How Does a Sump Pump Work?

According to the statistics, around 60% of American homes experience basement flooding. This can be a result of heavy rains, snow melt, or ground water seepage from the water table. This makes a sump pump a vital part of any home in an area prone to flooding or in low-lying areas where the water table may rise to the level of a basement or crawl space below the floor. While these seemingly insignificant machines are an important part of most homes, how many of us understand how a sump pump works?

As knowledge is the key to understanding, this article is going to help you understand how a sump pump works. These are the unsung heroes of the modern home, working 24/7, without us even noticing what an import role they play in ensuring our most valuable asset, our home, remains free from water damage. Knowing how a sump pump works could end up saving you loads of cash. You will be better equipped to diagnose sump pump problems and, perhaps, be more capable when it comes sorting these issues out before spending your hard-earned cash on a repairman.

Any home built in an area where flooding and damp may pose a risk, a has sump drain at the lowest point. This will be a low lying area of a basement or crawl space. The purpose of this sump or drain is to provide a low area where unwanted water can accumulate. Whether it’s runoff water from flooding, or rising water from the water table, by force of nature, gravity will cause the water to end up in this pit.

The pit is constructed in such a manner that it can accommodate a pump, or at very least, a pipe that can be connected to a pump. The drain will be anything from 6” to 18” in diameter and will generally be around 2-feet deep; but can be deeper. In order to remove this water, a purpose built sump is required and these pumps have specific design features that make them equal the task of removing water from under a home. A discharge pipe is connected to the sump pump which takes the water from the sump and dispels it to a point where it can flow away from the home, preventing water damage to the foundation and walls.

Understanding the Basic Fundamentals of a Sump Pump

While not all sump pumps work in the same way, they have certain elements that are common to all types of sump pump.

Electric motor
Impeller pump
Automatic controls

In order to understand how a basement sump pump works, I’ll explain how each of these essential elements combine to ensure that your home is protected from water damage. I’ll then go on to discuss the types of sump pumps, and how they work.

Electric Motor

Most sump pumps are powered a 110 -120V AC electric motor. They usually have a standard household plug that can be quickly connected to a domestic electrical outlet, with a ground connection. As these pumps are always in contact with water, a Ground Fault circuit Interrupter (GFCI) is recommended to protect you and your family from electric shock.

The size of the electric motor used to power a sump pump ranges from approximately ¹⁄₅ HP up to 1 HP. It is important that the electric motor is correctly sized for the specific application at hand. The power of the pump serves two purposes, to lift the water to a level where it can be safely disposed of and to move a sufficient amount of water as to prevent flooding.

The vertical height (head) requirement of a pump is determined by the height (in feet) from the bottom of the drain pit to the highest point to which the water needs to be lifted.
The flow rate of the pump is specified in Gallons per hour (GPH). This is the volume of water the pump is able move in one hour.
The GPH rate is affected by the vertical lift. As the head increases, the volume (GPH rate) will decrease. A pump will move much more water per hour along a flat surface, compared to lifting it.
The power of the pump, therefore, needs to be calculated taking into account how much water needs to be removed and to what height this water needs to be lifted.
If the pump is under-powered, it wont remove the water fast enough which will result in flooding.
If the pump is overpowered, water will be removed too quickly. This will result in frequent cycling, causing the pump to switch on and off more than it needs to. This will cause premature pump failure. It may also result in the sump running dry, which could cause damage to the pump.

The Impeller Pump

Most sump pumps use a centrifugal pump. This type of pump makes use of a disc with ridges used to channel the water from the center outwards. Water is directed into the center of the disc, which is connected to the electric motor by means of a shaft. As the motor spins the disc, water is forced outwards where it leaves the pump chamber into a discharge pipe. This movement of the water causes a vacuum in the center of the disc, sucking in more water.

The volume of water that the pump is able to move, is relative to the size of the disc and how fast it rotates. A smaller disc, with a high RPM, will generate more pressure and be able to lift the water to greater height but won’t move as much water. These pumps will have a lower GPM capability with improved vertical lift. In contrast, a larger disc, at a lower RPM, will move more water with less pressure and won’t have significant lifting ability but will have a higher GPM spec.

Automated Controls

For user convenience, a sump pump should switch on and off automatically. This means that when the water level increases to a point where there is a danger of flooding, the pump should switch on. The pump should run for as long as possible, to shorten the run cycle. This prevents the pump from switching on and off too often. The pump must switch off before the sump is completely dry. When a pump spins with no water entering it, excessive heat from the friction, can damage the pump.

The most common method used to control the pump cycle is a float switch. This consists of a plastic ball that floats on the surface of the water. As the water rises, the ball is lifted. When the water level drops, the ball moves downwards with the water.

There are two types of float switches used for sump pumps:

Mechanical Float Switches are probably the most common. This switch has a float ball connected to a lever which activates a switch inside the pump. When the ball reaches the highest position (the sump is full), it pulls the lever up and switches the pump on. As the water is pumped out of the sump, the water level drops, moving the ball downwards. At the lowest point, the lever is pulled down to a point where it switches the pump off.
Magnetic Float Switches use a metal ball inside the float. As the float moves up, the ball rolls down the float, activating a magnetic switch at the bottom of the float. Only when the float is at the highest point, will the ball reach the bottom of the float, switching the pump on. As the water level drops, the ball moves in the opposite direction, causing the pump to switch off when the float reaches the lowest point.

In most cases, a float switch can be set for a top limit and lower limit. This allows the user to determine when the pump is switched on or off, according to the depth of their sump drain.

Type of Sump Pump

There are two basic types of sump pump, a submersible or pedestal sump pump.

A submersible sump pump is placed inside the drain, the pump is always in the water. Whereas a pedestal sump pump is placed outside the sump, with a pipe leading into the drain.

While submersible sump pumps are the most popular, there are benefits and disadvantages to both types.

Submersible sump pumps are usually more expensive and tend not to last as long as pedestal pumps.
As they are surrounded by water, submersible pumps are quieter.
Submersible sump pumps are seen as the safer option because they are in the pit and cannot be touched or bumped into.

Battery Backup Pumps

In order to ensure continued operation during a power outage, a battery system can be used to provide emergency power to the sump pump.

This can be achieved by using two pumps. A dual pump backup system will use a primary AC pump when the mains power is functioning and a second DC pump, powered by a rechargeable battery, when the mains power fails. A battery charger will automatically recharge the battery when the mains power is restored. Battery-powered DC sump pumps are generally less powerful than the primary AC pump and will not be as effective for removing larger volumes of water.

An inverter can also be used to convert the DC power from a backup battery to AC power which will supply the same AC pump that is used when the mains power is functional. These systems will also recharge the battery when the mains power is restored. While an inverter system is often more expensive than a dual backup pump system, there are some benefits. Using only one pump means less space is required, which is of particular significance for a submersible sump pump which needs to fit into a confined space inside the drain. The pump will be just as effective whether using battery or utility power.