How a sump pump works

Water finds the low spot, so we give it one. A sump is a low spot made on purpose, with a pump standing in it, and the whole system is no more than a hole, a switch that floats, and a pipe that only lets water travel one way.

The pit

The City of Sturgis, South Dakota, describes a sump pump as a small pump installed in a specially designed pit in the lowest part of a basement or crawl space, whose job is to keep the area under the building dry and so prevent flooding and damage to the foundation. When rain or snowmelt raises the groundwater, the water flows into the pit instead of onto the floor.

It gets there through drain tile. The City of Fargo’s explanation: drain tile is a perforated pipe that directs the flow of water to the sump pit, and the pit holds the water until it reaches a level that can be pumped out.

The float

North Dakota State University’s extension engineers describe the control: a float-activated switch controls the on and off operation of the pump. When the water in the sump rises to a certain level the pump turns on, and when the water drops to a certain level it turns off. A float switch that cannot move cannot do its job, which is why Zoeller’s installation instructions for its Aquanot Fit battery backup system want the pump level and the float clear of the sides of the basin, and why the City of Ann Arbor’s maintenance checklist asks for a float that moves freely, not caught on the walls of the sump or the discharge pipe. That is a thing to watch, not to feel for: Zoeller’s instructions say to disconnect the pump from its power source before touching any component in the sump pit.

NDSU names the two basic kinds of pump. A submersible is designed to sit under water on the bottom of the sump. A pedestal pump, also called an upright, has its motor usually about a foot above the top of the sump, where it is not meant to get wet, with the pump itself at the base. Either works well with proper maintenance.

The check valve, and the small hole below it

A pump lifts a column of water up the discharge pipe and then shuts off. Without something to hold it, that column falls back into the pit and the pump has to lift it again. NDSU: either type of pump should have a check valve on the water outlet pipe so water does not flow back into the sump when the pump shuts off.

Zoeller, a pump maker, adds a detail that puzzles people who see it for the first time. For a submersible pump it recommends, in most cases, a weep hole in the discharge pipe just above the pump and below the check valve, to keep the pump from air locking, which is when air in the pipe prevents the flow of water. A small stream of water spraying from the weep hole while the pump runs is normal; the City of Ann Arbor’s maintenance checklist says the spray should be there; it is how the hole prevents air lock. The hole is there on purpose and is meant to stay clear; it is not a leak to plug. Zoeller’s guide drills it during installation, before the pump goes into the pit. The same checklist, which starts with the pump unplugged, says to add one if it is missing and to clear it if it is blocked. Neither is a job to do over a pump that is plugged in.

Where the water goes

Out, away, and not into the sanitary sewer unless your city says it may. Sturgis’s guide for homeowners says the groundwater must be discharged to the ground surface outside, to a storm drain, to a sump pump collection system or to a natural outlet, and that the end of the hose must be far enough from the building that the discharged water does not cycle straight back to the pit.

The sanitary sewer is the place it may not go in Sturgis, by city ordinance, and Fargo explains why cities care. The sanitary system does not have the capacity to carry that excess clear water; thousands of extra gallons arriving at once can back sewers up or force a treatment plant to release partially treated water into local streams and rivers. Sturgis lists other limits: not over a public sidewalk, where a hose is a hazard and blocks the walking path, and not onto a neighbor’s property. It also explains what a discharge running into the street can do: algae in spring and summer, and icy conditions in winter.

Rules on sump discharge are local, and they do not all agree: NDSU notes that some cities allow a sump pump to discharge into the sanitary sewer before March 31, for a fee. Before rerouting a discharge line, ask your city’s public works or stormwater office what it allows.

Why pumps fail on the worst night

FEMA puts the problem in one sentence: a basement can flood when the primary pump stops working, when the power goes out, or when torrential rain overloads the pump beyond its capacity. Zoeller’s installation guide names the catch: the power may go out when a harsh storm hits, and a pump that runs only on household current can stop just when flooding is most likely.

The Insurance Institute for Business and Home Safety (IBHS) found the same pattern. In its study, power outages caused 18 percent of the water damage incidents involving a sump pump, and another 40 percent were attributed to things such as a clogged inlet screen or a faulty float switch. NDSU adds the mechanical causes: damaged or rusted bearings in the motor, and a float switch that does not make good electrical contact and turns on slowly or not at all. The City of Ann Arbor’s list of reasons a primary pump fails adds a check valve that fails and a discharge pipe blocked by debris or by freezing.

The standard answer to the power problem is a battery backup pump. FEMA calls it an inexpensive safeguard that goes to work when the water in the sump reaches its float switch, and IBHS says to choose a system with a battery replacement warning and to replace the batteries on a schedule; the maker’s manual sets it. The swap itself follows the maker’s instructions; Zoeller’s manual has the charger unplugged from the wall before a lead comes off the battery.

For the night the water wins anyway, FEMA’s basement guide says to mark the breaker box for the circuits that feed the basement and to shut off the power before entering a flooded basement. Ready.gov’s flood page sets the limits on that: turn off the electricity only if it is safe to do so, and do not touch electrical equipment if it is wet or if you are standing in water. A flood safety notice from the City of Fargo, Cass County and three electric utilities goes further: never work a breaker or switch that is wet or under water, even just to turn it off, and call the electric utility, which will disconnect the service at your request. If the breaker box is wet or has been in the water, or the only way to it is through the water, stay out of the basement and make that call. The notice is stricter than FEMA in one more way: once the electrical system has been flood damaged or even exposed to flood water, it says not to try to disconnect your own power at all. We take the stricter side: if the water has reached the outlets, the wiring or the panel, leave the switching off to the utility.

