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Ultimate Guide To How Bell Siphons Work

Bell siphons are a vital part of the water management in any media based aquaponics system. This device is responsible for regulating the height and flow of water through the grow bed, which via a continuous flood and drain cycle, provides fresh water and oxygen to the plants and bacteria. This part of the process can be thought of as a type of stabilizing function that not only ensures the grow bed gets filtered clean water on a regular basis, but that it does so long enough for the nitrification process to take place and for the plants to uptake sufficient nutrients.

What Is A Bell Siphon 1
Image Credit: Bob's Aquaponics & Backyard Farm.

Bell siphons are an extremely popular solution for media based aquaponics systems since they provide numerous benefits for the grower. These units are affordable, easy to build and set up, they also optimize growing conditions as the flood and drain mechanism aids filtration, improves nutrient uptake, and prevents waterlogged conditions where the stagnant water can cause root rot, all without the need for an electronic timer.

Commonly constructed from PVC piping, and originally installed as a water regulating solution for Ebb & Flow Hydroponics setups, the bell siphon also sees application in media-based aquaponics since the two systems share the same irrigation and drainage concept. The process that occurs inside the siphon is rather complex physics-wise, however the device is simple to put together and relatively straight forward to understand.

How a Bell Siphon Works

A bell siphon works by leveraging the properties of water, pressure and gravity. As the grow bed fills, so too does the bell siphon as water enters from slits cut into the bottom of siphon pipe. Inside the siphon pipe chamber sits a standpipe; essentially a drainage pipe that has its opening near the top of the chamber. This sets the max height of the water in the grow bed as the water eventually drains once it reaches this threshold.

First the water fills, as it reaches the top of the stand pipe it push air out, which creates a negative pressure pocket that prevents water from flowing out. As the water fills more it overcomes the negative pressure and equalizes through the standpipe, which forces water to flow out. The water then drains until the level in the grow bed dips low enough for air to enter the standpipe, this effectively breaks the siphon to refill the grow bed and start the cycle over again.

Bell Siphon Alternatives

Bell siphons are proven drainage solution shown to withstand the test of time. With a little tweaking and adjustment, even the most simplistic DIY builds can be configured to the correct sufficient to handle the flow rate of water draining from any size media bed. However, bell siphons are not the only type of flood and drain solution available to media based aquaponics systems.

Other bell siphon alternatives include:

Floating Siphons: Floating siphons rely on a floating chamber that rises with the water until it reaches the predetermined max height. Water continues to rise till it breaches the open top of the chamber where it then drains

U Siphon: Works similar to standard siphon, instead of a stand pipe installed in the grow bed, it utilizes an upside down U pipe on the outside of the grow bed, This essentially functions in the same manner with the apex of the inverted U acting as the top of the stand pipe; marking the threshold at which the siphon is triggered once the water level attempts to rise past it.

Loop Siphons: Loop siphons are simply a different configuration of U siphon, which instead of utilizing an inverted U outside of the tank uses a loop. The loop functions in the exact same way as the U, with the apex of the loop marking the max height of the water level and the point at which the siphon fires.

Siphon Snorkel: Some systems can have difficulty breaking the siphon when the flow rate is too high. By adding a ‘snorkel’ to a standard bell siphon more air can enter the drain, stand pipe and bell, so it can then break the siphon.

Larger grow beds regularly encounter more problems as there are more factors to potentially disrupt the consistent flow rate required to keep bell siphons firing and breaking. This may be less of an issue for smaller home systems, but in larger setups, could require the use of alternative siphon solutions. For example, where snorkel siphons may be more suitable for large systems, loop siphons are typically better for smaller media based aquaponics setups.

Benefits of Installing Bell Siphons

The type of drainage you chose to install in your aquaponics setup will largely be determined by the method of aquaponics you’ve selected. For example, DWC and NFT aquaponics do not require the use of a bell siphon, or similar component, since the water maintains a constant flow as opposed to the flood and drain method.

For those setting up media based aquaponics, the choice will need to be made as to which of the four aforementioned siphon options should be installed. Although some may claim that one of the alternatives happens to be most suitable for their specific application, standard bell siphons are generally one size fits all and typically provide a number of advantages over other options, some of these benefits include:

  • Regulates water automatically: The main advantage of installing a bell siphon, automated drain and fill, at the correct intervals
  • Simple construction: Utilizes a few basic parts that are affordable and easy to put together
  • Easy maintenance: In addition to being easy to build, bell siphons are also straightforward to maintain, fix and/or replace where necessary
  • Optimal nutrient uptake: Draining and refilling the grow bed at specific intervals can maximize plants exposure to nutrient rich water
  • Optimized filtration: Ensures grow bed is saturated long enough for plants and bacteria to break down fish waste into nutrients, which provides optimal filtration conditions
  • Additional filtration: The outer casing of a bell siphon provides an extra layer of natural filtration by capturing mid-to-large-sized pieces of debris
  • Promotes oxygenation: The drain outfall helps to enrich the water with oxygen to increase the concentration of dissolved oxygen
  • Saves Power Consumption: The automated flood and drain action that occurs without the need for electrics saves on power consumption and cost

Disadvantages of Bell Siphons

Although there isn’t really any disadvantages specific to bell siphons over other siphon/drainage options, there are a few important points that can help overcome a couple of tricky

Getting Bell Siphon To Fire/Initiate & Break: Getting a bell siphon dialed in so it fires and breaks can take a little adjustment, especially if it’s your first time. If you’re having difficulty getting your bell siphon to fire, adjusting the flow rate of the water can resolve the issue.

