A sump pump is a submersible pump that sits in a pit, called a sump, at the lowest point of a building or site. When groundwater or surface water collects in the pit, the pump switches on automatically and pumps it away before it can flood the space. Sump pumps protect cellars, basements, lift pits, plant rooms and underground car parks across the UK.
This guide explains what a sump pump is, how it works, the main types, and how to tell whether you need one. It also covers sizing a pump to the water coming in, where the water can go, and the mistakes that cause most sump pump failures. It is written for facilities managers, contractors and property owners responsible for below-ground spaces.
What Does a Sump Pump Do?
A sump pump keeps a below-ground space dry by removing water as fast as it arrives. Water reaches the sump through the floor, through a drainage channel or cavity drain membrane, or as run-off from outside. The pump lifts it up and out to a drain at a higher level, where it can flow away by gravity.
Typical places you will find a sump pump include:
- Cellars and basements, especially those converted into living or working space.
- Lift pits, where groundwater or a leak would otherwise damage the lift equipment.
- Plant rooms and underground car parks that sit below the level of the surface water drains.
- Light wells and sunken yards that collect rainwater with no gravity outlet.
- Cavity drain membrane systems, where water behind the membrane is channelled to a sump and pumped away.
A sump pump is for clear or slightly dirty water only. It is not designed for toilet waste, which needs a sewage pump. Our comparison of sewage pumps and sump pumps explains why the two are not interchangeable.
How Does a Sump Pump Work?
Every sump pump installation follows the same simple cycle. Water drains into the pit, the level rises until it lifts a float switch, the pump starts and empties the pit, and the float drops and stops the pump. A non-return valve on the discharge pipe stops the water in the pipe from falling back into the pit when the pump stops.
Float Switches and Level Controls
The float switch is what makes a sump pump automatic. Tethered floats swing up on a cable and need room in the pit to move. Vertical floats ride up and down a fixed rod, which suits narrow pits. Some pumps use electronic level sensors with no moving parts. A pump without any float is a manual pump and only runs when someone switches it on, which suits temporary pumping but not unattended protection. Our float switch guide compares the options.
Where the Water Goes
The pump discharges through a rising pipe to a point where water can drain away by gravity: usually a surface water drain, gully or soakaway. Groundwater and rainwater should normally go to surface water drainage rather than the foul sewer. Check with your building control body and, for any connection to a public sewer, the water company before deciding on a discharge point. Discharging next to the building simply sends the water back into the ground around the space you are trying to keep dry.
Types of Sump Pump
Most sump pumps sold in the UK are submersible, but they vary in size, materials and controls. The table summarises the main choices.
| Type | How it works | Best for |
|---|---|---|
| Automatic submersible | Built-in or attached float switch starts and stops the pump | Permanent protection of cellars, lift pits and plant rooms |
| Manual submersible | Runs whenever switched on | Emergency pumping, site use, or pits controlled by a separate panel |
| Compact sump pump | Small body that fits narrow pits and can pump down to a few millimetres | Small pits, light wells and tight spaces |
| Stainless steel sump pump | Corrosion-resistant body for harsher water | Pits with aggressive or slightly dirty water, long service life |
| Duty/standby pair | Two pumps on a control panel that alternates them and covers a failure | Basements and buildings where flooding would be costly |
Pedestal sump pumps, with the motor mounted on a column above the pit, are common in North America but rarely used in the UK. If your pit is too small for a standard pump, a compact model such as the Pentair Flotec Compac is usually the answer. For harsher conditions, the Oreal CK stainless steel sump pumps are a durable choice.
Do You Need a Sump Pump?
You need a sump pump wherever water collects in a space that cannot drain away by gravity. The question is usually not whether water will arrive, but how often, how much, and what it would cost if the space flooded. These signs point to needing one:
- Water or damp in a cellar or basement after heavy rain, or a tide mark on the walls.
- A high water table, often found in low-lying areas, near rivers or on clay soils.
- A converted or new basement with a cavity drain membrane, which relies on a pumped sump to work.
- A lift pit, plant room or car park below the level of the nearest surface water drain.
- A property in a flood risk area. The Environment Agency's flood maps show the risk for any address in England.
You may not need one if the space has a gravity outlet that never backs up, sits above the water table and has stayed dry through wet winters. Even then, a portable pump kept on site is cheap insurance. Our guide to basement sump pumps goes into detail on cellars and basement conversions.
Do You Need a Sump Pump if You Have a Drain?
Often, yes. A floor drain or channel only helps if it has somewhere lower to drain to. If the drain itself falls to a sump below the level of the outside drainage, the sump still needs a pump. A drain connected to a sewer can also surcharge and back up in heavy rain, which is why many basements combine drainage channels with a pumped sump and a non-return valve.