Switching it back on is not the homeowner’s call either. The notice says that once any part of the electrical service has been damaged by or exposed to flood water, or the service has been disconnected because of the flooding, it must be inspected before it can be reconnected, and that repairs to flood-damaged electrical equipment must be done by a licensed electrician. That is the rule it gives for North Dakota; elsewhere, ask your local building department what it requires before the basement circuits go back on.

The electrical side

A sump pump is an electric appliance standing in a pit of water, so the outlet matters. Zoeller’s instructions are blunt: do not use an extension cord with a sump pump. They recommend connecting the pump to a separately protected and properly grounded circuit. Zoeller’s consumer guide says the pump should have its own GFCI outlet, and where there is no such outlet near the pit, to have an electrician set one up. The code is less tidy, and our sources do not agree. A summary of the 2020 edition of the National Electrical Code by the U.S. Virgin Islands Department of Planning and Natural Resources lists sump pumps rated 150 volts or less to ground and 60 amperes or less among the appliances that need GFCI protection, which can sit in the breaker, in a device or outlet on the circuit, in the plug, in the cord within 12 inches of the plug, or in the pump itself. The City of Ann Arbor’s maintenance checklist says a GFCI is generally not required on a dedicated single receptacle for a sump pump, without naming the code or the edition it has in mind. The code is a model: CPSC’s GFCI fact sheet notes that it may be adopted into law by states, counties or local jurisdictions and that it is revised every three years, and Zoeller’s manual says the National Electrical Code and all applicable local codes must be followed, so the edition your town has adopted, with any local rules, is the one that applies; ask the local building department which that is. Ann Arbor adds a caution that holds either way: a GFCI can trip and leave the pump off, so if the pump is on one, check before a storm that the GFCI is on and the pump runs. Our room-by-room guide covers where else the code asks for it.

One more rule from Zoeller’s manual, before you ever reach into the pit: always disconnect the pump from its power source before handling it, and with a battery backup system, unplug its power supply too before handling either pump. Do none of it standing in water: Ready.gov’s flood page says not to touch electrical equipment if it is wet or if you are standing in water.

Testing it

NDSU’s spring routine is the one the Almanac uses in March:

1. Check that the discharge pipe outside is not plugged and that it directs water away from the house.
2. Slowly pour water into the sump until the float turns the pump on.
3. Make the pump turn on and off at least twice.
4. Listen. If the pump sounds like it is pumping gravel, or the float is sluggish, NDSU says you may need a new pump or float. Watch with your hands out of the pit: Zoeller’s manual says to disconnect the pump from its power source before touching any component in the sump pit. When you are done, put the cover back on: the same manual calls for one on every installation, to keep debris out and to prevent accidental injury.

The City of Ann Arbor’s maintenance checklist goes one level deeper. It puts the manufacturer’s instructions first and has the pump unplugged before the inspection starts: look in the pit for silt or debris that could block the float or clog the pump, check that the weep hole is open, and check that there is a check valve on the line just above the cover; if there is none, Ann Arbor says to have a licensed plumber add one. IBHS adds clearing any debris that may be blocking the pump’s inlet screen. Then plug the pump back in, check that its breaker is on, and run the pour test again.

Zoeller says that with proper installation and maintenance its pumps can last 7 to 10 years on average, with actual life depending on use.

What is yours, and what is a pro’s

Yours, on a dry floor and never in a flooded basement (the power rules for that night are above, and they leave the switching off to the utility once the water reaches the outlets, the wiring or the panel): the pour test, keeping the cover on, checking that the outside end of the discharge line is open, clearing debris from the pit and the inlet screen only after the pump is unplugged, and replacing a backup battery on the maker’s schedule and by the maker’s instructions, with the backup system unplugged from the wall first; Zoeller’s manual warns that batteries contain acid, that caution must be taken in handling them, and that a wrench or any other metal object must never short across the terminals.

A pro’s: a pump that fails the test (IBHS says to contact a plumbing professional); a missing check valve (Ann Arbor says a licensed plumber should add one); putting in a new pump or a backup system; repair and service of a battery backup system itself, which Zoeller’s Aquanot Fit manual sends to an authorized service station; and any new circuit or outlet for the pump, or an outlet that a circuit tester shows is wired wrong, which Zoeller’s manual and FEMA’s basement guide hand to a licensed electrician. On putting in a pump our sources do not agree, so here is who says what. Zoeller’s own installation guide calls it a project you can take on yourself, and Ann Arbor’s guide to backup options says a handy person can usually swap in a spare pump, kept ready with its own riser pipe and check valve, in just a few minutes. Zoeller’s FAQ allows that some homeowners feel comfortable doing it, but recommends professional installation, since a wrong installation can lead to pump failure and water damage; and Ann Arbor’s guide says a battery backup installation generally requires a permit from the local building department. We take the stricter side and leave it to a pro. NDSU adds that a homeowner who is not familiar with sump pumps can have a licensed plumber check that the pump is ready before the spring thaw. Sewage backing up through toilets, sinks and other drains is a different problem with a different device: FEMA says to have a licensed plumber install sewer backflow valves.

Related

• Almanac: March, test the sump pump before snowmelt and spring rain
• How a GFCI trips, for the outlet the pump plugs into
• Glossary: sump pump, float switch, check valve, GFCI