How Does a Sump Pump Work?

Written By Adam Paul – Waterproofing Expert

The sump pump is an electrically operated piece of equipment. A sump pump consists of an electric motor and an impeller. A sump pump consumes electricity using a motor and generates mechanical energy to pump out water in the sump basin using an impeller. This process happened with 7 major steps.

How does a sump pump work in 7 steps?

Water collects in the sump pit
The float rises with the water level
Float activates the pump motor
The impeller spins, pushing water into the discharge pipe
The check valve prevents backflow
Water is discharged away from the foundation
The water level drops, the float lowers, and the pump turns off

In each of these steps, different parts of the sump pump are engaged. Each part of the sump pump has a specific task. So at Aqviz, we’ve installed, repaired, and maintained countless sump pumps, and we know their operation inside and out. So we will explain the process by breaking down every step of a sump pump mechanism.

1. Water Collects in the Sump Pit

The first stage in the sump pump’s operation is water accumulation. Every basement waterproofing system includes a sump pit, and a specially designed hole (usually 18-24 inches deep and 15-18 inches wide) in the lowest part of the basement or crawl space. This pit acts as a reservoir that collects water from drainage systems like French drains, perimeter drain tiles, or natural groundwater seepage.

When heavy rainfall, rising groundwater, or foundation leaks occur, water funnels into the sump pit rather than spreading across the basement floor. The sump pit prevents flooding by concentrating the water in one location, allowing the sump pump to handle it effectively.

2. Float Rises with Water Level

Inside the sump pit there is a float switch is the key component that determines when the pump activates. As water fills the pit, the float mechanism rises along with the water level. This float is similar to the one inside a toilet tank. It moves freely and triggers the sump pump when the water reaches a specific height.

There are different types of float switches:

Vertical float switches (most common) move up and down a rod.
Tethered float switches float on top of the water and tilt when the level rises.
Electronic sensors detect water levels without a physical float.

Once the float reaches its activation level, it closes an electrical circuit, signaling the sump pump to turn on and start pumping water out of the pit.

3. Float Activates the Pump Motor

Once the float rises to the activation level, it completes an electrical circuit and trigger the pump motor to start. This is the moment when the sump pump actually begins its job of removing water from the pit.

Most sump pumps run on 120V AC power. As well as there are battery backup systems and water-powered pumps which are also available for areas prone to power outages. The motor, starts running as soon as the float switch signals the need for operation.

There are two main types of sump pumps at this stage:

Submersible pumps (installed inside the sump pit, placed underwater)
Pedestal pumps (motor is mounted above the pit, keeping it dry)

At this point, the pump is actively preparing to remove water and prevent basement flooding due to accumulated water.

The sump pump floating switch is moving upward and downward according to the water level in side the sump basin.

4. Impeller Spins, Pushing Water into the Discharge Pipe

As the pump motor runs, it powers a spinning impeller, which is the heart of the sump pump’s pumping action. The impeller is a fan-like component that rotates at high speed, creating centrifugal force.

Here’s what happens:

The rapid spinning of the impeller creates low pressure at its center, drawing water in from the sump pit.
This motion pushes water outward through the housing and into the discharge pipe.
The force generated by the impeller ensures water moves efficiently out of the basement.

A properly working impeller ensures smooth water flow, but over time, debris or sediment can clog or damage the impeller blades, reducing efficiency.

5. Check Valve Prevents Backflow

Once the impeller forces water into the discharge pipe, the check valve plays a crucial role in ensuring that the water only moves in one direction out of the sump pit.

A check valve is a one-way valve installed on the discharge pipe, typically 1-2 feet above the pump. It prevents water from flowing back into the sump pit when the pump turns off. Without this valve, the water in the discharge line would flow backward due to gravity, forcing the pump to work harder and cycling on more frequently, which can wear out the motor prematurely.

How the check valve works:

When the pump is on, the force of the moving water opens the valve and allows water to escape.
When the pump shuts off, gravity tries to pull the water back down, but the flap inside the check valve snaps shut, blocking the backflow.

