What does a sump pump do?
It keeps groundwater from rising into your basement. Water that soaks into the soil around the foundation, or collects in drain pipe around the footing, runs into a pit called a sump basin. The pump empties that pit before it overflows.
A sump pump does not stop a burst pipe, a leaking water heater or a sewer backup. It handles water coming from the ground. That is why a house can have a working sump pump and still get a wet basement.
How does a sump pump work?
It is a simple cycle that repeats every time the pit fills.
- Groundwater drains or seeps into the sump pit.
- A float switch rises with the water level.
- When the float rises high enough, the pump turns on.
- The motor spins an impeller that pushes water up the discharge pipe.
- A check valve in the pipe stops that water from falling back into the pit.
- The water exits outside, away from the foundation.
- As the level drops, the float falls and the pump shuts off.
The main parts
| Part | What it does |
|---|---|
| Sump pit (basin) | Collects the water at the lowest point |
| Pump and motor | Moves the water up and out |
| Float switch | Turns the pump on and off as the level changes |
| Discharge line | Carries water outside the house |
| Check valve | Keeps pumped water from flowing back into the pit |
| High water alarm (optional) | Sounds when the level gets too high |
What are the types of sump pumps?
There are two main styles of primary pump, plus two kinds of backup that run when the main pump cannot.
| Type | How it works | Good to know |
|---|---|---|
| Submersible | Sealed pump and motor sit inside the pit, under water | Quieter and out of the way |
| Pedestal | Motor sits on a column above the pit | Easier to service; often lasts longer; louder |
| Battery backup | Second pump runs on a battery when power fails | Battery needs testing and eventual replacement |
| Water-powered backup | Uses city water pressure to pull water out of the pit | Needs municipal water; uses fresh water to run; local code may require a backflow device |
Water-powered backups work without electricity, but they use more fresh water than they remove. The City of Ann Arbor, for example, describes one using about 2 gallons of fresh water for every gallon it pumps out.
What size sump pump do you need?
The right size depends on how fast water enters your pit during heavy rain and how high the pump has to lift it to get it outside. A plumber or waterproofing contractor sizes it by watching how quickly the pit fills.
- Measure the inflow. During a hard rain, time how long the pit takes to fill once the pump shuts off. Fast refills mean you need more capacity.
- Check the lift. Pumps are rated by how much water they move at a given height. The higher the discharge pipe has to climb, the less water the pump moves.
- Watch for warning signs. A pump that runs almost nonstop in storms, or that lets the water rise near the top of the pit, is likely too small.
Bigger is not always better. A pump that is far too large for a small pit can switch on and off constantly, which wears it out early.
Where should a sump pump drain?
Outside, and far enough from the house that the water does not soak back down to the foundation. Point the outlet toward ground that slopes away from the house, not at a neighbor's yard or a walkway that ices over in winter. Rules on where a sump pump can discharge vary by locality, so check with your county before you connect it to anything other than open ground.
How long does a sump pump last?
Most sources put a typical sump pump at around 7 to 10 years. Pedestal pumps can last longer because the motor stays out of the water.
How hard it works matters more than the calendar. A pump that cycles all spring in a wet basement wears faster than one that runs a few times a year. Sand, gravel and debris in the pit shorten its life too. If yours is near the end of that range, replace it before the wet season rather than after it fails in a storm.
Why do sump pumps fail?
Most failures come down to a handful of causes.
- Power outage. The most common one. Heavy storms knock out power at the moment the pit fills fastest.
- Stuck float switch. The float gets caught on the pit wall, the cord or debris and never turns the pump on, or never lets it shut off.
- Clogged intake. Silt, gravel and dirt block the screen.
- Undersized pump. In a heavy storm, more water comes in than the pump can move.
- Age. Motors and switches wear out.
- Frozen or blocked discharge line. Ice or debris in the outside line traps the water, and the pump works against the blockage until it overheats.
- Bad check valve. Water falls back into the pit after each cycle, so the pump runs far more than it should.
What are the signs a sump pump is failing?
- Grinding, rattling or loud humming
- It runs constantly, even in dry weather
- It turns on and off every few seconds
- It does not start when water fills the pit
- Water flows back into the pit after the pump stops
- Rust or corrosion on the pump or the pipe
- Damp spots or a musty smell around the pit
How do you maintain a sump pump?
A few minutes, a couple of times a year, prevents most failures. Unplug the pump before you clean it.
- Pour a bucket of water into the pit and confirm the pump starts and shuts off.
- Clear debris from the pit and the intake screen.
- Check that the float moves freely and is not tangled in the cord.
- Walk outside and make sure the discharge outlet is clear and points away from the house.
- Test the backup pump and battery, and replace an old battery.
- Before winter, make sure the outdoor part of the discharge line slopes so it drains, and insulate any exposed section. How to prevent frozen pipes in Northern Virginia
What should you do if your sump pump fails and the basement floods?
Make it safe first. Do not touch the pump or step into water near outlets until the power to that area is off, and only turn it off at the panel if you can reach it from a dry spot.
- Shut off power to the basement from a dry spot, or call an electrician.
- Take photos of the water, the pit and the pump.
- Get the water out with a wet vac or a rented pump, or call for extraction.
- Pull wet carpet pad and start drying with fans and a dehumidifier.
- Have the pump checked or replaced before the next storm.
For the full checklist, read Basement flooded? What to do first. If water sat for more than a day or two, watch for mold in the basement.
Do Northern Virginia homes need a sump pump?
Not every house does, but many with basements have one. They are most useful where groundwater rises after long rains or the soil holds water against the foundation.
Parts of Fairfax County sit on marine clay, a high shrink-swell soil the county has published a homeowner guide about, and clay holds water near foundations. If your basement gets damp after storms, or you see water seeping where the wall meets the floor, ask a waterproofing contractor or plumber about a sump system.
If your pump already failed and you have water downstairs, our flooded basement cleanup crew pumps it out, dries it, documents it for your insurer, and rebuilds the finished space. Does homeowners insurance cover water damage?