7, 2017 at 23:01
  • the installer called it a "drain" and it is routed to the sink drain as well. (under the sink)
    – Denzo
    Commented Jan 7, 2017 at 23:02
  • To clarify: It is not the clean water line to the spout. It is a separate line coming out of the manifold that the three filter canisters hang from. Tank pressure is at 6 psi when drained.
    – Denzo
    Commented Jan 7, 2017 at 23:18

1 Answer 1


Most likely you have a plugged flow restrictor. The flow restrictor regulates the flow of waste water away from the membrane. If it is plugged for very long, the membrane can become fouled so you might need to replace the membrane and also clean/replace the flow restrictor to restore production.

From the Watts Premier WP-4v manual:

enter image description here

The procedure might be somewhat different if you have a slightly different model.

Regarding the handyman, I have no idea what he thought could be back flushed.

  • Your comment "flow restrictor" flipped on a switch in my head. It is an in-line unit. Because my system is 16 years old, maybe it isn't too common anymore. I will get a new one and it should do the trick. I found this just FYI: purewater4u.com/store/flowrestrictors.shtml Thanks for the help!
    – Denzo
    Commented Jan 8, 2017 at 2:50
  • In-line restrictors are very common and they are reasonably cheap. They come in all different flow rates so double check that you are ordering one that is the correct size. If the restrictor has been plugged for a long time then you will also need to replace the RO membrane.
    – user39367
    Commented Jan 8, 2017 at 15:22

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