Common check valve issues:

If the valve fails, the pump will keep turning on repeatedly, known as short cycling.
A stuck check valve can prevent water from exiting, causing the pump to run dry.
If you hear a loud “thunk” noise after the pump shuts off, it’s likely the valve closing abruptly. Installing a valve with a soft-close mechanism reduces noise.

sump pump check valve — Sump pump check valve

6. Water is Discharged Away from the Foundation

Once the check valve ensures that water only moves forward, the discharge pipe carries the water away from the home’s foundation.

Where should the sump pump discharge water go?

The sump pump discharged water should be,

At least 10-20 feet away from the foundation to prevent seepage.
Toward a storm drain, dry well, or designated drainage area.
Avoid areas too close to the house, where water can soak into the soil and return to the sump pit.

What are the key components of a Sump pump discharge system?

PVC or corrugated discharge pipe: This pipe directs water out of the basement.
Freeze-proof discharge line: In colder climates, an anti-freeze adapter prevents water from freezing inside the pipe and causing blockages.
Downspout extensions: If the discharge is near the foundation, extensions help move water farther away.

sump pump discharge pipe — Sump pump discharge pipe

7. Water Level Drops, Float Lowers, Pump Turns Off

As the sump pump continues to discharge water, the water level in the sump pit begins to drop. This is the final stage in the sump pump’s operational cycle. Here’s what happens step by step:

Water exits through the discharge pipe, lowering the water level inside the sump pit.As the water level drops, the float switch gradually lowers along with it.
Once the float reaches its deactivation level, it breaks the electrical circuit, turning the pump off automatically.
The check valve prevents any remaining water in the discharge pipe from flowing back into the pit, keeping the system efficient.

At this point, the sump pump remains idle until the next time water fills the pit and raises the float again.

These parts can be varied with the types of sump pumps. If you need to select a sump pump, our guide “5 Types of Sump Pumps and their Efficiency” will help you. This is prepared by the Aqviz expert team with more than 10 years of experience.

Where Does Sump Pump Water Go?

Sump pump water goes 3-6 m (10-20 ft) away from the basement. The best places to discharge sump pump water are downhill grade, storm drainage systems where local code allows, yard pop-up emitters, and dry wells or soakaway pits, all positioned beyond 3 m (10 ft) from the foundation to prevent flooding your neighbor’s yard, which can lead to legal disputes or fines.

Discharging to these locations moves water away from the building envelope, reduces soil saturation around footings, prevents basement dampness, and protects the structural integrity of the foundation walls.

If the water drops near the basement, it can seep back into footing drains, raise hydrostatic pressure on foundation walls, overload the pump motor, and cause frequent on-off cycling that shortens sump pump life.

Why Does a Sump Pump Not Work?

If a sump pump does not work properly, it can be due to these 11 reasons

Power failure: No electricity to the sump pump or tripped circuit breaker.
Stuck or faulty float switch: The pump doesn’t turn on or off properly.
Clogged or blocked sump pit: Clogged debris in the sump pump prevents water intake.
Broken or jammed impeller: The pump runs but doesn’t move water.
Failed check valve: Water flows back into the sump pit, causing short cycling.
Frozen or clogged discharge pipe: Water can’t exit from the sump pump properly.
Undersized sump pump: The pump isn’t powerful enough for the volume of water.
Old or worn-out pump: Mechanical failure in the sump pump due to age.
Improper installation: Incorrect sump pump positioning or missing components.
Excessive water inflow: Heavy rain or high groundwater overwhelms the sump pump.
Lack of maintenance: Dirt buildup, rust, or sump pump component failure due to negligence.

Aqviz tips to Maintain your sump pump properly

As waterproofing experts, you can follow these 5 tips to maintain and run the sump pump properly.

Regularly inspect the sump pit to ensure debris or sediment doesn’t clog the inlet pipes, reducing the system’s efficiency.
If your sump pump frequently turns on and off, the float switch might be positioned incorrectly or stuck. Always check for obstructions and make sure it moves freely.
If your pump motor hums but doesn’t start, it could be due to a stuck impeller or clogged intake screen. Regular maintenance prevents these issues.
If the sump pump runs but doesn’t pump water, the impeller might be broken or jammed. Cleaning and inspecting the impeller every few months can help maintain efficiency.
Ensure your check valve is installed in the right direction. Most have an arrow indicating the water flow direction. Installing it backward will prevent water from leaving the pump.