What is a Bell Siphon

In the wonderful world of Aquaponics simple mechanical and biological theories are put to the test to create a product from a process of nature. The most important control in Hydroponics systems for the process to function is called a Bell Siphon. What is a Bell Siphon?

A Bell Siphon improves the oxygenation to the roots of the hydronic plants. As the bell siphon drains out the water, the full lengths of the roots are exposed to air regularly keeping the system aerobic. This promotes oxygen intake, aiding in plant growth, & the overall health of the plants.

Using no powers or timers a Bell Siphon controls a necessary function critical for the Hydroponic and Aquaponic systems. A  simple function that is genius and effective.


How Does a Bell Siphon Work

A Bell Siphon is an easy way to drain grow beds in Aquaponics or Hydroponics systems. It can regulate the flow without the need for human or mechanical intervention. The fundamental principles of a Bell Siphon are based on hydrostatics or the relationship between the height of a water column and its pressure. At the highest point in a siphon, the pressure is actually lower than atmospheric pressure, also known as a vacuum.

This allows the atmospheric pressure to push water up over the peak of the siphon so gravity can carry it the rest of the way. For most situations, that’s enough theory to describe your typical siphon. It’s a very convenient way to drain a reservoir or transfer a liquid without a pump, electric power, or timers. The Bell Syphon is used in different applications but for this article, we show how important it is in the field of Hydroponics and Aquaponics.

A Bell Syphon has three parts:

  • Reservoir
  • Bell
  • Riser

External Bell Siphon


  • The drained grow bed is first filled with water using a water pump.
  • When the water level reaches the top of the standpipe situated inside the siphon pipe, water will start flowing through the reducer placed at the top of the standpipe and out through the standpipe into the fish tank at low pressure.
  • As the water slowly drains out of the standpipe, water builds up inside the bell and pushes air out through the standpipe. As a result, the pressure inside the bell eventually falls.


  • The resulting low pressure in the bell leads to a pressure difference between the bell and the atmosphere which forces the siphon to fire. Thus, the siphon rapidly pushes out and dumps the water through the standpipe at a much higher pressure until the water level reaches the base of the grow bed.
  • As the water level approaches the base of the siphon pipe, air enters the bell through the slits at the bottom of the siphon pipe which relieves the pressure difference between the bell and the atmosphere, causing the siphon to break and halt the drain of water.

A Bell Siphon is a simple device used in Hydroponics that regulates the flow of water in aquaponic and hydroponic systems easily and efficiently, with no need for human intervention.

The siphon first allows the grow bed to flood, then it automatically drains out the water when it reaches a certain level so as not to flood the system. The siphon also maintains a minimum water level as the excess water is drained automatically.

In Hydroponics and Aquaponics flooding and draining the grow beds is a constant operation that keeps the plants in the system healthy during the growth cycle. It keeps them in contact with the right amount of nutrients and water.

It will also provide oxidization which is also important in plant development. It will slowly fill the tanks and provide plenty of time for the plant’s roots have plenty of time to uptake a good amount of nutrients and water.

When it drains it will do so rapidly that it provides a large amount of air that is used up by bacteria in the bed and at the roots for the plants to grow. This keeps the plants and the system aerobic and healthy. Air is important in the system where bacteria use it for breaking down and processing the fish waste turning it into nutrients that the plant used for food. Without the oxygen in the growth bed that acts as a filter and can’t operate properly. Soon the system would turn anaerobic and fail.

In an Anaerobic state, bacteria break down organic material, and fish waste, without oxygen.  This process gives off methane and hydrogen sulfide, which produces the rotten egg smell of anaerobic digestion. 

In its basic function, the bell siphon regulates the health of the bacteria, foremost, and plants in the media bed.  This, in turn, regulates the health of the entire system, plants, bacteria, and the fish keeping a fine-tuned ecosystem working in tangent with each other naturally.


Hydroponic Agriculture

Bell Siphon Diagram


The Aquaponic System is a recirculating system, water is continuously running through the system.  There are four tanks, a Fishtank, a Float Bed, a Sump tank, and a Media bed.  Water is pumped from the sump tank into the float bed and then drains to the fish tank. The fish tank drains into the media bed and the bell siphon drains the water to the sump tank.



  • As the water fills the Grow Bed
  • the Standpipe limits the maximum fill height of the water.
  • When the water rises it pushed air out of the bell housing and down the Standpipe.
  • Once the water level reaches the top of the Standpipe it begins to drain down the pipe.  This is the beginning stage of the siphon. 


  • The water as it drains pulls the rest of the air out of the bell housing and begins to pull water out of the Media Bed up into the Bell housing and down the Standpipe. This draining is faster than the incoming water from the fish tank.  The water acts like a chain and pulls water from above due to gravity and cohesion, or the attraction of water molecules to each other.
  • The water continues to drain from the Media bed to the Sump tank until it reached the bottom of the snorkel tube.  The snorkel end rests in a small reservoir and as the water level lowers it reaches the height of the snorkel reservoir.


  • At this point, the snorkel is only sucking water from the reservoir and drains it faster than the siphon drains the tank.
  • When the reservoir is empty the snorkel sucks air, this air breaks the vacuum in the siphon, shutting it off and allowing the tank to refill.
  • The end of the snorkel determines the minimum water height in the media bed.


The Bell Siphon does have its limits even though they are far apart.  If the water comes into the media bed too fast or too slow the siphon will not be engaged.  You need to set the Siphon in place and take it through a few operating times. Make some adjustments to iron out the pump levels.

The Bell Siphon is not only an integral part of Aquaponics but a fascinating feat of science.  With the ability to flood and drain a tank in a system with one pump greatly lowers the risks of failure.  The siphon runs automatically and consistently giving the plants a stable growing environment.  By having even flood drain cycles the plants receive even amounts of air and nutrients


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JimGalloway Author/Editor





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