What a sump pump does, and when you need one

How a sump pump works

Water under a basement floor or a crawl space follows the drain tile to the lowest point, which is the sump basin. When the water in the basin rises, it lifts a float switch, and the pump turns on and pushes the water up through a discharge pipe and out of the house. A check valve on that pipe stops the water falling back into the basin when the pump shuts off.

The pump is only half the system. The drain tile around the perimeter is what collects the water and brings it to the basin. That is why a sump pump goes in with basement waterproofing or crawl space waterproofing.

When you need one

• Water on the basement floor after rain, usually at the joint where the floor meets the wall.
• Standing water or mud in the crawl space after a storm.
• A basement or crawl space getting an interior drainage system: the drain needs somewhere to send the water.
• An old pump that runs constantly, a basin with no lid, or a discharge that dumps water right back at the foundation.

Louisville sits on clay over limestone. After a big storm the clay stays saturated for days and groundwater keeps moving toward the lowest point it can find. Near the river, the creeks and in the lower parts of town, the water table is high after wet weather.

What a proper installation looks like

• A basin set at the low point of the perimeter drain, not just where there was room.
• A sealed, gasketed lid so humidity and debris stay out and the float stays free.
• A quality pump sized to the flow, a check valve on the discharge, and a union so the pump can be pulled for service.
• A discharge line that runs away from the foundation to a place the water cannot come back from.
• A battery backup pump on its own float.

How to test yours

Pour a bucket of water into the basin and watch a full cycle: the float should rise freely, the pump should run smoothly, the water should leave and not come back, and the pump should shut off cleanly. Then walk outside and find where that water came out. If the pump runs a lot, the sump pump guide walks through the usual causes.