Sump pump guide
How Does a Sump Pump Work? The Whole System, Pit to Outlet
A sump pump sits in a pit at the lowest point of the basement. Drain tile around the footing carries groundwater into that pit. As the water rises it lifts a float switch, the pump's impeller spins and pushes the water up a discharge pipe, through a one-way check valve and outside. When the level drops, the float turns the pump off.
In one line each
The eight parts of a sump system
- Drain tile collects water; the basin (pit) holds it.
- The float switch decides when the pump runs.
- The impeller pushes water up the discharge pipe.
- The check valve stops it falling back; the outlet sends it away.
- A backup and an alarm cover power cuts and failures.
The pump only moves water that reaches the pit. It does not stop water getting in.
Short answer
One path, eight parts
A sump pump is the last step in a drainage system, not the whole system. Groundwater around the foundation is collected by drain tile, a perforated pipe at the footing, and runs downhill to a sump basin (the pit) set into the basement floor. When the water in the pit rises, it lifts a float switch. The switch starts the pump, whose spinning impeller throws the water up the discharge pipe. The pipe passes through a one-way check valve, leaves the house and ends at an outlet that sends the water away from the foundation. When the level falls a few inches, the switch drops and the pump stops.
That is one cycle. In a dry month it may happen a few times a week; in a wet spring or on a high water table it can happen every few minutes. A backup pump and a high-water alarm sit beside the main pump for the times it cannot keep up or loses power.
Every sump pump problem is a problem with one of these eight parts, which is why the diagram numbers them. The sections below take them in order, then look closely at the part most people ask about: the basin.
Last reviewed: September 24, 2026
Step by step
What each part does
Two to four sentences per part, and where to go when that part is the problem.
1Drain tile: where the water comes from
Drain tile is perforated pipe laid in gravel beside the footing, either outside the foundation (installed when the house was built) or inside, under the edge of the slab (usually added later as interior waterproofing). Water in the soil moves toward it because it is the easiest path, enters through the holes and runs along the pipe to the pit. Groundwater pushing up under the slab also drains through the gravel to the pit. This is what relieves the hydrostatic pressure that would otherwise push water through the floor and wall joint.
Some pits have no drain tile at all and only collect water rising through the floor under them. Those pits can lower the water table close by, but they do little for a wet wall 30 feet away. See basement drainage systems and sump pump vs. French drain for how the collection side is built.
2The basin (sump pit): where the water waits
The basin is a plastic or fibreglass liner, most often 18 or 24 inches across, set into a hole in the slab so that its rim sits at floor level. Drain tile enters through the side, often through a knock-out or a rubber grommet. The basin gives the pump a clean, stable place to sit and holds enough water for a proper cycle. The basin section below covers sizes, depth and lids in detail.
3The float switch: the part that decides
The switch is the brain of the pump, and its most common point of failure. InterNACHI notes that a jammed float is one of the most common reasons sump pumps fail. As the water rises it lifts the float until the switch closes and starts the motor; as the pump draws the level down, the float falls and opens it. The distance between the on and off points (the swing or differential) sets how much water each cycle moves. There are four common types, compared in a table below.
4The pump and impeller: the part that lifts
Inside the pump an electric motor spins an impeller, a small wheel with curved vanes. Water enters at the centre through a screened intake at the bottom, is flung outward by the vanes and forced up the discharge outlet. Most residential pumps are submersible: the motor sits sealed in the water, which cools it. A pedestal pump keeps its motor on a column above the pit and only the impeller end sits in the water. The submersible vs. pedestal comparison covers the trade-offs.
Pumps are rated in horsepower, but the useful number is flow at your lift: the height from the water in the pit to the highest point of the discharge plus friction in the pipe. A pump's flow at zero lift can drop sharply by the time it is lifting water ten feet. The sump pump sizing calculator works this out from your pit and lift.
5The discharge pipe: the way out
A rigid pipe, usually 1¼ or 1½ inch PVC, rises from the pump and turns toward the nearest wall, typically leaving the house through the rim joist above grade. Every foot of height and every elbow reduces the pump's flow, so the shortest, straightest route is best. Outside, the pipe may be extended to a splash block, a buried line to daylight, or a dry well, depending on the lot and local rules.
