Aquaponics — 28 February 2013

A bell siphon is the magic behind an ebb and flow aquaponics system.  It allows the water to fill the grow bed, then once it is full it quickly drains the bed, and the process starts over again.  This fill and drain action allows the roots of the plants to get water and nutrients as well as oxygen.  As I started researching for my aquaponics system project, I discovered that there are several different methods of constructing a siphon.  It sure would have been nice to have a place where all of these different methods were presented in one place.  I hope that this will help you as you build your siphon system.  It seems clear that the materials that are available for each method vary in availability for different parts of the world.  The best advice that I can give is to see examples of what has been done, then build a similar system from the pipe/fittings/materials that are available to you.

Here are the major components of the bell siphon.

Bell Siphon Components

Bell Siphon Components


The purpose of the bulkhead is to allow the drain pipe to pass through the grow bed container without leaking any water.  For my system I used a 3/4″ PVC bulkhead fitting made by Watts.  I’ve seen these fittings at the Home Depot as well as Lowes.

3/4" Watts Bulkhead Fitting

3/4″ Watts Bulkhead Fitting

The bulkhead has a threaded hole all the way through the middle of it.  The main piece looks like a large bolt.  There are two washers (look like rubber) and a large plastic nut.  They are installed by drilling a hole (much larger than the pipe going through the tank).  The hole doesn’t need to be perfectly circular, since the washers will seal up the imperfections.

Another option that I have seen being used is a Uniseal.  This is a flexible grommet that fits into a perfect circular hole to provide watertight seal for a pipe with the same outside diameter as schedule 40 pvc pipe.

Uniseal fitting

Uniseal fitting

There are also many people that are using electrical conduit as the bulkhead.  They provide a small shoulder or sealing surface (unlike the Watts type bulkhead fittings).  A rubber washer helps to seal up any leaks.

Electrical conduit and rubber gasket as a bulkhead (image from )

Electrical conduit and rubber gasket as a bulkhead (image from )


The standpipe connects the bulkhead to the reducer.  I used 3/4-inch sch 40 PVC pipe.  This really doesn’t need to be sch 40 or even PVC, its is just what is common cheap material around here (USA).  The standpipe length can be adjusted to raise or lower the water surface in the growbed.

Some aquaponics systems just use a standpipe without the reducer.  To create the ebb and flow, the pump is put on a timer.  When the water reaches the top of the standpipe, it flows out.  When the pump stops, the water slowly drains out through a small hole drilled at the base of the standpipe.


The reducer at the top of the standpipe helps create the siphon in the discharge pipe.  A reducer with a 2:1 ratio is a good rule of thumb.  So, if your standpipe is 1 inch, a reducer that goes from 2 inches to 1 inch would be ideal.

I had trouble finding a reducer with a smooth transition.  I stood in front of all the fittings at Ace Hardware, scratching my head wondering what I was going to and I ran into this reducing adaptor.


It is a 1.5″ x 3/4″ threaded x socket reducing adaptor.  I have no idea where else it would be used other than for an aquaponics siphon.  It works great.

An interesting idea that I have seen is to build the reducer from the top of a soda bottle.

Soda Bottle Bell Siphon (image from Brett Cooper's YouTube video

Soda Bottle Bell Siphon (image from Brett Cooper’s YouTube video


Siphon Pipe and Cap

The job of the siphon pipe and cap is to prevent air from entering from the outside.  When water starts flowing down the standpipe, the velocity of the water increases, and the pressure drops.  The negative pressure creates a vacuum that keeps the water flowing in the siphon.  If atmospheric air is entered into the system, the pressure rises, and the siphon stops.

I used 3″ PVC pipe with a 3″ PVC cap.  A good rule of thumb is to has the reducer diameter twice as big as the standpipe, and have the siphon pipe twice as big as the reducer (mine was 3/4″ standpipe, 1.5″ reducer, and 3″ siphon pipe).

Siphon Pipe and Cap

Siphon Pipe and Cap

You may try using a sewer pipe cap which is a lot less expensive than a sch 40 pvc pipe cap.  It doesn’t have as smooth an interior, but this doesn’t really matter.

Holes or slots are cut along the bottom to allow air in once the growbed is drained.  The air will break the siphon.

Media Guard

The sole purpose of the media guard is to keep rocks (or whatever media you use) off the siphon pipe and allow water in.  This will allow you to take out and inspect the piping without taking out all the media.

I used 4-inch 3,000 lb crush strength pvc (commonly used for septic drainfield pipe) for my media guard.  I drilled a bunch of hole to allow water in.  I put a 4-inch pvc cap on top.  (I have to be real careful on to push down on the cap.  It is really hard to take off once it is put on.)

2013-02-16_16-30-31_960Cutting slots in the pipe will also work, and may be easier to do.

One idea that I had for a media guard is a wire mesh.  If you can find a wire mesh with holes that are small enough to keep the media out, roll it in a tube shape (or bend it into a box shape) to form the media guard.

I hope this helps with your next siphon project.  Good luck!




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(3) Readers Comments

  1. Very cool. Thanks for the explanation of this siphon and guidelines for building it. I appreciate the time you’re taking to share this info with other aspiring independents.

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