We have the float for the water backup located above the normal high water level set by the electric sump, so unless the electric fails the water powered float is resting in its low or closed position. The float mechanism connects to what looks like a diaphragm style valve via a 1/8" tube, the diaphragm valve is about 18" above the floor so not submerged. I'm not sure how that 1/8" tube actually triggers the valve ( water, vacuum)?
Your system sounds different from what we had. So I cannot really comment on how it works.
What I can say is that my system failed because of crust like calcium deposits formed from the normal high calcium in the ground water that drains into the sump pump. In my case, that water comes from the gravel foundation inside and outside of the basement wall concrete footings. It is standard construction practice. But some homes may use crushed granite or beach stone which would not produce the calcium like limestone gravel does.
The crust deposits did two things. It froze up the valve, and it choked up the venturi nozzle.
Need to keep an eye out for 12V pump, what size battery do you use?
I use a regular marine deep cycle battery sitting in a plastic box on a wood platform near the sump-pump. NEVER put a battery directly onto a concrete floor. It's probably 500 to 600 CCA and 50 to 75 AHrs. It's just a standard car size - nothing special. Even the deep cycle rating isn't that important because it isn't used often enough to be damaged by the deep draw.
You can buy pumps that are designed to use in a 12V backup sump pump system. But given how horrible my last flood was, I went into hyper defense mode and did everything overkill. Hence the large expensive high lift marine bilge pump.
I also didn't plumb the backup system output into the regular sump output either because I didn't trust a check valve. Instead, it goes into a 2" abs drain pipe of its own that just goes out onto the lawn where there is a downhill slope away from the house. I didn't worry about erosion because it hardly ever gets used and what is a little lawn repair?
I phased my system similar to the way you did with a few extra elements. I decided to use the subfloor gravel as an accumulator. So my electrical switch is located about a foot above the regular sump pump float switch. That way, the backup pump doesn't come on until after the water fills the pump hole and the gravel under the floor. Even in a heavy downpour it takes several hours to fill the gravel.
My alarm levels are also 2 phased. The first is a simple alert that the gravel is starting to fill, and the other is an alarm set to go off if the water goes above the backup trigger point.
Our electric sump does not seem to operate very often.
Sounds like you are lucky and your location is not prone to flooding. This might be a very good thing for your water powered backup system. I'd still check it regularly though. At our previous home (the one that flooded) I would actually wake up in the middle of the night if the pump
DIDN'T come on regularly! My wife used to say that it proved I was either wired wrong from birth or dropped on my head as a baby.
Where we are now, the pump works fairly often in the spring and goes on vacation all summer and fall. Winters are quite variable.
I think you would be well advised to figure out exactly how your system works, determine its weakness, and address them as appropriate. A flooded basement totally completely sucks!
One more thing. We had both storm and sanitary sewers at our last house. They can fail too. I broke up the concrete and installed big serviceable check valves in a valve pit. I checked and cleaned them once a year. I highly recommend that to anyone who has sewers.
