What a sump pump does and why basements need one
A sump pump removes water that collects in a pit beneath your basement floor and pushes it away from your house. When groundwater, rain, or melting snow seeps through your foundation walls or floor, it flows into a basin called a sump pit. The pump sits in that pit, detects rising water, and automatically turns on to send the water out through a discharge pipe—usually to a storm drain, dry well, or away from your foundation.
Without a sump pump, that water stays in your basement, causing mold, rot, and structural damage. The pump runs only when needed, so it sits idle most of the time and activates during heavy rain or spring thaw.
Key Takeaways
- A sump pump sits in a pit below your basement floor and automatically turns on when water rises to a set level.
- The pump forces water through a discharge pipe that leads away from your house, typically to a storm drain or daylight outlet.
- Most residential sump pumps are either submersible (sitting in the water) or pedestal (standing above it), and submersible models are more common in new basements.
- A float switch or pressure sensor tells the pump when to start and stop, and a check valve prevents water from flowing backward into the pit.
- Sump pumps need a backup power source because they fail during the power outages that often accompany heavy storms.
The sump pit: where water collects before the pump removes it
The sump pit is a hole dug into the lowest point of your basement floor, usually 2 to 3 feet deep and 18 to 24 inches across. Groundwater naturally flows toward low spots, so the pit catches water seeping through your foundation before it spreads across the floor. Some pits are lined with plastic or concrete; others are straightforward dug into the soil and left open.
A perforated drain tile or gravel layer around the pit helps water flow into it from the surrounding soil. The pit sits below the main basement floor, so you typically do not see it—it is covered with a lid or grate that keeps debris out while allowing water to enter.
How the pump motor and impeller force water out
Inside the sump pump housing is an electric motor connected to an impeller—a spinning wheel with curved blades. When the motor turns on, the impeller spins and creates a vacuum that pulls water into the pump from the pit. As the impeller continues to spin, it forces that water outward and upward through the discharge pipe.
The pump must be powerful enough to push water not just horizontally but also upward and away from your house. The height the water must travel (called "head") and the distance to the outlet determine what size pump you need. A typical basement pump moves 2,000 to 3,000 gallons per hour, though larger homes or wet basements may need a stronger model.
Float switches and pressure sensors: what tells the pump to turn on
The pump does not run constantly. Instead, a float switch or pressure sensor detects when water in the pit rises to a certain level and signals the pump to start. A float switch works like a fishing bobber—as water rises, the float rises with it. When it reaches a preset height (usually 8 to 12 inches of water), it triggers a switch that powers on the motor.
Once the pump empties the pit and water drops below that level, the float falls back down and the switch turns the motor off. Some newer pumps use a pressure sensor instead, which detects water weight at the bottom of the pit and activates at a set pressure. Both methods work automatically, so you do not have to turn the pump on manually.
The discharge pipe and check valve: keeping water moving away
Water leaving the pump travels through a discharge pipe, usually made of PVC plastic, that runs from the pump up and out of the basement. The pipe may exit through a basement wall, through the rim joist, or under the floor to an exterior outlet. The goal is to move water at least 4 to 6 feet away from your foundation so it does not seep back in.
A check valve sits in the discharge pipe just above the pump. This one-way valve lets water flow out but prevents it from flowing backward into the pit when the pump shuts off. Without the check valve, water would drain back into the pit as soon as the pump stops, forcing it to run more often and wearing out faster.
The discharge pipe should slope downward slightly and end above ground so you can see water flowing out. If the outlet is buried or submerged, water backs up and the pump works harder than it should.
Submersible versus pedestal pumps: the two main types
Submersible pumps sit inside the water in the sump pit. The motor is sealed and waterproof, so the entire unit is submerged. These pumps are quieter, take up less space, and are easier to hide. Most new basements use submersible models because they are reliable and fit neatly in a small pit.
Pedestal pumps stand above the pit with only the intake pipe reaching down into the water. The motor sits on a tall column above the pit, so it stays dry. Pedestal pumps are cheaper upfront and easier to service, but they are louder and more visible. They also cannot handle as much water as submersible models of the same horsepower.
The choice depends on your pit size, budget, and noise tolerance. If your basement is finished or you spend time there, a submersible pump is usually the better choice. If your basement is unfinished and space is not tight, a pedestal pump works fine.
Why sump pumps fail during storms and what to do about it
Sump pumps run on household electricity. During the heavy rain or snow melt that creates the most water, a power outage often happens at the same time. When the power goes out, your pump stops working—exactly when you need it most. Water then backs up in the pit and floods your basement.
The solution is a backup power source. A battery backup system includes a separate pump or a battery-powered motor that kicks in if the main pump loses power. Some systems use a car battery with an inverter; others use a dedicated sump pump battery. A backup pump should be sized to handle at least half the flow of your main pump and tested monthly to make sure the battery still holds a charge.
If you do not have a backup system and the power goes out, you can use a wet-dry shop vacuum to remove water from the pit manually until power returns. This is not a long-term solution, but it can prevent flooding in an emergency.
Frequently Asked Questions
How often should a sump pump run?
A sump pump should run only when water in the pit rises to the float switch level. During dry weather, it may not run for weeks. During heavy rain or spring thaw, it might run continuously. If your pump runs constantly even when it is not raining, the float switch may be stuck, the check valve may be leaking, or groundwater may be entering faster than the pump can remove it—call a plumber to diagnose the problem.
What does it mean if my sump pump is making a loud noise?
A grinding or rattling sound usually means debris like gravel or sand is inside the pump. Sucking air or gurgling sounds mean the intake is not fully submerged or the check valve is not sealing. A loud humming that does not stop means the motor may be stuck or the float switch is jammed. Turn off the pump and have it inspected before running it again.
Can I use my sump pump to drain a pool or flooded yard?
A sump pump is designed for the small volume of water that seeps into a basement pit, not for draining large bodies of water. Using it for a pool or major flooding will burn out the motor quickly. For large water removal, rent a submersible pump rated for that purpose from a tool rental company.
What happens if the discharge pipe freezes in winter?
If the outlet freezes, water backs up in the pipe and the pump cannot push water out. The pump will run but accomplish nothing, and water may back up into the basement. Insulate the discharge pipe where it exits the house, or run it to a location that does not freeze. If freezing happens, thaw the pipe with warm water or a heat lamp.
Do I need to do anything to maintain my sump pump?
Test your pump monthly by pouring water into the pit until the float switch triggers the pump. Check that water flows out the discharge pipe and that the pump shuts off when the water level drops. Clean the intake screen if debris builds up. If you have a battery backup, test that too and replace the battery every three to five years.