Two nights ago my pump was running every 5 minutes due to heavy rainfall. Water was pouring into the pit from the normal ground water sources (not the exit pipe).

My pump and float switch are both 6 months old, I have a bleed off hole in the recommended location. There is ample clearance around my float switch, nothing can conceivably touch it, it can't get stuck on anything.

The pit typically half-fills, then the pump takes it down to a few inches, maybe 2.

I checked on the pump today, (2 days after the on and off night) and the pit is dry. Like still damp, but no standing water in the pit.

How is this possible? Has my switch already failed? It's just been 30 hours since water was pouring in, could I possibly have gotten the pump to kick on with the last ounce of entering water, then had the damp basement air dry the or completely over the past day?

The pump and switch still work, I just tested it.

What should I be concerned with?


As a test, I poured a bucket of water into the pump to add about an inch to the bottom of the sump pit (insert).

After 4 hours all the water was still there.

After 18 hours, the water is gone!

On closer inspection, there is a small hole in the bottom of my pit, just a small triangular cut (only about 0.5" wide by 2" long). So it looks like even though I have what appears to be a normal plastic sump insert, there is, in-fact, a hole in the bottom!

  • Do you have a modern plastic sump barrel or something else?
    – isherwood
    May 22, 2017 at 19:46
  • Yes, modern plastic sump barrell. May 22, 2017 at 20:03

1 Answer 1


You should be concerned with wanting to be concerned about things that are not problems, if you're going to be concerned about anything, here.

Not that difficult - the local-to-your-sump-pit groundwater source level has receded below the floor of the sump pit. The groundwater has moved to refill local groundwater stores and is no longer high enough to enter your pit.

  • My concern is that a plumber is telling me my switch is likely defective and my pump could be on its way. There's no way for the last few inches to drain when operating normally, and it wouldn't have evaporated. May 22, 2017 at 18:30
  • Most sumps are not (even remotely) waterproof/sealed on the bottom - it's a normal source of water entry (in addition to any pipes), and is also a place of water exit when the groundwater level recedes. The sump is typically like a rather short well, and when the groundwater level is below it, there's no water in it.
    – Ecnerwal
    May 22, 2017 at 18:33
  • 3
    I suggest you pour in enough water to cover the bottom of the sump but not to activate the pump, and come back in 20-40 minutes (unless you want to stand there looking at it for 20-40 minutes) to see if it has either gone down or completely vanished, supporting a "sump not sealed" condition.
    – Ecnerwal
    May 22, 2017 at 18:52
  • I've never seen a sump pit that wasn't a sealed poly barrel. Mine will hold 2" of water for months. Where are you proposing the water would leak out?
    – isherwood
    May 22, 2017 at 19:27
  • @isherwood older sump pits in our area (before radon awareness) often have gravel bottoms and will drain when the water table is low. I'd just get buckets of water and/or drag in the garden hose and test til I was convinced it was working properly and make educated guesses where the rest of the water goes from there.
    – Tyson
    May 22, 2017 at 19:44

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