Sump Pump Float Switch Working Principle

How Does a Sump Pump Float Switch Work

A float switch

Figure 1: A float switch

A sump pump float switch controls the water level in a basement pit. The float switch alerts the sump pump to operate and push the water away from the pit when it fills up. This article discusses sump pump float switch types, repair, and selection criteria.

Table of contents

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What is a sump pump?

Basements contain a sump basin that accumulates excess water and moisture from the ground. A sump pump removes water collected in a sump basin. Sump pits solve dampness in areas where basement floodings are regular. The sump pump removes water from the sump pit and transfers it to dry wells or storm drains.

Float switch in a sump pump

A float switch attached to a submersible drainage pump.

Figure 2: A float switch attached to a submersible drainage pump.

A sump pump float switch detects flooding in the basement tank. The switch floats on the water's surface or within the water and moves according to the water level within the tank. When the water level rises, the float switch rises with it. When the water level rises high enough, the switch within the float closes and switches on the sump pump that discharges the tank. When the water level drops sufficiently low, the switch inside the float opens and stops the pump— this cycle repeats. Read our article on float switches for more details on the working and various types of float switches.

Sump pump float switch types

Float switch types are based on function and construction. The switch's function depends on its wiring, i.e., whether it is normally open or normally closed. The switch's construction can be tethered, vertical, diaphragm, or electrical. Any of these constructions can be wired to be normally open or normally closed.


Pump-up float switch

Pump-up float switches (normally closed) work with sump pumps that fill a storage tank. A pump-up float switch is at the top of the storage tank. As the water level increases, the float switch rises within the tank. The switch inside the float opens, turning the pump off and preventing further filling of the storage tank. The power for the sump pump first routes to the float switch at the top of the tank and then to the pump that fills the storage tank.

Pump-down float switch

Pump-down float switches (normally open) work with pumps that empty a tank. The float switch shuts off the pump before it runs out of water entirely and starts sucking air. The pump can lose its prime by sucking air, shortening its lifespan. Pump-down float switches are approximately 101 mm - 152 mm (4 - 6 inches) above the pump suction. The switch inside the float closes when the water level increases. The closed connection within the switch supplies power to the pump. The pump turns on and removes the water from the tank. As the water level drops, the switch inside the float switch falls down, the internal switch will open, and the pump will turn off.


Tethered float switch

A tethered float switch has a floating element that lowers and rises with the water level in the sump basin. The switch creates a closed circuit that triggers the pump when the level exceeds a preset height. The switch mechanism comprises a floating hollow body with a steel ball inside. The floating element attaches to the pump with a cord. When the float pivots, the internal ball moves due to gravity and operates an electrical switch. Changing the cord length determines the level range of the switch.

The main drawback of tethered switches is that they require a large basin to move up and down. The longer the cord, the larger the risk that the float tangles on things inside the basin. Read our article on float switch wiring for more information on how to wire float switches.

Tethered float switch

Figure 3: Tethered float switch

Vertical float switch

Vertical float switches are typical for smaller basins with a small level range. The most simple designs have a floating element that connects via a rod to a switch. When the floating element raises, the rod pushes against a lever that operates an electrical switch.

Other designs have a floating element that slides over the rod. The floating element contains a permanent magnet that operates the reed switches when it crosses predefined points in the rod.

Compared to tethered float switches, vertical float switches are suitable for small-sized basins with low depth. The space consumption is limited to the rod length. They can be more accurate than tethered switches.

Vertical float switch

Figure 4: Vertical float switch

Diaphragm switch

As the name suggests, a diaphragm switch has a diaphragm that deflects under pressure. When the level rises, the increase in pressure causes the diaphragm to deflect and the switch to operate.

Diaphragm float switch

Figure 5: Diaphragm float switch

Electronic switch

Electronic switches have almost no moving parts, increasing reliability significantly. The switch comprises probes at the length that the switch should turn on. A small current applies to the probes that start to flow when the probes are in the water. The sensor measures the electrical resistance and compares it with a preset value to detect water presence.

Electronic float switch

Figure 6: Electronic float switch

Choosing the right sump pump float switch

Apart from knowing whether the float switch should act in a normally open or normally closed position, there are several other considerations when selecting a suitable float switch for a sump pump.

  • Pump size: Check the float switch’s rating and ensure it matches the sump pump’s horsepower rating. Using a float switch on a sump power with a higher rating can shorten the lifespan of the switch. Pumps with higher horsepower ratings need a float switch with an internal switch capable of handling the higher power flowing through the circuit. To determine the necessary size of the float switch, check the horsepower/amperage rating of the sump pump.
  • Pump connector: Different voltages and countries have different connector styles, so it is necessary to verify that the connector for the float switch has the same style as the pump. A piggyback float switch connects the float switch’s plug to a standard power outlet. A piggyback plug has prongs on one side and an additional outlet on the other. A piggyback connector allows the user to unplug the cords from the wall, separate them, and plug the pump directly into the outlet without the switch, thereby allowing it to test the pump by itself.
  • Cord length: Connecting multiple cords to create an electrical connection within a wet environment can pose serious safety issues. Make sure to get a float switch with enough cord length.
  • Wide or narrow-angle float switch: Narrow-angle float switches can cause the pump to turn on or off, even with minor changes in water level, like ripples. Turning the pump on and off rapidly can damage and shorten its operating life. For a wide-angle float switch, the water level needs to change by several inches to cause the float switch to either turn on or off.
  • Quality and type of liquid used: A float switch in potable water should have a suitable rating. Also, get a float switch for the correct temperature if installing the switch in hot liquids.

What causes sump pump float switches to fail?

Sump pump float switches fail over time due to various reasons.

  • External interference: The floating element tangles in the basin. Often it gets trapped between the pump and the basin wall and cannot rise or drop to the desired levels. This issue can be resolved easily manually.
  • Float switch wears out: The float switch may stop reacting to the rise and fall of water in the sump after many lifting and lowering cycles. Also, the components of the switch can fail mechanically. Replace the float switch in this case.
  • Power cut: The float switch and pump are electromechanical devices. The equipment fails to operate accurately to drain the water during power failure.

Read our article on float switch replacement for more details on troubleshooting, fixing, and replacing a float switch.


Does a sump pump need a float switch?

Yes, the float switch ensures that the sump pump turns on and drains the water out of the sump pit. The float switch can also be wired to fill the sump pit when the water level goes low.

How long does a sump pump float switch last?

With routine maintenance, sump pump float switches can last up to ten years.

How high should I set my sump pump float?

Hang the float switch a few inches above the top of the sump pump motor. The exact height varies depending on the sump pit size and the motor's power.

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