The essentials in 30 seconds
A sump pump sits in a pit at the lowest point of a basement or crawl space. Groundwater from the foundation drains collects in the pit, a float switch starts the pump as the level rises, and the water is pushed up a discharge pipe and out, away from the house. Test it at least twice a year, ideally before the spring snowmelt: pour in water until it cycles, check that it starts and stops cleanly, and look at the check valve, the weep hole and the outlet outside. A battery or water-powered backup covers power cuts and a failed primary pump.
- The most common mechanical failure is a float switch stuck on or off.
- Plug the pump straight into its own outlet: manufacturers advise against extension cords.
- In most towns the water must go to the yard or the storm sewer, never into the sanitary sewer.
How a sump pump works
Rain and snowmelt raise the groundwater around your foundation. Footing drains carry it into the sump pit (also called the basin or crock). Rising water lifts the float, the float closes a switch and the pump starts. The water goes up the discharge pipe, through a check valve that stops it falling back, and out of the house. When the level drops, the pump stops.
| Part | What it does | What goes wrong |
|---|---|---|
| Pit (basin) | Collects water from the footing drains | Sand, gravel or debris jams the float or clogs the pump |
| Float switch | Turns the pump on and off with the water level | Sticks against the pit wall; poor contact; stuck on or off |
| Pump and motor | Lifts the water | Worn or rusted bearings, burnt-out motor, broken impeller |
| Weep hole | A small hole, 3/16 to 3/8 inch, in the pipe just above the pump that prevents an airlock | Blocked by mineral scale or debris |
| Check valve | One-way valve on the pipe just above the pit cover | Fails, so pumped water runs back into the pit |
| Discharge line and outlet | Carries the water outside | Cracked joints, frozen or buried outlet, blocked air gap |
Pedestal, submersible and backup pumps
Primary pumps
- Pedestal (upright) pump: the motor stands on a column about a foot above the pit, with the pump at the base and a ball float on a rod. The motor is designed to stay dry. The switch is easy to see and reach.
- Submersible pump: the whole unit sits on the bottom of the pit and is built to run underwater, out of sight under the pit lid. On some models the on and off levels can be adjusted.
Either type works well when it is maintained. What matters more is that it can keep up with the water entering your pit: a pump that is too small runs for long stretches and burns out.
Backup options
- Battery backup pump: a second, battery-powered pump in the same pit, set so its float only triggers if the primary pump fails to. North Dakota State University Extension advises a deep-cycle lead-acid battery: its ampere-hour (AH) rating decides how many hours of protection you get, and a rating near the maximum the backup maker recommends, typically around 120 AH, is a sensible choice. In an emergency, a standard car battery can run the backup.
- Water-powered backup: uses the pressure of your household water supply. Water rushes through a nozzle into the discharge pipe, creating suction that draws water out of the pit (the Venturi effect). It runs without electricity, but it uses about 2 gallons of fresh water for every gallon it removes, and it needs a backflow prevention device so sump water can never enter the drinking water supply. Ask a plumber whether your water pressure suits one; on a private well, the well pump itself stops in a power cut.
- Generator: a properly connected generator keeps the primary pump running during an outage.
| Situation | Good fit | Points to check |
|---|---|---|
| Storms often cut the power | Battery backup | Charge and corrosion every six months |
| Long outages, city water | Water-powered backup | Backflow device testing under local rules |
| Pump runs often, high flood risk | Any backup plus a high-water alarm | Test the alarm with the pump |
How to test a sump pump, step by step
The City of Ann Arbor and NDSU Extension both recommend checking the pump at least twice a year, preferably before the spring melt. Allow 15 minutes.
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01
Check outside first
Walk to where the discharge pipe ends. Clear leaves, mulch, snow or soil so the water can flow away from the house.
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02
Check the power
Make sure the pump is plugged directly into its outlet and the breaker is on. If the outlet has a GFCI, press its test and reset buttons: a tripped GFCI leaves the pump silent.
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03
Unplug and open the pit
Remove the lid and look in with a flashlight. Lift out stones, mud and bits of pipe, and check that the float can move freely without touching the wall or the pipe.
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04
Look at the pipework
Find the check valve just above the pit and the small weep hole above the pump. Clear the weep hole if it is blocked.
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05
Plug in and add water
Pour water slowly into the pit, three to four gallons is usually enough, at roughly the speed water normally arrives. On a dry pit you can instead lift the float for about 10 seconds and lower it slowly.
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06
Watch two full cycles
The pump should start at the set level, empty the pit smoothly, spray a thin stream from the weep hole and stop cleanly. Make sure it turns on and off at least twice.
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07
Listen
A sound like pumping gravel, or a sluggish float, means the pump or float may need replacing.
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08
Check the pipe while it runs
With about 5 gallons going through, look along the discharge pipe for drips, cracks or loose joints.
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09
Test the backup
Unplug the primary pump, add water and confirm the backup starts. For a battery unit, check the charge and, on a serviceable battery, that the water covers the cells. Plug the primary back in.
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10
Close up
Refit the lid, reseal a sealed or clear cover with new sealant, and confirm every plug and breaker is back on.
