What is a sump pump?
Updated August 2026
A sump pump is an electric pump sitting in a pit at the lowest point of a basement or crawl space that pushes groundwater outside before it reaches the floor. A float switch, either a tethered ball or a vertical rod, starts the motor once water in the pit rises a few inches and cuts it off when the level drops. Most homes are served by 1/3 horsepower, which moves roughly 30 to 40 gallons a minute. Step up to 1/2 hp for a high water table or a long vertical lift.
Where it is
The pit is a lined hole 18 to 24 inches deep and about 18 inches across, cut into the slab at the low corner of the basement and normally covered by a plastic lid. Perforated drain tile around the footing feeds it. From the pump, a 1-1/2 inch PVC discharge line climbs through the rim joist with a check valve in the vertical run, and the outdoor end should release at least 10 feet from the foundation, never into a downspout and never into the sanitary sewer. Power comes from a dedicated GFCI outlet overhead. Submersible pumps sit underwater and run quieter; pedestal pumps put the motor on a column above the pit, cost less, and are simpler to service.
What it does
Groundwater rising against a foundation builds hydrostatic pressure, and the pump keeps that water below the slab so it never finds the joint where the floor meets the wall. During a spring storm a pump may cycle every 60 to 90 seconds for hours. Five things end that streak: a float wedged against the pit wall (the most common failure of all), an intake screen packed with silt, a check valve that lets the discharge column fall back and short-cycle the motor, a discharge line frozen or blocked outside, and the grid dropping in the exact storm that needed the pump. Cleanup math argues for redundancy. Drying and treating an unfinished basement runs $1,000–3,000; a finished one runs $3,000–10,000 and up once drywall, flooring, and contents are counted. Pumps last 7 to 10 years.
Maintenance
Pour 5 gallons of water into the pit every 3 to 4 months and watch the entire cycle: the float lifts, the pump starts, the level drops, the check valve thumps once, and everything goes quiet. Clear silt and gravel out of the pit yearly (pit cleaning) and confirm the outdoor discharge is open before the first hard freeze. A battery backup buys 5 to 8 hours of pumping when the power goes, and its battery needs swapping every 3 to 5 years (backup battery intervals). Replace the pump on age rather than on failure: how long sump pumps last and when to replace one set the timing.
What it costs to replace
A replacement sump pump runs $150–400 for a 1/3 hp unit you install yourself, or $400–1,200 installed with a new check valve.
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Frequently asked questions
How often should a sump pump run?
During heavy rain or spring melt, a cycle every 1 to 3 minutes is normal work rather than a warning. Running that often in dry weather is not: look for a check valve that has failed and is letting the discharge column drain back into the pit, a float stuck partly open, or outside water from a leaking supply line or a downspout emptying beside the foundation. A pump that never runs at all deserves the 5-gallon bucket test before you assume the basement is simply dry.
Do I really need a battery backup?
If the basement is finished, if the water table is high, or if your area loses power in the same storms that bring the water, yes. The failure mode is not subtle: the pump is fine and the outlet is dead. A battery backup system costs $300–800 installed and pumps for 5 to 8 hours on a full charge. A water-powered backup, which runs on municipal pressure instead of electricity, is an alternative at $400–900 where city water stays on through outages.
Can I replace a sump pump myself?
It is a moderate DIY job, usually 1 to 2 hours. Unplug the pump, unscrew or cut the union in the discharge pipe, and lift the old unit out (expect it to be heavier and filthier than it looks). Set the new pump on a brick or the flat bottom of the pit so it does not draw silt, reconnect with a fresh check valve, and test with 5 gallons of water. Choose cast iron over plastic if the pump runs often, because it sheds motor heat better.