How to Choose a Sump Pump
Three figures decide which pump you need: how much water comes in, how high the pump must lift it, and what is in the water.
Measure the Inflow
The most reliable way to size a sump pump is to measure how fast water enters the pit. With the pump switched off, time how long the water level takes to rise by a set distance, then work out the volume. The pump needs to clear that inflow comfortably at the head it will work against. Use the calculator below to turn your measurement into a flow rate.
Sump inflow calculator
Switch the pump off, measure how far the water rises in the pit and how long it takes.
For a round pit, enter the diameter as both length and width and the result will slightly overstate the volume. Measure in the wettest conditions you can, as inflow varies with the season.
Work Out the Head
Head is the height the pump must lift water, from the stop level in the pit to the highest point of the discharge pipe, plus an allowance for friction in the pipe and fittings. A pump's maximum head is the point at which it delivers no flow at all, so choose one whose curve gives your required flow at your actual head. Our guide to what size sump pump you need covers sizing in more depth, with worked examples.
Check What Is in the Water
Clear groundwater suits a standard sump pump. Water carrying silt, sand or leaves needs a pump with a larger free passage, such as a vortex drainage pump, and a pit that is cleaned regularly. Anything containing toilet waste needs a sewage pump, covered in our guide to how sewage pumps work.
Plan for Failure
A single pump is a single point of failure. Where a flooded basement or lift pit would be expensive or dangerous, fit a high level alarm as a minimum, and consider a second pump on a duty/standby panel or a battery backup for power cuts.
Common Sump Pump Mistakes
Most sump pump failures we see come down to installation rather than the pump itself:
- A pit too small for the float, so it snags on the wall or the pump cable and never switches on.
- No non-return valve, so the discharge pipe drains back into the pit and the pump cycles constantly.
- Discharging next to the building, so the same water soaks back through the ground into the pit.
- Silt and debris left in the pit, which blocks the intake and wears the impeller.
- No alarm, so a failed pump is only discovered when the floor is under water.
Sump Pump Maintenance and Lifespan
How long a sump pump lasts depends on how often it runs and what it pumps. A pump that runs a few times a year in clean water can last many years, while one cycling constantly in silty water will wear much faster. Test the pump by filling the pit with water every few months, clear debris from the pit and intake, and check the float moves freely. Our guides to cleaning a sump pump and cleaning a sump pit cover the job step by step.
Recommended Sump Pumps from AES Rewinds
Looking for a sump pump? At AES Rewinds, we stock a comprehensive range of sump pumps suitable for commercial, industrial and residential applications.
Browse our full range:
- Sump pumps in manual and automatic versions
- Sump pumps with float switch for unattended protection
- Small sump pumps for narrow pits and light wells
Our team can help you size a pump to your pit and discharge route. Contact us for expert advice on sump pump selection.
Frequently Asked Questions
What is a sump pump used for?
A sump pump removes water that collects in a pit at the lowest point of a building or site, such as a cellar, basement, lift pit or plant room. It switches on automatically as the water rises and pumps it away to a drain at a higher level.
Where does a sump pump get its water from?
Mostly from groundwater rising through the floor or walls, water channelled to the pit by a drainage membrane or floor channels, and rainwater running into light wells or sunken areas.
Where should a sump pump discharge to?
To a surface water drain, gully or soakaway well away from the building, so the water cannot soak back into the ground around the space. Check with building control and, for a public sewer, the water company before connecting.
Can a sump pump pump sewage?
No. A sump pump is designed for clear or slightly dirty water and will block on toilet waste. Foul water needs a sewage pump with a larger free passage or a cutting mechanism.
Is a sump pump the same as a submersible pump?
Most sump pumps are submersible pumps, but not every submersible pump is a sump pump. A sump pump is a submersible drainage pump chosen to suit a pit, usually with an automatic float switch.
How often should a sump pump run?
That depends entirely on the inflow. A pump in a dry cellar might run only after heavy rain, while one on a high water table may run several times an hour. A pump that runs continuously without lowering the level is undersized, air-locked or blocked.
Key Takeaways
- A sump pump sits in a pit at the lowest point of a space and automatically pumps out water that collects there.
- A float switch makes it automatic, and a non-return valve stops the discharge pipe draining back into the pit.
- You need one wherever water collects below the level of a gravity drain: cellars, basement conversions, lift pits and plant rooms.
- Size the pump from the measured inflow and the real discharge head, not from maximum head figures.
- Fit an alarm, and use duty/standby pumps where flooding would be costly.
Related Articles
- Basement Sump Pump Guide: protecting cellars and basement conversions.
- What Size Sump Pump Do I Need?: flow, head and worked sizing examples.
- Sewage Pump vs Sump Pump: choosing the right pump for the water.
- Flood Pumps: How to Choose the Right Pump: portable and emergency options.
- Everything You Need to Know About Float Switches: level controls explained.