What Is a Sump Pump?

What is a sump pump, in plain words

A sump pump is the basement's way of bailing itself out. Water that soaks into the ground around your foundation follows drain lines or gravel to the lowest point, which is a pit in the basement floor called a sump. The pump in that pit lifts the water up and sends it outside before it can spread across the floor.

FEMA describes it this way: sump pumps draw in groundwater from around the house and direct it away through drainage pipes, and they can be an excellent defense against basement seepage and flooding.

Here's the thing most people miss. A sump pump doesn't fix a wet basement by itself. It's one part of a system: the drains that feed the pit, the pit, the pump, the switch, the pipe and the spot where the water ends up outside. If any one of those quits, the whole system quits.

Down in our basement, the pit sits in the corner under the stairs. I've spent 34 years in other people's crawl spaces, so I know a good pit when I see one. Ours is a good pit. It also has opinions about rain.

How does a sump pump work

The cycle repeats every time water comes in:

1. Water drains into the pit. Groundwater from around the foundation collects at the low point.
2. The switch turns the pump on. A float rides up on the rising water. At a set height it switches the pump on.
3. The pump pushes the water up and out. It goes up a discharge pipe, out through the wall and away from the house.
4. The check valve holds it. A check valve is a one-way flap on the discharge pipe. It stops the water in the pipe from running back down into the pit when the pump stops.
5. The pump shuts off. As the level drops, the float drops, and the pump turns off.

Community Association Underwriters of America, an insurer, notes that without a check valve, pumped water can flow back into the pit and make the pump turn on and off more than it needs to. The City of Ann Arbor's maintenance checklist adds one more small part: a weep hole, a small hole in the discharge pipe just above the pump that prevents an air lock. If you see a little spray from that hole when the pump runs, that's on purpose.

The parts of a sump system

Part	What it does	What to check
Pit (sump)	Collects groundwater at the basement's low point	Silt, sand or debris in the bottom
Pump	Lifts the water up and out	Sits level; debris kept below the intake
Float switch	Turns the pump on as water rises	Moves freely; doesn't touch the pit walls
Check valve	Stops water running back into the pit	Present on the discharge line above the pit
Weep hole	Prevents an air lock in the pipe	Clear, not plugged
Discharge pipe and outlet	Carries water out and away	No leaks, no blockage, outlet clear outside

Sources: City of Ann Arbor maintenance checklist; City of Superior, Wisconsin, maintenance tips.

Pedestal vs. submersible: the two main types

Most home sump pumps come in two styles.

• Pedestal. The pump stands in the pit with the motor up on a column above the water, where it's meant to stay dry. Community Association Underwriters of America says pedestal pumps are noisier.
• Submersible. The whole pump sits down in the pit, under water, with a sealed motor. The same guide says submersibles are much quieter and tend to have a longer life expectancy, because the sealed motor keeps moisture and dirt out.

Both use a switch that turns the pump on when the water reaches a set level. The switch can be a float on an arm, a vertical float on a rod, or an electronic sensor; the maker's listing says which.

Rule of thumb: size a pump to your pit and to how high it has to push the water, not to the biggest horsepower on the shelf. Flow drops as the lift rises, so the number that matters is the flow at your lift, and makers list it by height. Our sump pump page lays those numbers side by side.

How often should a sump pump run

There's no fixed number. A sump pump runs when water comes in, so it works hard in a wet spring and may sit idle through a dry August.

A few signs tell you whether yours is running right:

• Runs after heavy rain or snowmelt, then rests. That's normal.
• Never runs, even after storms. Pour water into the pit to check that it still turns on. The City of Ann Arbor says three to four gallons is usually enough to start it.
• Runs all the time. Ann Arbor's checklist says a pump shouldn't have to. Check the float setting first; if water still reaches the top of the pit, the pump may be too small for the job.
• Short bursts, on and off. A missing or failed check valve can cause that.

The City of Superior, Wisconsin, adds a point about dry spells: if the pit stays dry for a long time, the pump's seals can dry out and crack. Its tip is to test once a month by adding enough water to start the pump.

(I test ours more often than that. The pit has never once complained about the attention.)

Where does sump pump water go

Outside, and away. The discharge pipe should carry the water to a spot where it drains away from the house instead of soaking back down to the foundation and into the pit again.

Where it's allowed to go depends on your town:

• The yard. The most common setup. The outlet should sit where the ground carries water away from the house.
• An underground pipe to the storm sewer or a buried drain, where your town allows it. Ann Arbor's checklist describes an air gap at the house wall where the discharge drops into a larger pipe.
• Not the sanitary sewer. Community Association Underwriters of America notes that local codes prohibit directing pump discharge into the municipal sewer system.

Check with your local building department before you change where your discharge goes. Ana's ruling: the water goes downhill toward the pond, the pond can have it, and nobody asks the neighbors to share.

Do I need a sump pump

Some houses do, some don't.

• Your house has a sump pit. Then you need a working pump in it. A pit with no pump is just a hole that fills up.
• Your basement gets water after rain or snowmelt. A sump system is worth asking a plumber about. FEMA lists installing a sump pump among its flood-protection projects, with a battery-operated backup in case the power fails.
• Your basement has never had water and there's no pit. You probably don't need a sump pump. You need a gutter check, downspouts that carry water away, and soil that slopes away from the foundation.

If you do have one, a backup and a high-water alarm deserve a look too. The City of Ann Arbor's backup guide lists power loss, a stuck float, a burned-out pump, a clog, a failed check valve, a blocked discharge pipe and more water than the pump can handle among the reasons pumps fail. Most of those happen during storms, which is exactly when you need the pump.

Simple care, twice a year

Ann Arbor suggests checking your sump pump at least twice a year, ideally before the spring melt. Unplug it first. Then:

• Clear silt and debris from the bottom of the pit.
• Make sure the float moves freely.
• Check the weep hole and the check valve.
• Look over the discharge pipe for leaks.
• Plug it back in and pour in water to watch it cycle.
• Walk outside and make sure the outlet isn't buried in leaves.

Read the plate first, then the manual. The maker's instructions come before any general list, this one included.

Keep it dry.