Space limitations on back up sump pump.


I finally realize that I need a battery powered back up sump pump, and finally have time to install it.
I want to use 2 check valves and then combine the output into the existing output pipe.
1) I'm not sure I will be able to do all the piping within the sump: The sump came with a perfect plastic plug that has room for the pedestal pump, the float rod, and the output pipe, but no room for anything more, no room for another pipe, and the plug is such that the notch in it can't be enlarged or the plug will fail, collapse or something. So I want to do the connections inside the sump. And have only one output pipe like it is now.
The sump is 17 inches in diameter and there is room on the floor of the sump for both the old pump and the new one.
The sump is 18 inches deep. Is that likely to be enough?
2) Is it a mistake to do it this way? Should I put a second output pipe in parallel with the current one, at least until I get out of the house? Once in 27 years there was so much water coming into the sump that my 1/2? HP pedestal pump wasn't enough to expel it all, and the sump overflowed.
The battery operated pump will run at the same time the AC pump runs, but maybe the standard diameter output pipe will restrict the output so that the total won't be much more than the one pump does now. Should I put a second output pipe in parallel with the current one?
Thanks.
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wrote:

The standard method is to join both output pipes with a wye a couple of feet above the pumps. You need a separate check valve for each pump above the pump and below the wye. Remember to drill a small (1/8) hole below each check valve to prevent air-lock.
You may be able to do this in 18" but it will be tight. Probably easier to drill another hole in the sump cover for the extra pipe, and then wye them together above the cover. (You will have to slit the cover, or be willing to dissassemble the piping at the check valves to get the cover off, though.)
You generally don't want both pumps to run together all the time. Set the switch for the backup so it comes on a few inches above the on point of he main pump. Then the backup will only run if the main fails or can't keep up with the flow.
The water carrying capacity of the output line depends on many factors, lift, pump pressure, how long the run is, etc. If it's cheap and easy to run a second line it's cheap insurance, but it wouldn't typically be necessary. But a single 1.5 inch line probably won't be able to handle the full rated capacity of both pumps running at the same time, so if you really think both pumps will run a lot of time, then you use double lines. But that is a *lot* of water.
HTH,
Paul
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If you have city water not well water a better option is a water powered pump www.basepump.com or www.zoeler.com water powered will outperform a battery unit in most every way, there is no comparison on an extended power failure, water powered wins. I would not use one discharge use 2. Your present pump may be restricted and get improved output by raising it a foot to reduce head, and increasing output pipe size. If your sump lid wont work make one , or cut new holes or buy one you can cut and modify. Remember batteries last 5-7 years, will you remember to test and replace them before they go bad, to test a battery properly you need to do a load test, something few homeowners can do as the test equipment is expensive.
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