Sump Pumps and Backwater Valves, How They Work and How to Keep Them Working

Two Devices, Two Different Jobs

People often mix these up, but they protect against different water.

Many homes need both. A basement can take on groundwater through the walls and floor during a long rain, and at the same time the city sewer can surcharge and push water up through the floor drain. A sump pump does nothing to stop a sewer backup, and a backwater valve does nothing to stop groundwater.

If your basement flooded before, find out which kind of water it was. Clear water at the walls points to groundwater. Dirty, smelly water coming up through a floor drain or toilet points to the sewer.

How a Sump Pump System Works

A sump system has several parts, and a weak link anywhere can flood the basement.

• Drain tile. Perforated pipe around the footing or under the floor collects groundwater and carries it to the pit. Some homes have no drain tile and the pit only collects water under the slab.
• The pit. A basin sunk into the floor. It should have a lid to keep debris out and reduce odor and moisture.
• The pump. Submersible pumps sit in the water and are quieter. Pedestal pumps sit above the pit with the motor out of the water.
• The float switch. Turns the pump on as water rises. A float that snags on the pit wall or the cord is a common failure.
• Check valve. On the discharge pipe, it stops pumped water from falling back into the pit.
• Discharge line. Carries water outside. It should end well away from the foundation, often 10 to 20 feet, and never where the water runs back toward the house.

Do not connect a sump pump to the sanitary sewer or a sink drain. Many cities prohibit it because it overloads sewers during storms, which is exactly when sewer backups happen.

Most sump pumps last roughly 7 to 10 years, and less if they run constantly. If yours is older than that, replace it on your schedule, not during a storm.

Testing and Cleaning a Sump Pump

Test the pump twice a year, before the wettest season and before winter, and after any long period without running.

• Check that the pump is plugged in and the outlet has power. If it is on a GFCI outlet, press test and reset.
• Slowly pour a bucket of water into the pit until the float rises.
• The pump should start on its own, empty the pit and shut off.
• Listen for grinding or rattling and watch that the check valve closes without much water falling back.
• Go outside and confirm water comes out of the discharge line.

Once a year, clean the pit. Unplug the pump, lift it out if you can, rinse the intake screen and remove silt, gravel and debris from the basin. Check the float moves freely and the cord is not in its way.

A pump that runs constantly in dry weather may mean a stuck float, a failed check valve or high groundwater. A pump that hums but moves no water may be clogged or air locked. Many submersible installations have a small hole drilled in the discharge pipe between the pump and the check valve to prevent air lock. Do not plug it.

In cold climates, check that the outdoor end of the discharge line cannot freeze shut. A frozen line forces the pump to run with nowhere to send the water.

Backup Power and Alarms

The storms most likely to flood a basement are the same storms that knock out power. A pump without power is just a weight in a hole.

• High water alarm. A battery alarm that sounds when water rises above the normal level in the pit costs little. Wi-Fi models send a phone alert.
• Battery backup pump. A second pump runs off a deep cycle battery when the main pump fails or power drops. Check the battery each season and replace it as the maker directs, often every few years.
• Water powered backup. Uses city water pressure to pull water from the pit. It needs no battery, but it uses a lot of water, needs good water pressure and often needs a plumber and backflow protection. It will not work with a private well that loses power.
• Generator. A portable generator can run a sump pump during a long outage. Run it outside, far from doors, windows and vents, because generator exhaust can kill.

Many electrical codes now require GFCI protection for basement outlets. A tripped GFCI stops the pump silently, which is one more reason to have an alarm. Put the pump on its own circuit if you can, so a tripped breaker from another appliance does not shut it off.

How a Backwater Valve Works

When heavy rain overwhelms a city sewer, sewage can flow backward up building lines and come out at the lowest drain, often a basement floor drain, a basement toilet or a shower. A backwater valve on the main sewer line blocks that reverse flow.

Most valves have a flap that normally stays open or rests lightly closed. Waste from the building pushes it open and flows out. When sewage flows back, the flap closes against it. Some installations add a manual gate that you can close by hand during a storm.

• Location matters. The valve must protect the fixtures that sit below the level where sewage would rise. A plumber decides where it goes.
• Access matters. The valve needs an access cover so it can be inspected and cleaned.
• Permits. Installing one means cutting into the building sewer, so most cities require a licensed plumber and a permit.

While the valve is closed during a backup, your own wastewater has nowhere to go. If you run the washer or flush toilets, that water can back up in your own basement. During a heavy storm, limit water use until the city sewer drains.

For a smaller step, a plumber can fit floor drain plugs that float closed when water rises. They help at that drain but do not protect toilets or showers.

Maintaining a Backwater Valve

A backwater valve that is stuck open protects nothing. Debris like wipes, grease and paper can keep the flap from sealing.

• Open the access cover once or twice a year, wearing gloves, and look at the flap. It should move freely and close fully.
• Remove any buildup with a gloved hand or a brush. Rinse with a hose if the maker allows it.
• Check the cover seal before closing it again.
• If the valve has a manual gate, practice closing and opening it so you know how.

Never pour grease down drains and never flush wipes, even ones labeled flushable. They clog the building line, foul the valve and add to city sewer blockages.

After a sewer backup, have the line camera inspected. Tree roots, a collapsed pipe or a sag in the building sewer can cause backups that a valve cannot fix. The property owner is usually responsible for the line from the building to the city main.

Check your insurance. Standard homeowners policies often exclude sewer and drain backup unless you add a water backup endorsement, and flood damage needs a separate flood policy. The endorsement is usually inexpensive compared with a single basement cleanup.

Step by Step

1. Check Power and GFCI

   Confirm the pump is plugged in and the outlet has power. Press test and reset on any GFCI outlet.

2. Fill the Pit

   Pour a bucket of water slowly into the sump pit until the float rises and the pump starts.

3. Watch the Cycle

   Make sure the pump empties the pit and shuts off on its own. Listen for grinding and check that little water falls back through the check valve.

4. Check the Outlet Outside

   Walk outside and confirm water flows out of the discharge line well away from the foundation and not back toward the house.

5. Test the Alarm and Backup

   Lift the alarm float or pour in more water to trigger it. Check the backup battery charge and date.

6. Clean the Pit

   Once a year, unplug the pump, clean the intake screen and remove silt and debris from the basin.

7. Inspect the Backwater Valve

   With gloves on, open the access cover and check that the flap moves freely and seals. Clear debris and close the cover tightly.

Mistakes That Make It Worse

• Never testing the pump until a storm. A stuck float or dead motor shows up at the worst moment.
• Relying on one pump with no alarm or backup when storms often cut power.
• Running the washer during a storm with the backwater valve closed. Your own water backs up into the basement.
• Assuming homeowners insurance covers sewer backup. Many policies need a separate endorsement.

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