A sump pump sits in a pit and waits. When groundwater fills the basin, a float rises and trips a switch, and the pump pushes the water up and out of the building. Use the buttons to look inside and follow the water from the inlet to the discharge.
A sump pump is a small pump that stands in a pit, called the sump basin, at the lowest point of a basement or crawl space. Water that would otherwise spread across the floor drains into the pit instead, and when enough has gathered the pump sends it out through a pipe. It is a flood defence that works automatically, one basin-full at a time.
From the outside it is just a lid on a hole with a pipe leaving it. The interesting part is the little control loop hidden inside: the float, the switch it trips, and the impeller that does the work. Cutting the basin open shows how the water level itself becomes the signal that turns the pump on and off.
Water from the ground around the foundation seeps in through a low inlet and collects in the basin. The basin is simply a receiving pit: it gives the water a single place to gather so the pump always has something to work with.
A float hangs at the end of an arm joined to the switch. As the basin fills, the float rides up with the surface and the arm swings upward. The float is the sensor — the water level itself is what the pump responds to.
At a set height the tilted arm closes the switch. That completes the circuit and starts the motor. Because the arm only trips at the top of the travel, the pump waits until the basin is properly full instead of switching on at every ripple.
The motor spins an impeller that takes water in at the bottom through a strainer and drives it up the discharge pipe. The strainer keeps gravel and debris out of the impeller so it cannot jam.
A check valve in the discharge pipe lets water flow out but not back. It keeps the pipe full between runs, so when the pump stops the water does not drain back into the basin and start the cycle all over again.
As the basin empties the water level drops, the float comes down, and the arm swings back until the switch opens. The motor stops and the pump waits quietly for the next time the water rises.
The float switch. As groundwater fills the basin, the float rises on its arm. At a set level the arm tilts enough to trip the switch, which closes the circuit and starts the motor.
Because it only drains what has collected. The float rises as the basin fills and falls as the pump empties it, so the switch turns the motor on near the top level and off again near the bottom.
It lets water leave the pump but not run back. Without it, every time the pump stopped the water still in the discharge pipe would drain back into the basin and make the pump restart over and over.
The strainer is a slotted base that lets water in while keeping out gravel and debris that could jam the impeller. It is the pump's intake, and it must stay clear to work.
The basin gives the water somewhere to gather and the pump a place to sit below floor level. The pump can only lift water it has collected, so the pit is what turns a trickle of groundwater into a usable pool.