6The check valve and weep hole: the one-way door
When the pump stops, the column of water in the pipe wants to fall back into the pit. A check valve on the riser, usually just above the basin lid, closes and holds it there. Without one, the pump re-pumps the same water and starts more often. Below the valve, inside the basin, many manufacturers call for a small relief or weep hole: Zoeller's installation guide specifies 3/16 inch, drilled between the pump and the check valve, so trapped air can escape and the pump does not air-lock. The check valve guide shows exactly how much water a failed valve sends back.
7The exterior outlet: where the water should end up
The point of the whole system is to put the water somewhere it will not come straight back. InterNACHI suggests discharging at least 20 feet from the foundation and never toward the house or a neighbour's property. Many towns do not allow sump water in the sanitary sewer, and some have rules about discharging to the street; check locally. A discharge that dumps next to the wall simply recirculates water through the soil to the drain tile and back to the pit. In freezing climates the outside line needs to drain fully or it can freeze and block the pump.
8Backup pump and high-water alarm: the insurance
The main pump fails for three reasons: it loses power, a part fails, or the water arrives faster than it can pump. A battery backup pump sits in the same pit, set to start a little higher than the main pump, and runs off a deep-cycle battery. A water-powered backup uses municipal water pressure instead and needs no battery. The battery vs. water-powered backup comparison covers which suits which house. A high-water alarm is a cheap separate sensor that sounds when the water passes the point where the main pump should have started; if it is going off, see sump pump alarm going off.
The pump itself plugs into a nearby outlet; Zoeller's installation guide calls for its own GFCI outlet and no extension cord. Electrical work belongs to an electrician.
The pit
The sump basin (sump pit): sizes, depth and lids
Most sump pump problems that are not the switch start with a basin that is too small, too shallow or open to the room.
Pit or basin? People use both words for the same thing. Strictly, the pit is the hole in the slab and the basin is the liner that sits in it, but a “sump pit” in a listing almost always means a lined basin. Old houses sometimes have a bare concrete or clay-tile pit, which works but lets gravel and silt reach the pump.
How water gets in. Drain tile enters through the side of the basin, below the slab. Many basins also have perforated walls or holes in the bottom so water in the surrounding gravel can seep in; those are normally wrapped in filter fabric or set in washed stone to keep soil out. A solid-wall basin only fills from the drain tile inlets. If your pit has water coming in faster than you expect in dry weather, the sump pump running constantly guide shows how to tell groundwater from a failed check valve or a leaking supply line.
| Basin (diameter × depth) | Gallons per inch | Nominal capacity | Where it fits |
|---|---|---|---|
| 18 × 22 in | 1.1 gal | ≈ 24 gal | The small retail basin. Fine for a light-duty pit; little room for a tethered float or a second pump. |
| 18 × 30 in | 1.1 gal | ≈ 33 gal | A common upgrade on the 18-inch diameter. Extra depth gives the pump more water below the inlet. |
| 24 × 24 in | 2.0 gal | ≈ 47 gal | Wide and shallow. Room for a backup pump; used where digging deeper is difficult. |
| 24 × 36 in | 2.0 gal | ≈ 71 gal | A frequent choice for new interior drainage systems: room for primary plus backup and a longer cycle. |
| 24 × 48 in | 2.0 gal | ≈ 94 gal | Deep pits for wet sites or deep drain tile inlets; more reserve before the alarm level. |
Nominal volumes for a straight-sided round basin: π × radius² × depth ÷ 231 cubic inches per gallon. Real basins taper toward the bottom and the pump takes up space, so usable volume is lower. Sizes are those most often listed by manufacturers such as Zoeller.
How deep should a sump pit be?
InterNACHI suggests a pit at least 24 inches deep and 18 inches wide. Zoeller's basin guidance frames it by the inlet: the total depth is the depth of the drain tile inlet plus the working depth needed below it, and it suggests about 24 inches of depth in the bottom of the pit so the pump gets enough run time without short-cycling. That is why a 22-inch retail basin often ends up as the shallow compromise in a quick install, and why drainage contractors commonly set 30- to 36-inch basins.
Two practical rules follow from the arithmetic. First, the pump should sit fully submerged when it is off; Zoeller advises choosing a basin that lets the pump submerge entirely. Second, the pump should switch off before the water drops to the intake, and switch on before the water backs up into the lowest drain tile inlet. If the on point is above the inlet, water stands in the drain tile every cycle and silts it up.
Should the pump sit on bricks? Some installers raise the pump on a brick or paver to keep it above silt on the pit floor; others leave it on the basin bottom so it pumps the pit lower. Follow the pump's manual, keep it level, and make sure the float has room on every side to move freely, which Zoeller's installation guide also stresses.
