What you need to know about Check Valves and sump pumps.
Check valves are one-directional flaps that keep water from flowing back into your sump pit.
The Basement Defender™ battery backup pump comes with a check valve already installed in the assembly.
The Basement Defender™ backup system requires a sump pump in line check valve, included in the kit, mounted on the outlet on the base of the AC sump pump, so when the backup pump turns on, it will not send water back into the sump pit through the AC pump.
If a second check valve is mounted above the backup pump, it could cut its capacity in half because the backup pump now has to push water through two check valves. Another thing it can do is recirculate water in the pit, running continuously until your sump pump burns out.
A simple rubber coupling should be used to connect the assembly with the discharge line.