Sump pump not working: causes and fixes
Sump pumps fail for four broad reasons: power outages, lack of maintenance, age and poor installation. Use the table to narrow it down, and always unplug the pump before touching it.
| Symptom | Likely cause | What to do |
|---|---|---|
| Runs constantly, pit stays full | Pump too small for the inflow, or a high water table after heavy rain | Check whether the level ever drops; a larger pump or a second pump may be needed |
| Runs constantly, pit is empty | Float switch stuck in the on position | Unplug, free the float; replace the switch if it sticks again |
| Starts and stops every few seconds | Failed check valve letting water fall back, or float set too low | Watch the pipe after it stops; replace the check valve; raise the float or the pump |
| Will not turn on | Tripped breaker or GFCI, damaged cord, switch stuck off, float jammed | Reset power, free the float, then test with water; replace a faulty switch |
| Hums but moves no water | Clogged intake screen, broken impeller, airlock | Unplug, clean the screen, clear the weep hole, check the impeller |
| Shuts off after running hard | Motor overheating from overwork or worn bearings | Plug it straight into the outlet, check pump size; a burnt-out motor means a new pump |
| Clanging or banging | Discharge pipe hitting the pit or wall; many elbows | Wrap the pipe with insulation, fit rubber grommets; ask a plumber to straighten the run |
| Gurgling after each cycle | Water draining back down the pipe | A spring-loaded check valve and a switch set to stop before the pit is bone dry |
| Grey water with a sewage smell | Sewer backup, separate from groundwater | Call your city or a plumber; ask about a backflow valve |
| Musty or earthy smell from the pit | Open or poorly sealed lid, damp pit | Fit or reseal the lid; in radon areas, a sealed lid also blocks soil gas |
Groundwater that reaches your basement is usually clear and odorless; sewage backup is greyish and usually smells. That difference matters for who you call and for your insurance claim.
A sealed lid also keeps radon from entering through the footing drains and an open sump: see radon testing.
Where a sump pump can discharge
Discharge rules are set locally, so check your city or county code. The rules below appear again and again in municipal ordinances, such as those of Sturgis, South Dakota:
- Never into the sanitary sewer. Connecting a sump pump, downspout or foundation drain to the sewer is banned in many towns, because the extra clean water overloads treatment plants and can cause sewer backups into homes. A few cities allow it seasonally under permit.
- Far enough from the house that the water cannot run straight back to the foundation and into the pit again.
- Onto your own land, or into a storm sewer where a connection is offered. Directing water onto a neighbor's property causes disputes.
- Off sidewalks and streets. Water across a public sidewalk is a trip hazard; water running into the street causes algae in summer and ice in winter.
- With a clear air gap where the inside pipe drops into an underground outlet, so leaves and soil cannot block it.
Backflow valves and insurance
FEMA recommends a sump pump with battery backup in basements that flood often, and a backflow valve fitted by a plumber to keep sewer backup out. Flood damage is often excluded from standard homeowners policies, so ask your insurer about water backup cover; a new NFIP flood policy generally starts only 30 days after purchase.
A water leak detector or high-water alarm by the pit warns you when a pump fails while you are away. For a crawl space that stays damp, see crawl space encapsulation.
When to call a plumber
- The pump runs nonstop and the pit still fills: it is undersized or the drainage needs a look.
- Sand or sediment keeps building up in the pit: the footing drains may be letting soil in.
- You want a check valve, a backflow valve, a water-powered backup or a new discharge line.
- The motor is noisy and old, or you have already replaced the switch once.
Ask what flow the pump is sized for, where the discharge will end and whether a permit is needed.
Sump pump: frequently asked questions
How does a sump pump work?
Water from the foundation drains collects in a pit. A float switch starts the pump as the level rises, and the pump pushes the water outside through a discharge pipe with a check valve.
Why is my sump pump running constantly?
Either the float switch is stuck on, the check valve lets water fall back, or water is arriving faster than the pump can move it. Check whether the pit ever empties.
Why won't my sump pump turn on?
Start with power: breaker, GFCI and cord. Then look for a float jammed against the wall or by debris, and a switch stuck in the off position.
How often should I test a sump pump?
At least twice a year, ideally before the spring snowmelt and before the stormy season, and after any long dry spell.
Do I need a battery backup sump pump?
It is worth having if storms cut your power, if the pump runs often or if your basement holds valuable things. Check the battery every six months.
Can a sump pump drain into the sewer?
In many towns it is prohibited. Discharge to your yard, away from the house, or to a storm sewer if your city offers a connection.
Sources
Official and university guidance, checked on 27 October 2026.
- NDSU Extension · Now is the time to check your sump pump ↗Pedestal and submersible pumps, the float-lift test, two-cycle check, bearing failure and backup advice.
- NDSU Extension · Electric backup sump pumps for houses: quick tips for battery selection ↗Deep-cycle battery, ampere-hour rating, six-monthly checks.
- City of Ann Arbor · Sump pump resources ↗Maintenance checklist, weep hole size, causes of failure, noise fixes, water-powered backups and backflow devices.
- City of Sturgis, SD · Guide to sump pump discharge ↗Sanitary sewer ban, neighbors, sidewalks, streets and storm sewer connections.
- FEMA · Be flood smart: protect your property ↗Battery backup, backflow valves and the 30-day wait for flood insurance.
- CDC · Protect yourself from electrical hazards ↗Standing water, main breaker and electrician inspection.
Storm season coming?
Pour a bucket of water into the pit this week and watch two full cycles before you need the pump.