There are arguments for both approaches -- two pipes are better than one in terms of flow capacity, but adding a second pipe may be a more-difficult plumbing job.

If using the existing pipe, you will need to cut it and install a check valve and T-connector. The check valve goes below the T-connector between the T and the existing sump pump. Make sure the flow direction for the check valve is correct! A second check valve will be installed between the side of the T-connector and the new bilge pump.
If installing a new bilge pump outflow pipe, run that to the same place that your current sump outflow exits. For such a long run, you should use electrical ties to keep the pipe from sagging when full of water.;
, The bilge pump secondary level control switch needs to be installed a few inches above the existing sump level control switch. There are many possibilities for mounting this switch. The main consideration is that the switch must be mounted so that nothing interferes with its operation and that it is solidly mounted., This will entail finishing the plumbing by either attaching the bilge pump to the anti-backflow valve or to the independent outflow pipe. (No backflow valve is required for the independent pipe approach.)
, Make all connections above the water level. Use wire nuts and anti-oxidation compound for long-term reliability. Polarity is important for DC circuits!
, If the sump pump runs but does not pump, you may have the polarity reversed., You should see at least a trickle-charge if not a several-amp charging current. Test the pump again with the charger and battery., Also, the bilge pump should work properly with no house power available.,
Determine whether you are going to use the existing outflow pipe for both pumps or add a second pipe for just the bilge pump.
Know where it needs to be installed.
Install the bilge pump in the sump.
Connect the battery, fuse, control switch (optional), bilge level control switch, and bilge pump with heavy-duty wiring.
Test the battery-only operation.
Attach the charger to the battery and plug in the charger.
Final testing should include making sure the original sump pump and the new bilge pump sequence properly and work simultaneously.Make sure all connections are taped and all wires are tied or taped and you should be done.Make sure the charger is set to automatic mode (it will maintain the battery normally but switch to charge mode if there is a draw on the battery or it is discharged after an outage).
{"src": "wikihow-all_validation", "title": "How to Repurpose a Bilge Pump As a Sump Pump"}