What It Is
A backup sump pump is a secondary pump installed in the sump pit to take over when the primary pump fails or is overwhelmed. Two main types exist: battery-powered DC pumps that run independently of grid power, and water-powered pumps that use municipal water pressure to create suction and move pit water out. Each type has different strengths and installation requirements.
What It Does
A backup sump pump monitors water levels in the pit independently of the primary pump. Its float switch is set to activate at a water level higher than the primary's float, so it only runs when the primary isn't keeping up. This could happen because the primary pump motor burned out, the primary float switch failed, the circuit breaker tripped, or the incoming water volume simply exceeds the primary pump's capacity during a major storm.
The backup pump shares the same discharge line as the primary (or uses a separate one), routing water away from the foundation and out through a discharge point away from the house.
Where It Is Located
The backup pump sits in the sump pit alongside or above the primary pump. Water-powered backup units often mount higher in the pit. The backup's float switch hangs slightly higher than the primary's to maintain the correct activation order. Battery units have their control module and battery mounted outside the pit on a nearby shelf or wall bracket.
How It Works
Battery-Powered Backup
Uses a 12-volt deep-cycle battery to power a DC pump motor. When grid power fails or the primary is overwhelmed, the backup unit senses the rising water level and activates. Battery capacity determines how long it can run - typically several hours of continuous pumping on a full charge. The battery charges continuously when AC power is present.
Water-Powered Backup
Uses the venturi effect - a constriction in the water line creates a pressure drop that draws pit water up and out through the discharge line along with the municipal supply water. No electricity is needed. The trade-off: it uses municipal water to pump pit water, which increases water usage and requires adequate municipal pressure. Not suitable for homes on well systems.
Common Problems or Signs of Failure
- Battery backup fails to activate during a test - discharged or failed battery
- Backup runs constantly in normal conditions - primary pump may have failed and backup is doing all the work
- Water-powered backup uses excessive water - check valve or solenoid issue
- Alarm sounds but no water in pit - float switch may be stuck in the activated position
- Backup pump hums but doesn't move water - clogged intake screen or failed impeller
Can a Homeowner Check It?
- Battery backup: check the charger status indicator light for a "charged" signal
- Pour water into the pit to confirm the backup activates after the primary stops (or when you manually hold the primary float down)
- Check the battery age - most need replacement every 3-5 years
- Water-powered unit: verify municipal water supply is open and the inlet screen is clear
- Check that both pumps share or have separate unobstructed discharge lines to the outside
When to Call a Plumber
Call a plumber if the backup pump is running as the primary pump regularly - that means the primary has failed and needs replacement. Also call if you want to switch types (battery to water-powered or vice versa), or if the discharge lines need to be reconfigured for two pumps. Sizing a backup for your specific pit volume and groundwater load is worth getting right.
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