Water collects in a pit, a pump turns on automatically, and pipes carry it away from your house

A sump pump sits in a basin dug into your basement floor and removes water that pools there. When water rises to a set level, a float switch or pressure sensor triggers the pump motor. The pump pushes the water up through a discharge pipe that runs outside your foundation, away from your house. Once the water level drops, the pump shuts off automatically. The whole cycle repeats whenever water accumulates — which in wet basements can happen several times a day.

The system works because water naturally flows downward and collects at the lowest point in your basement. Instead of letting that water sit and damage your foundation, a sump pump intercepts it and moves it to a spot where it cannot seep back in. Most systems run on electricity, though some have battery backup for power outages.

Key Takeaways

  • A sump pump activates when water in the pit reaches a certain height, then pushes that water out through a pipe that extends away from your foundation.
  • The float switch or pressure sensor that triggers the pump turns it off once water level drops, so the pump does not run constantly.
  • A check valve in the discharge pipe prevents water from flowing backward into the pit after the pump stops.
  • Most sump pumps need electricity to run, and a power outage will stop the pump unless you have a battery backup system.

The sump pit and how water gets there

The sump pit is a hole in your basement floor, usually 18 to 24 inches deep and 18 to 30 inches across. It is typically lined with plastic or concrete to keep soil from collapsing into it. Water reaches the pit through foundation cracks, seepage through the basement walls, or drainage systems installed under your floor.

In newer homes, a perimeter drain system runs around the inside or outside of the foundation and directs water toward the pit. In older homes without such a system, water straightforward seeps through the walls and floor and pools at the lowest point — which is where the pit is located. Either way, gravity does the work of getting water into the pit; the pump's job is to get it out.

The pump motor and how it moves water upward

The pump itself is a small electric motor connected to an impeller — a spinning wheel with curved blades. When the motor runs, the impeller spins rapidly and creates pressure that forces water upward into the discharge pipe. The pump can only move water in one direction because of the way the impeller is shaped and because of a check valve installed in the discharge line.

Most basement sump pumps are either submersible or pedestal models. A submersible pump sits inside the pit, submerged in water, and is sealed to prevent electrical hazards. A pedestal pump stands above the pit on a column and draws water up through a pipe. Submersible pumps are quieter and take up less space; pedestal pumps are easier to service and typically last longer because the motor stays dry.

The float switch and automatic on-off cycle

A float switch is a straightforward mechanical device that looks like a ball on an arm. As water level rises in the pit, the float rises with it. When the float reaches a preset height — usually 8 to 12 inches of water — it triggers an electrical switch that turns the pump on. The pump then runs until the water level drops back down, at which point the float descends and the switch turns the pump off.

Some newer pumps use a pressure sensor instead of a float. The sensor detects water pressure at the bottom of the pit and activates the pump when pressure reaches a certain point. Pressure sensors are less likely to jam or stick than floats, but they cost more. Either way, the goal is the same: turn the pump on when water accumulates and turn it off when the pit is mostly empty.

The discharge pipe and check valve

The discharge pipe is typically 1.25 to 2 inches in diameter and carries water from the pump to the outside of your house. It must slope slightly downward as it runs away from the foundation so water does not pool inside the pipe. The pipe usually exits through the basement wall or floor and extends at least 4 to 6 feet away from the foundation — far enough that water cannot seep back toward the house.

A check valve is installed in the discharge line, usually just above the pump. It allows water to flow out but prevents it from flowing backward into the pit when the pump shuts off. Without a check valve, water in the discharge pipe would drain back into the pit by gravity, forcing the pump to run more often and wasting energy. The check valve is a one-way door that closes as soon as the pump stops.

Power supply and what happens during an outage

A standard sump pump plugs into a regular household outlet and draws between 500 and 1,500 watts while running. During heavy rain or snowmelt, the pump may run for hours at a time. If your power goes out, the pump stops when ready, and water will continue to accumulate in the pit. If the outage lasts long enough, water can overflow into your basement.

A battery backup system is a separate pump and battery unit that activates when the main pump cannot keep up or when power fails. Battery backup pumps are smaller and run on a rechargeable battery, usually a deep-cycle marine battery. They can handle the water that accumulates during a power outage, but they will eventually drain if the outage lasts more than a few hours. Some homeowners install a generator as a backup instead, which can power the main pump indefinitely as long as fuel is available.

Maintenance and common problems

A sump pump needs little maintenance, but a few basic checks keep it working. Test the float switch every few months by pouring water into the pit and watching the pump turn on and off. Make sure the discharge pipe is not blocked by ice, debris, or a closed valve. Check that the pump is plugged in and that the outlet has power — many pump failures are straightforward unplugged units or tripped circuit breakers.

The most common failure is a stuck or corroded float switch, which prevents the pump from turning on. The second most common is a clogged discharge pipe, which forces the pump to work harder and wear out faster. If your pump runs constantly even when the pit is empty, the float is likely stuck. If water backs up into the pit, the discharge line is probably blocked. Both problems are fixable without replacing the pump.

Frequently Asked Questions

How often should a sump pump run?

In a dry basement, the pump may run only a few times per month or not at all. In a wet basement, it can run several times per day, especially during rain or snowmelt. If your pump runs constantly, the float switch is likely stuck or the discharge pipe is blocked. Constant running wears out the pump faster and signals a problem that needs attention.

Can a sump pump fail without warning?

Yes. A stuck float, a tripped circuit breaker, or a clogged discharge pipe can stop the pump without any obvious sign. The best protection is a battery backup system and regular testing of the float switch. Test your pump monthly by pouring water into the pit and confirming it turns on and shuts off correctly.

What size sump pump do I need?

Pump size is measured in gallons per hour (GPH). Most basement pumps range from 2,000 to 5,000 GPH. A wetter basement needs a larger pump. If your pit fills faster than the pump can empty it, water will overflow. A plumber or pump installer can measure your water inflow and recommend the right size for your situation.

Does a sump pump add to my electric bill?

A pump that runs a few times per day uses minimal electricity — usually less than $10 per month. A pump that runs constantly because of a stuck float or blocked pipe will cost more. The cost depends on how often it runs, the pump's wattage, and your local electricity rates.

What happens if the power goes out?

Without power, the pump stops and water accumulates in the pit. If the outage lasts several hours during heavy rain, water can overflow into your basement. A battery backup pump or a generator can keep water moving during an outage, but neither will work indefinitely without power or fuel.