How to Repurpose a Bilge Pump As a Sump Pump

Sump pumps are prone to failure of two sorts: Power outages and excessive flow. By adding a battery-powered bilge pump as a secondary/emergency sump pump, one can achieve a low-cost solution to both problems.


  1. 1
    Determine whether you are going to use the existing outflow pipe for both pumps or add a second pipe for just the bilge pump. 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.
    1. 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.
    2. 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.
  2. 2
    Know where it needs to be installed. 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.
  3. 3
    Install the bilge pump in the sump. 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.)
  4. 4
    Connect the battery, fuse, control switch (optional), bilge level control switch, and bilge pump with heavy-duty wiring. Make all connections above the water level. Use wire nuts and anti-oxidation compound for long-term reliability. Polarity is important for DC circuits!
  5. 5
    Test the battery-only operation. If the sump pump runs but does not pump, you may have the polarity reversed.
  6. 6
    Attach the charger to the battery and plug in the charger. You should see at least a trickle-charge if not a several-amp charging current. Test the pump again with the charger and battery.
  7. 7
    Final testing should include making sure the original sump pump and the new bilge pump sequence properly and work simultaneously. Also, the bilge pump should work properly with no house power available.
  8. 8
    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).


  • If electrically challenged, there are turn-key systems from Basement Watchdog that provide many of the benefits of the DIY approach. Using the above system allows one to pick items sized to your particular needs but does require a bit of electrical and mechanical/plumbing skill. A comparable (but somewhat "smarter") system to the above will run about $500.
  • Some instructions for this type of installation warn of the possibility of an air lock (air bubble preventing proper pump operation). This does not seem to be a problem for bilge pumps on boats, but if you do experience an air lock in testing or are concerned about the possibility of an air lock, simply drill a 1/8" hole in the outlet pipe between the pump and the check valve and angle the hole down to direct water back into the sump pit.


  • Test! You don't want to have your test be a case where it is vital that everything work. Use a hose or buckets of water to make sure that the bilge pump cuts-in in cases of excess flow (when water levels exceed the capacity of the main pump). Test to see that the bilge pump works properly with no power other than the battery.
  • The system will only work when all the pieces are operating properly. It is a good idea to schedule a check of the system, say, monthly to make sure nobody has tampered. Warning signs stating the purpose and importance of the installation cannot hurt. Enclosing the system with at least a minimal barrier will also help.
  • Even 12 volts can kill in the right (very wrong) situations. Wear rubber gloves and/or make sure everything is very dry when working on the wiring.
  • The check valves are mandatory! Without them, you will simply pump water back into your sump through the (probably) non-functioning pump.

Things You'll Need

  • Boat Bilge Pump. These are not expensive so get a good one. Most will have a capacity greater than your sump pump but are not designed for long-term pumping. For example a Rule 1500GPH pump costs approximately $100 at a boating supply store or on-line. This pump draws 5Amps.
  • Battery connectors suitable for the battery selected.
  • Wire nuts, anti-oxidizing paste, electrical tape, wire strippers, etc.
  • Deep Cycle Battery. Car batteries are not designed for this usage but can be used with reduced service life and pumping capacity. Batteries are rated in Ampere-hours -- hence, a 55 Ampere-hour battery could power the above pump at full volume for 11 hours and costs approximately $200. The advantage of larger batteries is simply to provide for more pumping time if the power is off.
  • You will generally not need continuous operation unless in a flooding situation (and all bets may be off then). 11 hours of intermittent operation (as above) may be more than sufficient for several days-operation even with heavy inflows.
  • Battery Charger. You need one with sufficient output to power the bilge pump without the battery for excess flow situations as well as having a trickle charge mode (virtually all do). A typical charger is a Battery Tender available for about $50. This type of charger has multiple automatic modes for charging and maintaining a battery at full charge for idle periods.
  • Outflow Pipe. You can use an independent pipe for only the bilge pump. This is the simplest approach.
  • PVC T- or Y-Connector (Y is somewhat better for flow) and two check valves if you want to connect into the existing outflow pipe. You will probably need small saw, PVC primer and cement, too. There are two main kinds of check valves available: PVC and rubber. Either will work fine. Boating stores carry flexible outflow pipe for bilge pumps if you choose the separate outflow option -- do not use the corrugated hose (or use a larger size), it will reduce flows up to 30%.
  • Wiring for the bilge pump to the battery. Use wire designed for at least the current requirement of the pump -- 10 gauge or heavier (lower numbers are heavier!).
  • Level control switch. The same stores selling bilge pumps will have switches for turning on the pump automatically. Choose one that fits your needs.
  • Bilge Pump Control Switch (optional). This switch provides for manual, off, and automatic operation of the bilge pump. It makes wiring a bit more complex and opens the opportunity for someone to play with the switch and leave it off or in manual mode.
  • Fuse. An in-line fuse sized for the pump amperage requirement times 2 (i.e., a 10-amp fuse for a 5-amp pump). This allows for start-up current surges while still preventing the burn-out of the pump.

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Categories: Plumbing Drains Waste and Vents | Home Security