Quick answer
A sump pump is an electric pump that sits in a pit, called a sump, in the lowest part of a basement or crawlspace and pumps out groundwater that drains into the pit before it can rise onto the floor.
A float switch turns it on as the water rises. The water goes out a discharge pipe to the yard or a storm drain, not into the sanitary sewer.
Quick facts
The key facts about Sump Pump, with sources:
Key takeaways
- A sump pump removes groundwater that collects in a basement pit, often fed by drain tile around the footings.
- A float switch starts the pump as water rises, and a check valve keeps pumped water from falling back.
- Water should go outside and away from the house, never into a floor drain or the sanitary sewer.
- Test it by pouring water into the pit until it cycles at least twice, and do it before the wet season.
- Power cuts and overloads are the usual failure points, which is why FEMA points to a battery backup pump.
Sump pump meaning: the last step in basement drainage
A sump pump is the last step in a basement drainage system.
NDSU Extension describes the sump as a pit that may be connected to drain tile around the footings, under the whole basement, or only near the pit.
The water that drains into the sump has to be removed, and the pump does that 1.
The word “sump” just means a low pit where liquid collects, and a sump pump handles only clear groundwater. A pump that lifts toilet and sink waste is a different machine, the sewage ejector pump, and the two are not interchangeable.
FEMA lists the sump among the four ways water gets into basements, along with sewer backups, cracks, and overland flooding through windows and doors 2. A working sump pump deals with the first; it does not stop the others.
- Drain tileCollects water at the footings and feeds the pit
- Sump pit and linerAbout 2 feet across; plastic liner over gravel
- Float switchTurns the pump on and off
- Check valveStops pumped water falling back
- Discharge pipeCarries water outside, away from the house
- GFCI outletPowers the pump near water
Sump pump vs sewage ejector
A sump pump moves groundwater to the yard or storm drain. A sewage ejector moves waste from a basement bathroom up to the sewer. Mixing them up breaks code and can spread sewage.
How a sump pump works, from rising water to shutoff
- Water collects: Groundwater from drain tile or the soil runs into the pit. FEMA notes heavy rain saturates soil and pushes groundwater in through the sump 2.
- The float rises: A ball float or a sealed float switch rises with the water and turns the pump on 1.
- The pump lifts water: The impeller pushes water up the discharge pipe and out of the house.
- The check valve closes: When the pump stops, the check valve keeps the water in the pipe from flowing back into the pit. NDSU notes that without it, the pump cycles more often and wears out sooner 1.
- The float drops: The pump shuts off and waits for the next rise.
A small hole is often drilled in the discharge pipe between the pump and the check valve; Zoeller’s instructions call for a 3/16-inch hole level with the top of the pump 4. It lets trapped air escape so the pump does not run dry.
Types of sump pumps: pedestal, submersible and backups
| Type | How it differs | Good to know |
|---|---|---|
| Pedestal | Motor on a column above the pit; pump at the bottom 1 | The switch and float are easy to see; the motor must stay dry 1 |
| Submersible | Whole pump sits in the water at the bottom of the pit 1 | Switch is a float on a pressure switch or a sealed float switch; NDSU calls the sealed switch generally more reliable 1 |
| Battery backup | A second pump on a battery that starts when water reaches its float 2 | FEMA calls it an inexpensive safeguard for power cuts and overloads 2 |
| Water-powered backup | Uses city water pressure to move pit water | Needs municipal water pressure; see that page |
| Floor “skimmer” pump | Small portable pump that works in 1/4 to 1/2 inch of water 1 | For homes with no sump; often used with a garden hose 1 |
Example: checking a sump setup against published numbers
This example compares one typical setup with NDSU Extension and Zoeller figures. It is a checklist, not a design for your home.
