Backstory. I own a home that has a driveway that slopes towards the garage. The garage has a grill in front of it to catch all the excess water drainage and dump it outside the house, I presume to main drainage on the street. The garage grill drain is on the same plane as the sump pump pit.

The crux of the problem is that if there is really heavy rain then the sump pit will fill up above the inlets even with the sump pump running and cause the garage grill drain to begin accumulating water, so much so that the water flows into the garage causing flooding.

Right now I have installed a 3/4 hp sump pump - which ensures that the sump pit doesn't overflow but unfortunately can't keep up and the water fills above the inlets. I think its only a matter of time.

So long story short, I'd like to add another sump pump in 'parallel' to push water out.

Currently this is my solution:

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My question is two fold. Should the 1hp pump be at the bottom or the top? I will be keeping the 3/4hp and would like to utilize it.

Secondly, when both pumps are activated, will there be an issue with flow? Will the PVC explode?

  • I went the cheapo method - I put 2 identical pumps in my basement sump pit, one on a brick so it was slightly higher than the other. I ran them into a Y before heading out through the foundation wall. No check valves, no fancy stand. I believe they're both 1/2 HP and I think it's 1.5" pipe. I used to have the control board for my furnace short out annually at least due to water (root cellar, not a finished basement), but haven't had anything beyond a lightly damp floor since installing the 2nd pump. Oh, and no exploding PVC.
    – FreeMan
    Jul 21, 2020 at 11:19

2 Answers 2


I did something similar using two pumps with 2” copper pipes.

Pumps and connections were designed to run in series and individually.

What is needed is non-return valves for each pump so they cannot backfeed.

  • Having the pumps at the same intake level (if they will fit) and using a proper pump controller like this (which are available) that normally alternates operation of each pump individually, and fires them both up if one cannot keep up (or the one "up first" has failed) is far better in the long run of pump life than having one that ONLY runs (possibly quite rarely) when the first is overwhelmed.
    – Ecnerwal
    Jul 21, 2020 at 16:05
  • @Ecnerwal does depend on the type of pump though - a piston type will not allow any reverse flow while a centrifugal one will. I had to use two pumps in series to overcome an initial static pressure head due to the wavy roof causing airlocks in the pipes on a 60m^2 set of solar thermal panels. Re-hanging the pipes was not an option. One the flow was started then only one pump was needed to continue the flow. It was also a drainback system. So every start needed two pumps and that could be once per day or several times per day depending on the panel temperature.
    – Solar Mike
    Jul 21, 2020 at 16:09

The PVC won't explode, but the performance of two pumps dumping into one (possibly already undersized, just based on many sump installations I've seen where pipe size was cheaped out on) pipe the same size will be much lower than individual pipes, or a pipe of increased diameter after the join.

Examine your pump(s) - if there is any reducer on the outflow piping, you might want to start with simply increasing the pipe attached to the maximum diameter that the pump is natively designed to connect. If you propose to join two discharges together, you should be increasing the discharge size to compensate for that (as well as having a check valve on each individual pipe so that one does not pump backwards though the other.)

If joining two 2" pipes, you need a 3" pipe to take the full flow of both (pipe capacity is proportional to the square of the diameter.) Likewise, using a 1.5" pipe when your pump will take a 2" pipe is not 75% capacity, it's 56%

  • Some times one is not in a position to be able to replace pipes... and have to accept a less than perfect solution.
    – Solar Mike
    Jul 21, 2020 at 16:11
  • Then again, sometimes all you need is new pipes, not a second pump.
    – Ecnerwal
    Jul 21, 2020 at 22:29

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