Monitoring and Maintenance: Once running, bell siphons will literally run forever as long as they remain unobstructed. Unfortunately this isn’t the reality of running an aquaponics system, so you will need to monitor and maintain the device to ensure it stays clean and working efficiently.

What Is A Bell Siphon 2
Image Credit: Bob's Aquaponics & Backyard Farm.
What Is A Bell Siphon 3
Image Credit: Bob's Aquaponics & Backyard Farm.
What Is A Bell Siphon 4
Image Credit: Bob's Aquaponics & Backyard Farm.

Components of Bell Siphon

As noted, bell siphons are rather straightforward to construct and thus, make for fun, useful DIY project that should take no more than an afternoon to build. Whether you’re a budding DIY enthusiast or not, deciding to build your own bell siphon isn’t just a money saving endeavor that’ll reduce you’re overall budget, it’s also a great hands-on lesson that teach you all about the mechanism that makes a bell siphon do its thing.

Bell siphons can take on one of a few different configurations; largely determined by the parts used in the build, e.g. PVC piping is popular due to the availability of different size parts, connectors and accessories that can easily be repurposed to construct a bell siphon with minimal, if any, modifications.

Be it off-the-shelf, DIY, or professionally custom built, every bell siphon is made of the same basic components, these parts include:

Siphon Pipe: The outer casing in which process takes place. Creates the vacuum that allows water to keep filling until pressure builds enough to forces it out through the standpipe, where gravity takes over to continue draining the system

Stand Pipe: The internal pipe that connects the reducer to bulk head, acts as the discharge pipe through which water is drained. Also regulates max height of the water by setting the level at which drainage automatically occurs

Reducer: This sits top of the standpipe, it creates a smooth transition from fill to drain. The dimension ratio between the reducer and standpipe should be 2:1 to ensure a stable operation, e.g. if the reducer is 2” the standpipe should be is 1”.

In fact, this same 2:1 ratio also applies between the standpipe and siphon pipe. Therefore, the dimensions should be as follows:

– 1” standpipe > 2” reducer, > 4” siphon pipe

Bell Cap: Essentially the lid/roof securely caps off the unit so the pressure can build up and equalization can take place

Media Guard: This offers an additional stage of filtration by preventing grow media and other debris from clogging up the standpipe and bell siphon

Bulkhead: This holds the unit secure to the grow bed, and allows water to drain through the standpipe without leaking

Outlet Pipe: Also called a drainpipe, this directs water drained from the grow bed, through the bulkhead, and into the fish tank/reservoir/sump tank.

How to Build and Install Your Own DIY Bell Siphon

When it comes to building your own DIY bell siphon there are several tried and tested methods that hobbyists have shown to work great. One particularly informative resource we came across was the playlist from Rob Bob over at Rob Bob’s Aquaponics & Backyard Farm. His comprehensive video library contains almost anything you could ever wish to know about aquaponics, and his series looking into bell siphons is a great example of how much knowledge is packed into his enjoyable content.

If you’d like to see a highly informative deep dive into what it takes to build your own DIY bell siphon, then check out these fantastic tutorials from Rob. Or if you’d like a step-by-step guide, with both text and video all in one place so it’s easy to follow, then watch this space for our comprehensive ‘How To DIY Bell Siphon For Media Based Aquaponics’. Or subscribe to our newsletter and receive updates direct to your inbox.

Advice, Tips & Tricks

Rough Sizing Guidelines

Standpipe Size: The height of the standpipe directly determines the max water level height in the grow bed, whilst the diameter relates to the ideal flow rate the bell siphon will regulate based on the volume of water and size of the grow bed: e.g. a bigger grow bed will require a wider stand pipe.

Bell Size: The size of chamber within the bell siphon affects the speed at which the device initiates and breaks. Bells with larger chamber volumes will cause a slower cycle, while smaller bells will accelerate the process. The size of the bell should be determined by the system’s specific requirements such as the capacity of the water pump and dimensions of the grow bed.

Media Guard Size: The media guard provides additional filtration as it prevents grow media and lose debris from obstructing the bell siphon’s operation. It should be large enough to house the standpipe whilst allowing water to flow freely. Just like the other components, the larger the system, the bigger the media guard should be.

Again, the most optimal ratio between each component is 2:1, meaning that the outer housing should be twice the size of the reducer, which should be twice the size of the inner drainage pipe.