| Item | Published figure | What to look for |
|---|---|---|
| Pump size | 1/3 hp suits most houses 1 | A 1/2 hp pump moves more water and lifts higher, which helps only where inflow can be fast 1 |
| Pit size | About 2 feet across, holding 15–25 gallons between cycles 1 | A small pit makes the pump start and stop more often |
| Pit base | 3–4 inches of coarse gravel under the liner 1 | Helps keep mud and debris out of the pump |
| Minimum basin (one maker) | At least 10 in. wide and 18 in. deep for Zoeller model 1099 4 | Check your own pump’s manual |
| Discharge pipe | Rigid 1-1/2 in. pipe, union and check valve 4 | A union lets the pump come out without cutting pipe |
| Discharge point | At least 20 feet from the house, draining away 1 | Not toward a neighbor, a window well or a septic drainfield 1 |
- 1/3 hpPump size that suits most housesNDSU Extension
- 2 ftTypical sump pit diameterStores about 15–25 gallons
- 20 ftMinimum distance to discharge from the houseDraining away from the house
- 2 cyclesTimes to run the pump during a testPour about 5 gallons slowly
Source: [1]
The pump’s box or manual shows a chart of flow against height of lift, in gallons per minute or per hour (multiply GPM by 60 for GPH) 1. Read your lift height off that chart, not just the horsepower.
Where sump pumps show up on inspections, quotes and city notices
On a home inspection report, the sump pump usually gets a line for whether it runs, whether there is a check valve, where it discharges, and whether the outlet is GFCI protected. A note like “discharge connected to sanitary” is a red flag; in many cities it breaks the code 3.
On a quote, look for the pump model and horsepower, the switch type, the pit liner and cover, the check valve, the discharge route, and any backup pump or alarm.
On a city utility bill or notice, you may see “inflow and infiltration” programs that inspect sump connections. Lincoln, Nebraska says one sump pump running 10 minutes an hour into the sanitary sewer equals the normal sewage of 12 homes 3.
What a sump pump does well, and what it cannot do
What it does well
What it cannot do
- Stops when the power goes out, unless there is a backup 2.
- Can be overwhelmed by torrential rain beyond its capacity 2.
- Does nothing about sewer backups through floor drains; that needs a backwater valve 2.
- A frozen or plugged outdoor pipe can make it overheat 1.
- Does not fix wall cracks or overland flooding 2.
Limits: power, capacity and the discharge line
FEMA names three ways a basement floods even with a sump: the primary pump stops working, the power goes out, or torrential rain overloads the pump 2. Each needs a different answer: a spare or backup pump, backup power, or a larger pump and better drainage.
The discharge line is the weak spot people forget. NDSU says to check first that the outlet pipe is not frozen shut or plugged. Most pumps will not burn up if it is, but they can overheat and must be shut off to cool 1.
Sizing up is not always the fix. NDSU notes that where water flows in slowly, a larger pump gives no advantage; it helps only where inflow can become rapid 1.
What drives the cost of a sump pump
The pit, the discharge line and any backup set most of the cost of sump pump work. We do not publish sump pump prices, because fair numbers need dated local quotes.
A pump swap in an existing pit is the simplest job. Costs rise when a new pit must be dug through the slab, when drain tile is added, when a new discharge line must run outside, when an outlet needs adding, or when a backup pump, battery or alarm is included.
The pump type (pedestal or submersible, plastic or cast iron) and horsepower also move the price.
If a sump is tied into the sanitary sewer, redirecting it is part of the cost, and some cities help pay for that work. Our sump pumps and basement water service lists what a visit covers, and the plumbing cost guide explains how quotes are built.
How a sump pump is installed or replaced
These are the steps in Zoeller’s quick guide and NDSU’s answers, in order 41.
- Unplug first: Zoeller’s first warning is to disconnect the pump from power before handling it 4.
- Prepare the pit: Set a liner over 3–4 inches of coarse gravel; the pit should be about 2 feet across 1.
- Set the pump and pipe: Place the pump on a firm base and fit rigid discharge pipe, a union and a check valve 4.
- Drill the vent hole: A 3/16-inch hole in the discharge pipe, level with the top of the pump, per Zoeller 4.
- Run the discharge outside: Aim at least 20 feet from the house, draining away, and not onto a neighbor’s lot or a septic field 1.
- Plug into a GFCI outlet: Zoeller calls for a GFCI-protected receptacle 4.
- Test and cover: Fill the pit with water to check it cycles, then fit a sealed cover 4.
Water and electricity
A sump pump sits in water, so unplug it before touching it 4 and use a GFCI outlet 1. If the basement is flooded, do not walk into standing water while the power is on; stay out and call your utility or an electrician to shut power off.
