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How Sump Pump Works
Quick answer: A sump pump sits in a pit at the low point of your basement. Footing drains and gravel channel groundwater into the pit; when water lifts a float or trips a sensor, the motor spins an impeller that pushes water up the discharge pipe, through a check valve and away from the house. The pump stops when the level drops.
The drainage system around the pump
A sump pump is the last piece of a drainage system that starts outside your foundation. Many homes have perforated drain tile, often called weeping tile or footing drains, laid beside the footings. Groundwater that collects around the foundation seeps into those pipes and flows by gravity toward a collection point. In homes with a sump, that point is a basin set into the basement floor. Some homes also have interior perimeter drains under the slab that feed the same basin. Without a pump, the water would simply rise in the basin until it came over the top.
Inside the pit
The pit, or basin, is usually a plastic or fibreglass cylinder 18 to 30 inches across and roughly two feet deep. Holes or openings in the side let drain tile enter. Gravel around the basin helps water flow in and keeps soil out. A lid keeps debris, children and pets out and, in radon-prone areas, helps seal soil gas. The pump sits on the bottom or on a brick to keep it above settled silt.
The switch: deciding when to run
Pumps use several kinds of switch:
- Tethered float. A buoyant switch on a short cord floats up as water rises and tips to turn the pump on. It needs a wide pit so it can swing freely.
- Vertical float. A float slides along a rod attached to the pump. It suits narrower pits.
- Diaphragm or pressure switch. Senses water pressure through a sealed diaphragm; no moving float in the pit.
- Electronic sensor. Solid-state probes detect water level without mechanical parts.
Switch setting controls cycle length: the difference between on and off levels times gallons per inch is the volume pumped each cycle. The pit calculator helps you see that volume.
The motor and impeller
When the switch closes, an electric motor sealed in an oil-filled or air-filled housing spins a shaft. On a submersible pump the motor sits underwater, which helps cool it; on a pedestal pump the motor sits on a column above the pit. The shaft turns an impeller, a small wheel with vanes. Water enters at the centre and is flung outward by centrifugal force into a chamber called the volute, which channels it into the discharge. Many sump pumps use vortex impellers, which tolerate grit better than tight-clearance designs.
The discharge and check valve
From the pump, water travels up a vertical pipe, usually 1½-inch PVC. A check valve just above the pit lets water flow out but stops the column of water in the pipe from falling back into the pit when the pump shuts off. Without it, the same water would return and the pump would cycle more often. A small weep hole drilled in the pipe below the check valve, recommended by many manufacturers, prevents air lock. Learn more in the check valve guide.
The pipe then runs across the ceiling joists, exits through the rim joist or wall, and directs water to a location that drains away from the foundation — a splash block, buried line, dry well or storm connection where allowed.
Why head matters
Every foot the water climbs and every foot of pipe it travels takes energy. The total resistance is called total dynamic head, and it is why pumps move less water in a real basement than the headline numbers suggest. The TDH calculator estimates yours.
Worked example of one cycle
Picture an 18-inch pit holding about 1.1 gallons per inch. The float turns the pump on when water reaches 10 inches and off at 3 inches, a 7-inch drawdown, so each cycle removes about 7.7 gallons. During a storm, inflow is 4 GPM. The pit fills those 7 inches in about 1.9 minutes. The pump, delivering about 30 GPM at the home's 11 feet of head, empties the pit in roughly 15 seconds — net removal is 30 minus 4, or 26 GPM. So the pump runs about 18 seconds and rests about 1.9 minutes, roughly 28 cycles an hour, which is why switch quality and setting matter so much.
Illustrative note (2026-10-29): The pit size, inflow and pump output in this example are hypothetical values prepared on 2026-10-29 to illustrate the cycle. Real pumps and pits will differ.
Backup and alarms
Because a sump pump depends on electricity and on a switch that can stick, many homeowners add a battery backup or water-powered backup that sits higher in the pit, and a high-water alarm that sounds if the level climbs past normal.
What a sump pump does not do
A sump pump only removes water that reaches the pit. It does not stop water from coming through cracks above the drains, overland flooding, or sewage backing up through floor drains, which require backwater valves and other measures. If you see water elsewhere in the basement while the pump runs normally, ask a licensed plumber or waterproofing professional to look at the system as a whole.
Electrical safety
Sump pumps run in wet conditions, so electrical protection matters. Many codes call for GFCI protection on basement receptacles, and some manufacturers and codes have specific guidance for sump pumps. Avoid extension cords. A licensed electrician can advise on dedicated circuits and the right protection for your installation.
Frequently asked questions
Why does my pump run when it has not rained?
Groundwater can rise after snowmelt or rain days earlier, or a high water table keeps feeding the drains. Constant running in dry weather can also mean a stuck switch or a failed check valve.
Where does the water come from?
Mostly from groundwater collected by footing drains. Roof runoff should be directed away from the foundation and not into the pit.
How do I know if mine is working?
Test it. See how to test a sump pump.