A sump pump removes water from your basement by sensing it, turning on automatically, and pushing it out through a pipe
A sump pump is a small electric pump that sits in a pit dug into your basement floor. When water collects in that pit — from groundwater seeping through the foundation, heavy rain, or a burst pipe — the pump detects the rising water level, switches on, and forces the water up and out of your house through a discharge pipe that leads away from the foundation. When the water level drops, the pump shuts off. The whole cycle repeats as needed, usually several times during heavy rain or spring thaw.
Most sump pumps run on standard household electricity. They are not complicated machines — they have a motor, an impeller (a spinning part that moves water), and a float switch or pressure sensor that tells the pump when to turn on and off. The pump itself is submerged in the pit, so it can pull water directly from the bottom without needing to be primed.
Key Takeaways
- A sump pump sits in a pit below your basement floor and turns on when water rises to a set level, then pushes that water out through a discharge pipe.
- The pump uses a float switch or pressure sensor to detect water and trigger automatically — you do not have to turn it on by hand.
- Water is forced upward and outward through the discharge pipe, which should empty at least 10 feet away from your foundation to prevent the water from seeping back in.
- Most sump pumps run on standard household power, but a backup battery or generator is useful because heavy rain often causes power outages.
The pit: where water collects before the pump moves it
The sump pit is a hole in your basement floor, usually 18 to 24 inches deep and 18 to 30 inches across. It is lined with plastic or concrete and sits below the level of the surrounding basement floor. Water naturally flows downward and collects in this pit instead of spreading across your basement.
The pit has a cover or grate on top to keep debris, dirt, and small objects from falling in and clogging the pump. The cover also reduces odor and prevents someone from tripping into the pit. Water still seeps in from below and around the edges, so the pit fills slowly even when it is not raining.
The float switch: how the pump knows when to turn on
A float switch is a straightforward mechanical device that looks like a small ball or cup attached to an arm. As water rises in the pit, the float rises with it. When the water reaches a set height — usually 12 to 18 inches deep — the float arm tilts and triggers an electrical switch that powers on the pump motor.
Some newer pumps use a pressure sensor instead of a float. This sensor detects the weight of water pressing down on it and sends a signal to turn the pump on at a certain depth. Pressure sensors are less likely to jam with debris, but they cost more than float switches.
Once the pump starts running and the water level drops, the float sinks back down, the switch flips off, and the pump stops. This cycle can happen many times per day during heavy rain or spring thaw.
The motor and impeller: the parts that actually move water
The pump motor is an electric motor powered by a standard 110-volt or 220-volt household outlet. When the switch turns on, electricity flows to the motor, which spins a shaft connected to an impeller — a fan-like wheel with curved blades.
The impeller sits inside a sealed chamber at the bottom of the pump. As it spins, the blades push water outward and upward, creating pressure that forces water up the discharge pipe. The faster the impeller spins, the more water it can move per minute. Most residential sump pumps move between 2,000 and 5,000 gallons per hour, depending on the model and how much pressure is needed to push water uphill and away from the house.
The discharge pipe: where water goes after the pump moves it
The discharge pipe is a rigid or flexible tube, usually 1.25 to 2 inches in diameter, that connects to the outlet of the pump and carries water away from your basement. The pipe runs upward and out of the house, either through the basement wall, through the rim joist, or up and out through the floor.
The far end of the discharge pipe should empty at least 10 feet away from your foundation, preferably 20 feet or more. If the pipe empties too close to the house, the water soaks back into the ground around the foundation and returns to the sump pit, defeating the purpose of the pump. Some discharge pipes are buried underground; others run above ground. Both work, but buried pipes are less visible and less likely to freeze in winter.
Many discharge pipes have a check valve installed near the pump outlet. This one-way valve lets water flow out but prevents it from flowing back into the pit when the pump shuts off. Without a check valve, water would drain backward and refill the pit, forcing the pump to run more often and wear out faster.
Power and backup: what happens when the electricity goes out
A sump pump needs electricity to run, and heavy rain — the time you need it most — often causes power outages. If your pump loses power during a storm, water will collect in the pit and may overflow into your basement.
A battery backup pump is a second, smaller pump powered by a rechargeable battery. It sits in the same pit as the main pump and turns on automatically if the main pump loses power or cannot keep up with the water flow. Battery backup pumps typically run for 24 to 48 hours on a full charge, depending on how often they cycle. They are recharged by a charger plugged into a standard outlet.
A generator is another option. A portable generator can power your main pump if the electricity goes out, but you have to be home to start it and monitor it. A whole-house generator starts automatically when power fails, but it is expensive to install.
Maintenance: keeping your pump working when you need it
A sump pump has few moving parts and requires little maintenance, but neglect can cause it to fail. Check the pit once or twice a year to make sure it is not clogged with dirt, sand, or gravel. If debris builds up, the float switch may stick or the pump intake may become blocked.
Test the pump by pouring water into the pit until the float rises and the pump turns on. If the pump does not start, the float switch may be jammed, the outlet may be blocked, or the motor may have failed. If the pump runs but water does not flow out, the discharge pipe is likely clogged or the check valve is stuck.
Listen for unusual noises — grinding, squealing, or rattling — which can signal a failing bearing or a loose part. If the pump runs constantly even when the pit is not filling, the check valve may be leaking and allowing water to drain back into the pit.
Frequently Asked Questions
Why does my sump pump run so often?
Frequent cycling usually means water is entering the pit faster than normal, which happens during heavy rain or spring thaw. It can also mean the check valve is leaking and allowing water to drain back into the pit after the pump shuts off, forcing it to restart. A leaking foundation or a high water table can also cause constant refilling.
Can a sump pump fail without warning?
Yes. The float switch can jam with debris, the motor can burn out, or the discharge pipe can become blocked. This is why testing the pump once or twice a year is important — pour water into the pit and confirm the pump turns on and water flows out. A battery backup pump protects you if the main pump fails during a storm.
What size sump pump do I need?
Pump size is measured in gallons per hour (GPH). Most residential basements need a pump rated between 2,000 and 5,000 GPH. A larger basement, a high water table, or heavy clay soil that holds water may require a larger pump. A plumber or pump installer can assess your basement and recommend the right size.
Does a sump pump use a lot of electricity?
No. A typical sump pump uses 300 to 800 watts while running, similar to a microwave. Since it only runs when water is present — often just a few hours per month — the annual electricity cost is usually between $10 and $30. A battery backup pump adds minimal cost because it only runs when the main pump loses power.
What should I do if water is backing up into my basement despite the sump pump?
First, confirm the pump is actually running when water is present. Pour water into the pit and listen for the motor. If the pump runs but water still backs up, the discharge pipe may be clogged, the check valve may be stuck, or the pump may be undersized for the amount of water entering. Call a plumber to inspect the discharge line and test the pump's output.