A like-for-like swap is often a homeowner job, per NDSU 1. New pits, new outlets, new exterior lines and any sewer disconnection usually need a pro, and may need a permit, so check with your local building department.
How to test and maintain a sump pump
NDSU Extension gives this test, best done before the wet season 1:
- Make sure the outdoor discharge pipe is not frozen or plugged and points away from the house.
- Confirm the pump is plugged in.
- Remove the lid and check with a flashlight that the pit is clean and the pump inlet is clear.
- Slowly pour about 5 gallons of water into the pit, about as fast as water normally flows in.
- Watch the float and listen to the pump. Make it turn on and off at least twice.
- Clean the inlet screen if it is clogged; sump pumps have screens, not filters 1.
Watch how often it runs
Lincoln, Nebraska suggests keeping an eye on how often your pump cycles; frequent cycling can point to a problem, such as a sewer connection 3.
Signs a sump pump is failing, and when to call a pro
- The pump hums but water does not leave, or the outlet pipe outside is dry while the pit is full; either way the water has nowhere to go.
- The pump runs constantly or cycles every few seconds.
- Water is rising in the pit faster than the pump can clear it.
- The float sticks against the pit wall.
- The pump is old and has never been tested, and a storm is coming.
A flooding basement is an emergency. See our plumbing emergencies guide or the emergency triage tool.
Where sump water may legally go, and other rules
Discharge to the sanitary sewer: Many cities prohibit it. Lincoln, Nebraska says sump pumps, roof drains and foundation drains connected to the city’s sanitary sewer violate the Lincoln Municipal Code 3.
NDSU adds that putting extra water into the sewer can overload the public system and may break a local rule, and that a sump must never pump into a floor drain where the home has a septic system 1.
Why cities care: The EPA lists sewer defects that let stormwater and groundwater overload the system among the causes of sanitary sewer overflows 5.
Plumbing and electrical codes: Pits, discharge piping and outlets fall under the plumbing, residential and electrical codes your jurisdiction has adopted, with local amendments.
Zoeller’s guide tells installers to fit the pipe, union and check valve “according to code” 4. Ask your building department whether a permit is needed for a new pit or line.
Backwater valves: FEMA advises checking with your local official before installing check or backflow valves on drain lines, since communities differ on what they allow 2.
Sump pump vs related basement water devices
| Device | What it handles | Where water goes | Main point |
|---|---|---|---|
| Sump pump | Groundwater in a pit 1 | Yard or storm drainage 1 | Never to the sanitary sewer 3 |
| Sewage ejector pump | Waste from fixtures below the sewer line | The sanitary sewer | Sealed, vented basin |
| Battery backup pump | Groundwater when the main pump or power fails 2 | Same as the main pump | Starts when water reaches its float 2 |
| Floor drain plug or backwater valve | Sewer water trying to come up a drain 2 | Blocks reverse flow | Can jam open with debris 2 |
Our pages on the floor drain and the plumbing check valve cover the other pieces.
Sump pump myths
- Myth Draining the sump into the floor drain is fine.
- Reality NDSU says no, and many cities ban sump water in the sanitary sewer 13.
- Myth A bigger pump is always better.
- Reality Where inflow is slow, NDSU says a larger pump gives no advantage 1.
- Myth Sump pumps have filters to change.
- Reality They have intake screens that can plug, not filters 1.
- Myth A pump that ran last year is ready this year.
- Reality Test it each season by pouring in about 5 gallons and watching it cycle twice 1.
- Myth A sump pump stops all basement flooding.
- Reality FEMA lists sewer backup, cracks and overland flow as separate paths for water 2.
How climate and local rules change the picture
Cold winters are the regional variable that most affects a sump. NDSU, writing for North Dakota, puts a frozen or plugged outdoor discharge pipe first on its test list, because a pump pushing against a frozen line can overheat 1.
In those climates, check that the outdoor line drains fully between cycles.
City rules are the other variable: places with inflow and infiltration programs, such as Lincoln, Nebraska, ban sump connections to the sanitary sewer outright 3.
Check with your own city before you change where the pump discharges.