18-inch or 24-inch basin?
An 18-inch basin holds about 1.1 gallons per inch of depth; a 24-inch basin holds about 2.0, nearly twice as much, because volume grows with the square of the radius. The wider basin means longer, fewer cycles for the same inflow (see the next section), room for a backup pump beside the main one, and room for a tethered float to swing. The 18-inch basin is cheaper, needs a smaller hole in the slab, and is enough for a pit that runs a few times a week.
Does a sump pit need a lid?
Yes. A lid keeps children, pets and debris out, and stops the pit from adding moisture and odour to the basement air. A sealed, gasketed lid with sealed pipe and cord penetrations goes further: it is standard where the pit is part of a radon mitigation system and it stops sewer-like or musty odours from the pit reaching the room. If the pit smells, the sump pump smell guide covers the causes, and a pit that is open to the room is a common reason a basement stays humid (see basement humidity targets).
Real numbers
One pump cycle, in gallons and minutes
How much water one cycle moves, and how often the pump starts, depends on only three things: basin size, switch swing and inflow.
The arithmetic. Water in a round basin is π × r² per inch of depth. For an 18-inch basin the radius is 9 inches: 3.1416 × 9 × 9 = 254.5 cubic inches per inch of depth, and 254.5 ÷ 231 = 1.1 gallons. For a 24-inch basin: 3.1416 × 12 × 12 = 452.4 cubic inches, or 2.0 gallons per inch.
Multiply by the switch swing to get the water moved per cycle. Zoeller's data sheet for its M53 pump, a common vertical-float model, lists an on point of 7¼ inches and an off point of 3 inches: a 4¼-inch swing. In an 18-inch basin that is 4.7 gallons a cycle; in a 24-inch basin, 8.3. A tethered float can be set for a longer swing; the table uses about 10 inches as an illustration.
| Basin and switch | Water per cycle | Inflow 1 gpm (light seep) | Inflow 5 gpm (steady rain) | Inflow 10 gpm (heavy) |
|---|---|---|---|---|
| 18-in basin, vertical float, 4¼ in swing | 4.7 gal | 12 per hour | 53 per hour | 85 per hour |
| 18-in basin, tethered float, ~10 in swing | 11.0 gal | 5 per hour | 23 per hour | 36 per hour |
| 24-in basin, vertical float, 4¼ in swing | 8.3 gal | 7 per hour | 30 per hour | 48 per hour |
| 24-in basin, tethered float, ~10 in swing | 19.6 gal | 3 per hour | 13 per hour | 20 per hour |
Cycles per hour = 60 ÷ (fill time + run time). Fill time = gallons ÷ inflow; run time = gallons ÷ (pump flow − inflow), assuming the pump delivers about 30 gpm at its working lift. Illustrative: your pump's flow, basin taper and switch settings will differ.
Two things stand out. The same 5-gallon-a-minute inflow starts a pump in an 18-inch basin with a short vertical float about 53 times an hour, but a pump in a 24-inch basin with a long tethered swing about 13 times. And the run time for each cycle is short, often well under a minute, so a pump that seems to be “always running” is usually starting very often rather than running continuously.
That matters because starts are what wear a pump out: each one heats the motor and works the switch. It is the reason a bigger basin, a working check valve and a pump that is not oversized all make a pump last longer. The sump pump lifespan guide turns these cycle counts into years.
Switches
Float switch types compared
| Switch | How it senses water | Room it needs | Typical swing | How it usually fails |
|---|---|---|---|---|
| Tethered float | A float on a short cord tips up as the water rises; a switch inside closes at an angle. | Most: it swings in an arc, so it suits 18-inch basins only if nothing is nearby, and 24-inch basins best. | Long and adjustable by tether length. | Catches on the pit wall, the pump or the backup pump; cord tangles; lid knocks it. |
| Vertical float | A float slides up a rod and trips a switch at the top. | Little: it moves straight up and down. | Short and fixed (4¼ in on the Zoeller M53). | Float sticks on the rod from scale or debris; short swing means frequent starts. |
| Electronic | Solid-state sensor or probes detect water with no moving float. | Little. | Fixed by design (about 7 in on Zoeller's floatless switch). | Sensor fouled by scale or iron deposits; controller electronics. |
| Diaphragm (pressure) | A sealed diaphragm on the pump body flexes under the weight of water above it. | Little; built into the pump. | Short and fixed. | Diaphragm clogs or stiffens with sediment; not always replaceable on its own. |
Swings are examples from manufacturer data where stated; others vary by model. Check your pump's manual for its on and off points.
