Is there any downside to having two check valves in series? I'm installing a new sump pump that came with a new check valve. Several feet from the sump pump, there is an existing check valve on the discharge pipe.

Unless there's a reason not to, my plan is to install the new check valve lower on the discharge pipe leaving the existing check valve on the same discharge pipe.

  • Parallel or in series??
    – JACK
    Apr 18, 2020 at 15:30
  • Think OP said parallel. See my answer below.
    – SteveSh
    Apr 18, 2020 at 15:33
  • @JACK Oops! I meant in a series, not parallel!
    – DIYoutube
    Apr 18, 2020 at 16:25
  • @SteveSh Yes he did and I'm questioning his statement.
    – JACK
    Apr 18, 2020 at 16:42

2 Answers 2


With the two in series you will be protected against one valve being stuck open. I don't think it's necessary and I wouldn't install two in series but since one is already there, no sense in having to change the plumbing by removing the old one.

  • My edumacation on the supply side of plumbing makes me very wary of two in series - though it may be less of an issue with the typical rubber sump pump check valves. Problem (with common supply-side metal check valves) goes something like one slams shut, pressure wave reflected down the pipe, slams the other one, ringing ensues until things finally damp themselves out.
    – Ecnerwal
    Apr 18, 2020 at 17:31

The check valve is there to prevent backflow from the discharge pipe (which is usually vertical as it leaves the sump) from filling or partially filling the sump well back up.

There are two main failure modes for a check valve - stuck closed and stuck open.

If the check valve is stuck closed, then the sump pump will not discharge the water when it runs. Worst case the water will just circulate continuously and the sump pump will run continuously.

If the check valves fails stuck open, then the pump will discharge the water in the sump, but what remains in the vertical portion of the discharge pipe or the downward sloping (back to the sump) part of the discharge pipe will flow back into the sump pit.

So with two check valves in parallel, you protect against the stuck-closed scenario (as it is unlikely that both check valves will fail in the same manner), but are actually more likely to have a stuck-open fault, since if either check valve fails open the discharge water can flow back into the sump well.


OP changed his question to mean two check valves in series, not parallel. So what Jack said.

  • To add some clarity for those not clear about what it means when referencing parallel and series: a parallel install case is two different and separate pipes and valves exiting the pump; a series install case is one pipe with both valves located one after the other and the water must flow through both of them. This answer applies to the parallel case but not the series case.
    – Ack
    Apr 18, 2020 at 16:08
  • 1
    Sorry for the confusion, I'm proposing installing the check valves in a series
    – DIYoutube
    Apr 18, 2020 at 16:28
  • 1
    Thanks for the detailed answer, you gave the right answer, I just asked the wrong question.
    – DIYoutube
    Apr 18, 2020 at 21:17

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