When a sump pump matters, and your next step
If your home has a pit, the pump matters every wet season. Test it, check the discharge, and ask whether a power cut during a storm would flood the basement; if it would, look at a battery or water-powered backup.
If water comes in and you have no sump, NDSU notes a small skimmer pump can move water from the floor, but a lasting fix is a pit with drain tile, which is pro work.
The guide below pairs four common situations with the first thing to do.
- If Pump never tested
- Then Pour in 5 gallons of water today
- If Power cuts during storms
- Then Add a backup pump
- If Pump runs nonstop
- Then Call a pro
- If Discharge runs to a floor drain
- Then Have it redirected outside
Questions about Sump Pump
Do I need a sump pump?
You need one if groundwater collects under or around your basement floor, which is why many homes with drain tile already have a pit.
Signs include water seeping in at the floor and wall joint after heavy rain.
A sump pump does not stop sewer backups, wall cracks or overland flooding, which FEMA treats as separate problems.
How do I test my sump pump?
NDSU Extension says to first make sure the outdoor discharge pipe is clear and the pump is plugged in.
Then remove the lid, check the pit and intake with a flashlight, and slowly pour about 5 gallons of water into the pit. Watch the float and listen. Make the pump turn on and off at least twice.
What size sump pump do I need?
There is no single correct size. NDSU Extension says a 1/3 hp pump suits most houses, and that a 1/2 hp pump moves more water and lifts it higher. The larger pump only helps where water can flow into the pit quickly. Check the pump’s flow-versus-lift chart for the height your water must rise.
Where should a sump pump drain?
Outside, away from the house. NDSU suggests at least 20 feet from the house, draining away, and not onto a neighbor’s lot, into window wells or onto a septic drainfield. It should not drain into a floor drain or the sanitary sewer; many cities, such as Lincoln, Nebraska, prohibit that connection.
Can a sump pump drain into the sewer?
Usually not. Many cities ban sump pump connections to the sanitary sewer because the extra water can overload the system and cause backups. Lincoln, Nebraska says one pump running 10 minutes an hour equals the normal sewage of 12 homes. With a septic system, NDSU says never pump the sump into the floor drain.
Does a sump pump need a check valve?
Yes. NDSU Extension says either pump type should have a check valve on the outlet pipe so water does not flow back into the pit when the pump shuts off. Without one, the pump turns on and off more than it needs to, which shortens its life. Zoeller’s instructions also call for a check valve.
What happens to a sump pump when the power goes out?
A standard sump pump stops. FEMA lists power outages as one of the main reasons basements flood even with a sump. A battery backup pump starts when water reaches its own float switch, and FEMA calls it an inexpensive safeguard. Water-powered backups are another option on city water.
Why is my sump pump running constantly?
It may be keeping up with heavy groundwater, or something may be wrong. Common causes are a stuck float, a missing or failed check valve that lets water fall back, a pump too small for the inflow, or a blocked discharge line. If it runs nonstop in dry weather, have a plumber check it.
Should a sump pump be on a GFCI outlet?
Yes, in most cases. NDSU says a standard 15-amp, three-prong grounded outlet can run a sump pump, and that a GFCI is best because the pump sits in or near water. Zoeller’s instructions call for a GFCI-protected receptacle. Always unplug the pump before handling it.
Sources
- NDSU Extension, Sump Pump Questions (via North Dakota Department of Emergency Services), retrieved .
- FEMA, Mitigation and Basement Flooding (presentation), retrieved .
- City of Lincoln, Nebraska, Sump Pump Connections: Tips for Homeowners, retrieved .
- Zoeller Co., Model 1099 installation quick guide (SW1203), retrieved .
- U.S. EPA, Sanitary Sewer Overflows (SSOs), retrieved .
Expert review
Written by the HyrePlumber Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Types, sizing, pit, discharge, test and electrical guidance come from NDSU Extension’s sump pump Q&A. Failure modes and backup pumps are from FEMA’s basement flooding presentation.
Install steps are from a Zoeller quick guide for one model, labeled as such. The sanitary sewer ban is from Lincoln, Nebraska and the overflow link from the EPA. No prices are given.
Suggest a correction. We fix errors and say what changed.