Many pumps come with the switch built in; others use a separate “piggyback” switch whose plug the pump plugs into. A piggyback switch can often be replaced without replacing the pump. Zoeller advertises its solid-state floatless switch as tested to more than a million cycles, which gives a sense of how many starts a switch is expected to survive.
Types
Submersible, pedestal and backup pumps in brief
- Submersible. Motor sealed in the water; quieter, handles more debris, fits most basins. Commonly lasts about 7 to 10 years.
- Pedestal. Motor on a column above the pit; louder, easy to service, fits narrow pits, often lasts longer because the motor stays dry. Full trade-offs on the submersible vs. pedestal page.
- Battery backup. A second 12-volt pump in the same pit that starts if the water rises past the main pump's on point. Runtime is limited by the battery.
- Water-powered backup. Uses city water pressure through a venturi to lift pit water; needs no battery but uses water and needs adequate supply pressure, so it rarely suits a house on a private well. See battery vs. water-powered backup.
- Combination units. A primary and battery backup sold as one assembly on a shared discharge, with separate check valves.
Troubleshooting map
Which part fails, and what it looks like
Match what you see to the part, then go to the guide for that part.
| Part | What you notice | Where to go |
|---|---|---|
| Float switch | Water rises and the pump stays silent; lifting the float by hand starts it. Or the pump never stops. | Sump pump not working · running constantly |
| Check valve | The pit refills an inch or two the moment the pump stops; a bang in the pipe at shutoff; short cycles in dry weather. | Sump pump check valve |
| Discharge line | Pump runs, the pit barely drops; in winter, ice at the outlet or a pump that hums against a blockage. | Frozen discharge line |
| Motor or impeller | Humming without pumping, grinding, tripping the GFCI, or a pump that runs hot. Often age-related. | How long sump pumps last · replacement cost |
| Power | Everything stops in a storm outage, or a tripped GFCI leaves the pump dead with no warning. | What to do when the pump fails · backup options |
| Backup battery or alarm | Beeping or chirping from the backup panel or the alarm; a low-battery or fault light. | Sump pump alarm going off |
| Pit and lid | A rotten-egg or musty smell near the pit, a dry pit with a smell, or debris in the water. | Sump pump smell |
| Capacity | The pump runs flat out in heavy rain and the water still gains on it. | Sump pump sizing calculator |
The fastest way to check every part at once is a bucket test: pour water into the pit until the float lifts, and watch the pump start, the level drop, the check valve close and the water leave the outlet outside. The how to test a sump pump guide walks through it.
Limits
What a sump pump can’t do
A sump pump removes water that reaches the pit. It does not stop water getting into the basement by other routes, and it is sometimes the wrong answer:
- Surface water above the drain tile. Water coming through a window well, a wall crack high on the wall or over the top of the foundation never reaches the pit. Start with water in the basement after rain.
- A pit with no drain tile. It only drains the soil right around it; wet walls elsewhere need collection. See do I need a sump pump.
- Sewer or floor-drain backups. Sewage coming up a floor drain is a plumbing problem, not a sump problem; call a plumber.
- Humidity and condensation. A pump moves liquid water; it does not dry the air.
- Power outages. A primary pump with no backup stops exactly when storms are worst.
If you are not sure where the water is coming from, the basement water source diagnostic narrows it down before anyone quotes a pump.
Upkeep
Keeping a sump pump working
- Test it before each wet season with a bucket of water, and after any long dry spell. InterNACHI recommends inspecting sump pumps at least annually.
- Keep the float free. Nothing within its travel: not the pit wall, the cord, the backup pump or the lid.
- Clear the intake and pit floor of silt and gravel once a year, with the pump unplugged.
- Check the outlet outside is clear, draining away from the house, and not frozen in winter.
- Test the backup and alarm when you test the main pump; replace backup batteries on the maker's schedule.
- Know its age. Write the install date on the pipe. Plan a replacement before it fails, not after; the replacement cost guide has planning ranges.
Thinking about a first pump? Start with do I need a sump pump, then sump pump installation cost and what a sump pump installation involves.
Related pages
Where to go next
Sump Pump Not Working
Diagnose a pump that won't start, won't stop or won't pump.
Read nextSump Pump Running Constantly
Groundwater, a bad check valve, or a stuck switch.
