The Setup

Last June, we bought a 16 year old townhouse with a finished basement; the sump pump was broken at least since March, when we first saw the house, but I suspect it was broken for a much longer time based on the looks of it (it appeared to be rotted to pieces). Soon after we bought the house, my dad and I decided to replace the sump pump ourselves.

The Players

  • my father, with 30+ years of homeownership experience that includes minor plumbing work
  • myself, new to homeownership
  • the rain, which wasn't too strong, but had started about an hour before we did
  • vacancy - the home hadn't been lived in since at least December the year before; it was foreclosed and some remodeling was done to prep it for sale

Some Observations

  • the water level in the pit was the same in March as it was in June; there was plenty of rain in the meantime, but water never came in the basement from what we can tell
  • no water was coming into the pit while it was raining
  • our unit is the second of 4 connected in a row. Our pump ejects to a pipe that merges with the neighbors' pipe. The neighbors have never had flooding
  • our basement had flooded at least twice in the last 15 years

What Happened

  • after we cut the pipe (below the check valve) and removed the sump pump, my father and I took it outside to marvel at its deconstruction--we surmised it could've been broken for a year or more, but we couldn't obtain any confirmation or evidence
  • after we came back downstairs to continue working, we noticed the sump pit filled to nearly the brim
  • about a half hour after we cut the pipe and removed the sump pump, my 3 year old noticed water coming in from the other side of the basement
  • we rushed to finish installing the new sump pump

What I Want to Know

Why did the water start coming in after we cut the pipe connecting the sump pump to the outside world? I briefly talked to a plumber about it, but he didn't have an explanation. Anyone have a similar experience? I'd love to figure this one out! Thanks.

  • 2
    Did any water come out of the pipe you cut? If it's a communal pipe, and you did not have a check valve in place, pumping from any other unit on the pipe would cause water to exit the pipe in your space.
    – Ecnerwal
    Mar 20, 2015 at 14:22
  • Now that the new sump pump is in, does it run regularly? Did water continue to enter the pit after the new pump was installed? Did you do any other work on the property the same day? Are you certain the old pump was broken and not just ugly?
    – Hank
    Mar 20, 2015 at 18:45
  • @HenryJackson, yes, the sump pump has been running regularly--water enters and gets pumped out as needed. No, we didn't do any other work that day or before. Yes, the pump was completely split open and falling apart--it looked like it got hit by a sledge hammer. :) Mar 20, 2015 at 19:14
  • Could have just been chance. I redid the plumbing for my sump pit and added a backup pump a couple years ago, working on it over a couple days. When I started and through the last day, the sump had been completely dry. On the last day, I went back to my basement after a light rain to discover my sump pit had gone from empty to nearly overflowing in the span of a couple hours. Needless to say, I quickly finished things up to get the pump back in service.
    – Bitmapped
    Mar 8, 2023 at 14:40

1 Answer 1


You removed the 1 way check valve, likely part of the pump, when you cut the line to the pump.

Your sump being low, already had a syphon ready to go, it just needed one last push to get started.

The pump of one of your neighbors started and everything in the pipes of all 4 pipes came flowing into your sump.

Your son witnessed an excellent lesson in gravity and syphons. Far more impressive than anything he will be shown even in college physics.

  • Thanks for your quick response--I forgot to mention that we did cut the pipe below the check valve so our neighbor's pump would not flow into our pit. I've added that missing detail to my original post. Mar 20, 2015 at 18:01
  • Damn, I blinked and the underhand weekend league softball became a pro knuckleball. I will clearly have to improve my game.
    – Some Guy
    Mar 20, 2015 at 19:38

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