Dimension Ratios: Ready-made bell siphon kits are available and will come with instructions on how to set the ideal volume of your grow bed and media. However, as a general rule of thumb you can follow these guidelines.

    • 16-cubic-ft/120-gallon grow bed 4ft x 4ft x 1ft = 2” bell & 1” reducer & 0.5” stand pipe
    • 24-cubic-ft/180-gallon grow bed 4ft x 6ft x 1ft = 3” bell & 1.5” reducer & 0.75 stand pipe
    • 32-cubic-ft/240-gallon grow bed 4ft x 8ft x 1ft = 4” bell & 2” reducer & 1” stand pipe

Installation Placement of Your Bell Siphon

Optimal Positioning: The placement of the bell siphon can affect how efficiently it operates. Place it in the center of the grow bed to ensure equal distribution, also, being near the water inlet can improve overall effectiveness.

Avoiding Obstructions: Install the bell siphon away from any obstructions like plant roots. Also note the media screen will capture debris and therefore may need cleaning so the water flow remains unobstructed.

Proper Submersion: Both bell and standpipe must be adequately submerged in the growing media. This prevents air from entering the bell prematurely, and ensures the device can function as intended.

Bell Siphon Maintenance

Clean regularly: Inspect the bell siphon for blockage caused by any debris and/or sediment, and check for algae buildup on a regular. Keeping the siphon clean ensures smooth operation of the siphon and prevents potential clogs.

Maintain airtight seals: Check and immediately fix any air leaks around the bell and standpipe seals. Airtight seals are crucial for the proper initiation and termination of the siphon.

Inspect media guard: The media guard should be kept in good condition. Regularly cleaning is essential and if any damage occurs, a replacement should be installed immediately.

Adjust for system changes: Any modifications to the system will require reassessing the size and position of your bell siphon. Changes in flow rates and size of grow bed will likely need adjustments to keep things ticking over smoothly.

Troubleshooting Bell Siphon Issues

Bell siphon can encounter several issues that may cause it to stop working correctly. The most common issues include:

Bell siphon not centered or level: If the bell siphon is not level or centered it may cause the water to drain improperly; which can lead to inadequate drainage and/or flooding. Ensure your bell siphon is positioned correctly and installed level, if it isn’t, you may need to adjust or even reinstall the device.

Blocked, clogged and/or incorrectly positioned pipes: The slits in the siphon pipe as well as the drainage outlet through the stand pipe must remain clear at all times. If either get blocked it can prevent air from entering the siphon which will stop the water from draining. The positioning of the outlet pipe can also affect the airflow. A pipe with even a slight incline can trap water in the lowest point, which obstructs water and air. Instead the outlet should be on a slight decline so that gravity can aid the process.

Install overflow outlet correctly: An overflow outlet can help excess water flow out of the grow bed. If not installed in the correct position, water may not drain properly. This can also lead to inadequate drainage and/or flooding. Check the installation of the overflow outlet and adjust where necessary.

Height of the bell siphon is incorrect: The height of the bell siphon determines the water level in the grow bed. And the height of the standpipe determines at what level the water will drain. Setting these height too far apart can lead to improper water levels, in which case you can adjust the height each until.

Generally the height of the standpipe should be just below the height of the grow media, and the bell cap should be at least 0.5 to 2”, possibly 3”, above the standpipe.

When testing the siphon you’ll want to see it cycle through numerous times to ensure it will continue to flood and drain. Note that when doing this, you don’t need to wait for the system to drain completely, instead interrupt the prices to the reinitiate the siphon and save yourself some time.

Other Tips and Advice

If you’re having trouble with leakage from the bulkhead and outlet pipe, try installing washers on either side of the lock nut.

Alternatively instead of using a threaded bulkhead, it is possible to use Uniseal connections. You’ll simply need to drill the correct size hole, and push it through pipes for a tight friction fit. You can use Teflon tape where needed.

Standard 65mm/2.5 inch pipe is commonly used as outer piping, but a good alternative is 50mm drain pipe.

Even soda/pop bottles can be good enough to construct a bell siphon, but it can be tough to find ones tall enough to meet the top of the grow media

When installing make sure there’s enough room on the underside for the drains outlet pipe, as well as some room to work.

Add an elbow connection to the vertical outlet pipe so you drain water at a better angle. This helps regulate the flow of air back up the pipe; it allows you to adjust the location of the falling water.

In Conclusion

Bell siphons are like the beating heat of a media based aquaponics systems. Just like the body’s vital organ, ensuring your bell siphon remains functional is crucial in keeping the ecosystem alive and thriving. In addition to aiding the nitrification process and keeping the water clean, bell siphons also help to maintain ideal water levels, and by doing so, your plants, fish, and the beneficial bacteria that helps support them both, will all reward you with a healthy existence the produces plentiful yields.

How to DIY Bell Siphon
Image Credit: Bob's Aquaponics & Backyard Farm.

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