Read nextHow Long Do Sump Pumps Last?
Lifespan by type and how cycle counts shorten it.
Read nextSump Pump Check Valve
How much water a bad valve sends back.
Read nextSubmersible vs. Pedestal Sump Pump
Noise, lifespan, pit fit and cost.
Read nextBattery vs. Water-Powered Backup
Which backup suits your house.
Read nextSump Pump Smell
What an odor from the pit means.
Read nextSump Pump Sizing Calculator
Flow at your lift, from your pit measurements.
Read nextFAQs
How sump pumps work: FAQs
Where does a sump pump pull water from?
From the sump pit itself. Groundwater reaches the pit through drain tile (perforated pipe) laid around the footing, inside or outside the foundation, and through the gravel under the slab, which is why many basins have perforated sides. The pump only removes water that has already reached the pit; it does not pull water out of the walls or the yard.
How does a sump pump work in a basement?
The pit is set into the basement floor at the low point of the drainage system. Water drains into it, lifts the float switch, and the pump pushes it up the discharge pipe, through a check valve, and outside to a point that drains away from the house. When the level drops a few inches, the switch turns the pump off and the cycle repeats as long as water keeps arriving.
Can a basement still flood with a sump pump?
Yes. The most common reasons are a power outage during a storm, a stuck or tangled float switch, a pump that is too small for the inflow, a frozen or clogged discharge line, and water that never reaches the pit, such as surface water coming through a window well or a wall crack above the drain tile. A battery or water-powered backup and a high-water alarm cover the first two.
Is it a red flag if a house has a sump pump?
Not by itself. Many houses are built with a sump pit because the lot or water table calls for one; where foundation drains cannot drain to daylight by gravity, building codes allow them to discharge by mechanical means, which in practice means a sump pump. What matters is the history: ask how often it runs, how old the pump is, whether there is a backup, and whether the basement has ever taken water. A pump that runs constantly in dry weather is worth investigating.
How many years does a sump pump usually last?
Commonly about 7 to 10 years for a submersible pump and 10 to 15 for a pedestal pump, and InterNACHI's life-expectancy chart lists 7 years. How often the pump starts matters more than its age, so a pump in a pit that cycles all day wears out sooner.
Does a sump pump need a basin?
Yes. The basin holds enough water for the pump to run a proper cycle, keeps gravel and soil away from the intake, gives the float room to move, and supports a lid. A pump set in a bare hole clogs with sediment and short-cycles. InterNACHI suggests a pit at least 18 inches wide and 24 inches deep.
How deep should a sump basin be?
Deep enough that the pump sits fully under water with room below the lowest drain tile inlet for a full cycle. InterNACHI gives 24 inches deep and 18 inches wide as a minimum; Zoeller's basin guidance also points to about 24 inches of depth below the inlet so the pump gets enough run time and does not short-cycle. Deeper pits of 30 to 36 inches are common with interior drainage systems.
Can I use a 5 gallon bucket as a sump pump basin?
Only as a temporary measure. A 5-gallon bucket is about 12 inches across and holds roughly half a gallon per inch of depth, so a pump in it would start and stop every few seconds in any real inflow, and a float switch may not have room to move. A standard 18-inch basin holds about 1.1 gallons per inch, and a 24-inch basin about 2.
How does a sump pump float switch work?
A float rides on the water. On a tethered switch the float tips as the water rises and a switch inside it closes; on a vertical switch the float slides up a rod and pushes a switch at the top. Electronic switches sense water with probes or a solid-state sensor, and diaphragm switches sense the pressure of the water above them. In each case the pump starts at the high setting and stops at the low one.
Sources
Sources
- InterNACHI — Sump Pumps (pit at least 24 in deep and 18 in wide; jammed floats; check valves; discharge distance)
- Zoeller — How to choose the best size basin for your sump pump (18×22, 18×30, 24×24, 24×48; depth below the inlet)
- Zoeller — Sump pump installation guide (3/16 in relief hole; float clearance; dedicated GFCI outlet)
- Zoeller — Model M53 sump pump (technical data: on 7¼ in, off 3 in; auto-reset thermal overload)
- Zoeller — FLOATLESS switch sheet FM3095 (tested over 1 million cycles; about 7 in differential)
Basin volumes and cycle counts are calculated for straight-sided round basins and an assumed pump flow; your basin, pump and switch settings will differ. This page is educational and does not replace an on-site inspection or a licensed electrician.