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how deep should the water get in the sump basin before the pump starts?

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I have a submerged sump pump that has some sort of diaphragm switch mechanism that triggers it to start pumping. However, I get very anxious that it doesn't work properly or that the switch has failed, because I look in the basin and the water is maybe 6 - 8 inches deep and has covered the pump body entirely, yet the pump has not been triggered.

I should add that I have already had the switch fail on me once, and only the fact that I was home and basically beat the pump housing with a stick until it started saved my basement from flooding!

  • How deep does the water need to get in the sump basin before the pump should start?
  • Also, I've been told that if I'm worried, I should test the pump by filling the basin with water from a hose. But I wonder if that is a good test of a switch-operated pump? My concern is that water coming out a hose comes into the basin with a lot more turbulence than the gradual trickle of water coming into the basin when it rains. With a switch-type pump, is water turbulence a factor in getting the pump to start?

Because if so, then I'm worried that the pump might "pass" the hose test but fail when I need it during one of the steady 2-day rains we get in my area.

  • When I mention this concern to local plumbers, some of them tell me I should switch to a float pump anyway, while others say the switch mechanism is "better"--what do you think? Is one better than the other? Or is each type of pump better for certain situations and, if so, in what situations is afloat pump better and in what situations is a switch-operated pump better?

As you can probably tell, these questions are causing me a lot of worry! Thanks for any advice/guidance you can provide.

PS - I do have a water-powered backup pump on the system, but as it will pump out water more slowly, I don't want to rely on it because the switch in the main pump failed.

asked in Plumbing by emk (120 points)
edited by darekrudy
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1 Answer

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Hi Emk,
Your first question was: How deep does the water need to get in the sump basin before the pump should start? 
That would depend on the switch design itself. Diaphragm switches are factory pre-set to a pressure that is created by certain volume of water in the basin. This might be 2”-3”, 6”-7” or some different value. You need to check the manual that came with the pump or write down the manufacturer / model number of your sump pump and try to find ON-OFF level range for the switch on the manufacturer’s website. 
Sump pump basin turbulance
If you worry about turbulence you can prevent it by simply placing a small container (ie. a plastic jar, small bucket, etc.) inside the sump pump well and inserting the garden hose into it. Open the valve slowly to allow water fill-up the container and the well by flowing over the container’s rim. 
You can also minimize water turbulence by only partially opening the garden hose valve and pointing the water stream onto the sump pump body so it loses its impact. You always have a full control over the amount of water that comes out of the hose; just turn the valve to a minimum if you want to create a “real life” trickle. 
However, I don’t believe that diaphragm switches are susceptible to turbulence in the tank at all. The only way a stream of water could trigger the switch earlier is if you’d point it into the membrane of the diaphragm switch, this would act like a mechanical push.
Which switch is best? Sump pump diaphragm switch, Float switch, or electronic probe?
There are a couple of diaphragm switch components that might go wrong. 
  • The membrane which usually gets harder over time and doesn’t respond to water pressure as it supposed to
  • The air tube that sometimes gets clogged or partially clogged which prevents proper response 

How soon is that going to happen... unpredictable, the best bet is to stay within the warranty period and replace the switch.

Floating (tethered and vertical action) switches work best if there’s sufficient room in the well and if there’s no debris floating around (turbulance might affect those switches). These too fail, simply because they are mechanical devices and have moving parts inside them. Sump pump floating switches are fairly inexpensive so replacing one as soon as the warranty expires should keep the pump running properly.
Another group of switches called “flood free” has no movable parts and it’s activated by a probe that you can set at a desired level inside the sump pump well (you can simply attach it to the discharge pipe). As soon as the rising water touches the probe, it activates electrical outlet and activates the sump pump plugged into it. 
No mechanical parts, just a probe and a circuit board, which can also fail at some point… unfortunately there’s no perfect solution for anything (this particular switch is claimed to be perfect smiley - levelguardproducts.com/sump/.
Things will  eventually fail at some point of their life cycle, sometimes when they are needed most. 
answered by darekrudy (21,730 points)
This is an EXTREMELY helpful answer - thanks so much! Based on what you wrote, I contacted the plumber who put in my pump to get its model number (it's a Hydromatic D-A1), downloaded the manual for it, and found out that the pump was set to come on when there was about 10 inches of water in the basin--quite a bit higher than I thought it had to get to trigger the pump, so that made me feel much calmer about why it hadn't come during our last rain event. Also, your helpful suggestions on how to lessen water turbulence to better test the unit will really come in handy. I am going to talk to my plumber about the warranty period for the diaphragm switch, so I can plan on replacing it when the warranty is up. Finally, I read in the manual about an audible alarm called the Hydromatic Q-Alert, so I'll be checking into adding one to my system to give me more peace of mind. Thanks again for all of your help!

You are welcome Emk. Those alarms are inexpensive and very handy for as long as somebody is at home to take action as soon as one goes off.

I don't have a basement in my home but in case I had a finished one I'd add a battery backup sump pump (in addition to your water powered pump) to protect it from flooding... just in case. I get paranoid myslef over some things as well smiley