What a sump pump does and why you need one

A sump pump is a small electric pump that sits in a pit at the lowest point of your basement or crawl space and automatically removes water that collects there. When groundwater, rain, or melting snow seeps through your foundation walls or floor, it drains into the sump pit instead of pooling in your living space. Once the water reaches a certain level, a float switch triggers the pump to turn on, pushing the water up and out of your house through a discharge pipe that leads away from your foundation.

Without a sump pump, water that enters your basement has nowhere to go except deeper into your foundation, where it can cause structural damage, mold growth, and damage to stored items. The pump works 24/7 as needed, so you don't have to monitor water levels or manually remove water yourself.

Key Takeaways

  • A sump pump activates automatically when water in the pit reaches a set level, triggered by a float switch that rises with the water.
  • The pump pushes water up through a discharge pipe and away from your foundation, usually to a storm drain, dry well, or yard area at least 10 feet from the house.
  • Most sump pumps run on household electricity, so they stop working during a power outage unless you install a battery backup system.
  • A sump pump typically lasts 7 to 10 years and needs annual maintenance, including testing the float switch and cleaning the intake screen.
  • If your pump runs constantly or makes unusual noises, the discharge pipe may be clogged, the float switch may be stuck, or the pump itself may be failing.

The main parts and how they work together

A sump pump system has four essential parts: the pit, the pump itself, the float switch, and the discharge pipe. The pit is a basin dug into your basement floor, usually 2 to 3 feet deep and lined with plastic or concrete. Water naturally drains into this pit from the surrounding soil and foundation.

The pump sits at the bottom of the pit and contains an electric motor connected to an impeller—a spinning wheel with curved blades. When the motor runs, the impeller spins and forces water upward into the discharge pipe. Most residential sump pumps are either submersible (sitting underwater inside the pit) or pedestal (mounted above the pit on a stand). Submersible pumps are quieter and take up less space; pedestal pumps are easier to access for repairs.

The float switch is a small lever or ball attached to the pump that rises and falls with the water level. When water fills the pit and lifts the float to a set height—usually 8 to 12 inches—the switch closes an electrical circuit and turns the pump on. As the pump empties the pit, the float drops back down and turns the pump off. This cycle repeats automatically whenever water enters the pit.

The discharge pipe is a plastic or PVC tube that carries water from the pump outlet away from your house. It runs up through your basement wall or floor and out to daylight, where it empties into a storm drain, a dry well, a rain garden, or straightforward onto the ground at least 10 feet away from your foundation. If the discharge pipe is too short or empties too close to the house, water can seep back into your foundation and defeat the pump's purpose.

How the pump cycle works step by step

The sump pump operates in a straightforward repeating cycle. First, water from rain, groundwater, or melting snow seeps through your foundation and drains into the sump pit. This water may come from a perimeter drain tile system installed around your foundation, or it may straightforward flow downward through the soil.

As water accumulates in the pit, the float switch gradually rises. When the water level reaches the preset height—typically 8 to 12 inches—the float switch triggers the pump to start. The electric motor spins the impeller, which creates pressure and forces the water up and out through the discharge pipe.

Once the pump empties the pit and the water level drops, the float switch falls back to its resting position and cuts power to the motor. The pump stops running. The cycle then waits until enough water collects again to trigger the float switch a second time. During heavy rain or snowmelt, the pump may run continuously or cycle on and off every few minutes.

Why your pump needs electricity and what happens during a power outage

A sump pump requires household electricity to run because the electric motor cannot operate without power. During normal conditions, the pump draws power from a standard 110-volt outlet in your basement. If the power goes out, the pump stops working when ready, even if water is actively entering your pit.

This is a serious problem because heavy rain often coincides with storms that knock out power. Without the pump running, water can quickly fill your basement and cause flooding. To protect against this, many homeowners install a battery backup sump pump or a backup power system. A battery backup pump is a second, smaller pump that runs on a rechargeable battery and activates if the main pump fails or if power is lost. Some systems use a car battery or a dedicated sump pump battery; others connect to a whole-house backup generator.

Battery backup systems vary in cost and capacity. A basic battery backup pump costs between $300 and $800 and can typically run for 6 to 12 hours depending on how often it cycles. A whole-house generator is more expensive but provides power to your entire home. If you live in an area with frequent storms or power outages, a backup system is a worthwhile investment.

Common problems and what causes them

If your sump pump runs constantly without stopping, the most common cause is a clogged or frozen discharge pipe. When the pipe is blocked, water cannot exit the system, so the pump keeps running but never empties the pit. The float switch never drops, and the pump never shuts off. Check the outdoor end of your discharge pipe to make sure it is clear of ice, leaves, or debris. In winter, water freezing inside the pipe can also block flow.

A stuck or corroded float switch is another frequent culprit. If the float cannot rise and fall freely, it may stay in the "on" position even when the pit is empty, causing the pump to run dry. Dry running damages the pump's motor and seals. To test the float, gently lift it by hand—it should move smoothly up and down without resistance.

If your pump makes grinding, squealing, or rattling noises, the impeller may be damaged, the motor bearings may be wearing out, or debris may be caught inside the pump housing. A pump that leaks water around the motor seal or fails to turn on at all has likely reached the end of its lifespan. Most residential sump pumps last 7 to 10 years with regular maintenance.

Annual maintenance to keep your pump working

Sump pump maintenance is straightforward and takes about 30 minutes once a year. Start by testing the float switch. Pour a bucket of water into the pit and watch the pump turn on. If it does not start, the switch may be stuck or the electrical connection may be loose. Check that the power cord is fully plugged in and that the outlet is working by plugging in a lamp.

Next, inspect the intake screen or filter at the bottom of the pump. Sediment, sand, and debris can clog this screen and reduce water flow. If it is dirty, remove it and rinse it clean under running water. While you have the pump accessible, check the discharge pipe for cracks, leaks, or blockages. Make sure the outdoor end of the pipe is clear and pointing away from your foundation.

Finally, test the check valve, which is a one-way valve in the discharge pipe that prevents water from flowing backward into the pit when the pump shuts off. If water drains back into the pit after the pump stops, the check valve is failing and should be replaced. Listen for a gurgling sound in the discharge pipe after the pump shuts off—that is a sign of backflow.

When to call a professional plumber

You can handle basic maintenance yourself, but some problems require a licensed plumber. If the pump does not turn on after you have checked the power cord and outlet, the motor or electrical switch may be faulty. If the pump runs but water still pools in your basement, the discharge pipe may be undersized, improperly sloped, or draining too close to your foundation. A plumber can assess the system and recommend upgrades.

If you notice water leaking from the pump housing, the motor seals are failing and the pump should be replaced. If the pump is more than 10 years old and running constantly, replacement is usually more cost-effective than repair. A plumber can also install a battery backup system, upgrade to a larger pump if your current one cannot keep up with water volume, or relocate the discharge pipe to a better location.

Frequently Asked Questions

How often should a sump pump run?

During dry weather, a sump pump may not run at all for days or weeks. During heavy rain or snowmelt, it may cycle on and off every few minutes or run continuously. If your pump runs constantly even when it is not raining, the discharge pipe is likely clogged or the float switch is stuck.

Can I pour water down the sump pit to test the pump?

Yes. Pour a bucket or two of water into the pit and watch the pump set up. It should turn on within a few seconds and run until the pit is empty. This is a good way to verify the float switch and motor are working. Do this once a year as part of routine maintenance.

What should I do if the sump pump stops working during a storm?

First, check that the power cord is plugged in and the outlet has power. If the outlet is dead, a circuit breaker may have tripped—go to your electrical panel and flip the breaker back on. If the outlet has power but the pump still will not start, the motor or float switch has failed and you need a plumber. In the meantime, use a wet-dry vacuum to remove standing water from your basement.

Is it normal for the sump pump to smell bad?

A slight earthy or musty smell is normal because the pit collects groundwater and organic material. A strong sewage smell may indicate that your sump pit is connected to a septic system or that sewage is backing up into the pit—call a plumber when ready. A burning smell means the motor is overheating and the pump should be shut off and replaced.

Do I need a sump pump if my basement has never flooded?

That depends on your foundation type and local water table. If your basement is below the water table or in an area with poor drainage, water will eventually find its way in. A sump pump is preventive—it stops water before it becomes a visible problem. A plumber or foundation inspector can assess your home's risk and recommend whether a pump is necessary.