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February 20, 2024

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    Siphons (also known as syphons) are tubes that allow liquid to move upward to above the surface of an origin reservoir against gravity and atmospheric pressure, then back down to a lower level without the help of a pump.

    Siphons are essential in a toilet, enabling the system to work without a power source. The siphon offers critical benefits like water efficiency and hygiene, which are necessary for a properly functioning toilet.

    This article explores the basics of how toilet siphons work, how to troubleshoot common problems, and replacement processes, so continue reading to learn more.

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    Highlights
    • The siphon tube in a toilet pulls waste and water from the bowl and transmits it into the sewage system.
    • Common issues associated with toilet siphons, which are usually easily fixable, include clogs, leaks, and weak flushes.
    • Repairing the part of the toilet responsible for the siphoning action is doable in some cases, but not all.

    How a Toilet Siphon Works: The Basics

    A toilet siphon consists of a few crucial parts that allow it to work properly, including the flush valve, trap, bowl, and drain pipe. The pieces work together to move liquid levels between the toilet bowl and tank, creating low pressure to flush the toilet. These parts are essential for the siphon process, so issues with any particular piece can lead to problems with the siphon function.

    Siphon Location

    In a toilet, the siphon setup sits below the toilet bowl. Many traditional-styled toilets feature a curved tube-like shape underneath the toilet bowl — an integral part of the siphon setup.

    If you look at the tube-shaped portions at the base of your toilet, you’ll notice that the top of the tube starts lower under the toilet bowl (at the maximum height), arches upward to create a high point, then drops almost straight down from the high point. The short portion before the high point is called the short leg, and the long drop after the high point is called the long leg.

    Image Credit: Canva

    How the Siphon Works

    So, with the anatomy of the toilet siphon in mind, we can understand how it works. In a properly functioning toilet, the water level remains at a certain point. It remains below the high point in the tubing but high enough that a relatively small amount of water could push it over that point.

    When you flush the toilet, water drains down from the tank behind the toilet, moving into the toilet bowl. Additionally, it comes through a hollow portion between the bowl and the outside of the toilet, known as the siphon jack.

    Water flowing from both areas flows into the short leg, but the liquid from the siphon jack has a direct shot into the short leg. The rising water level fills the short leg with a column of water, quickly filling the tubing past the high point and down into the long leg.

    This results in the siphoning effect (which works due to Bernoulli’s principle), as the weight of the water from the force of gravity continues to “push” water down the long leg until there’s no water left. It essentially creates a vacuum (negative pressure) that pulls water and waste from the toilet bowl into the plumbing system.

    Illustration of Bernoulli's Principle

    Image Credit: Adobe Stock

    Once the flush is complete, the flush valve resituates itself at the bottom of the tank. This blocks the drain hole and stops the flow of water, allowing the tank to refill. When the tank is full, the toilet is ready for another flush, repeating the cycle.

    Given the nature of the water siphon, you don’t actually need to flush the toilet to kick-start the siphon. For example, if your home’s water main is off temporarily, you could still “flush” the toilet.

    By pouring a bucket of water into the toilet, you would raise the water level past the curve in the drainage tubing, creating a vacuum and causing the toilet to flush. Once you stop pouring, the water will flow out the end of the siphon (out of the toilet) and stop “flushing.”

    Siphon Effect

    Siphons don’t only flush toilets. You can also use a hose and the siphon effect to empty clogged toilet bowls, sinks, fish tanks, buckets, etc. Just put one end of the hose in the tank and the other end below the tank and add a small amount of suction to overcome the hydrostatic pressure. Then, remove the suction once the water gets past the highest point of the hose. The water will continue to flow due to the siphon effect.

    How Do You Troubleshoot a Toilet Siphon Not Working?

    When the siphon process in your toilet stops working, the entire toilet is essentially useless. If waste and water cannot drain into the plumbing system, you can’t use or flush the toilet, so it’s essential to identify and repair the toilet problem sooner rather than later.

    Three common problems lead to toilet siphon issues: clogs, leaks, and weak flushes (i.e., low flow rate).

    Clogs

    When troubleshooting your non-functioning toilet siphon, it’s important to consider the possibility of a clog. Clogs and blockages are all too common with toilets, given their function, so most folks have a trusty plunger nearby to tackle these issues.

    A clog will prevent the siphon from working by blocking water movement into the short leg. If water cannot move into the short leg, over the high point, and down the long leg, the toilet can’t create a siphoning effect, so it can’t flush.

    Flushing a clogged toilet often results in water backing up into the toilet bowl — and if you’re unlucky, the bowl may overflow.

    Luckily, clogs are usually relatively easy to tackle. Sometimes it’s as simple as waiting for the toilet paper to break down, as it’s designed to do in water. When the toilet paper breaks down, it might clear the clog by allowing water to move through the toilet and create a siphon.

    However, some blockages require more assistance, so you should enlist the help of a plunger. To use the plunger, fit the cup over the hole at the bottom of the toilet, ensuring you cover the entire drainage hole. Push down to create suction, then plunge vigorously four to five times.

    The suction should create air pressure in the toilet’s drainage system, dislodging the clog and allowing water to flow. If your attempts are successful, you should know as soon as you remove the plunger from the toilet, as water will rapidly swirl down the drain.

    If the toilet doesn’t drain immediately, you might need to wait a few minutes and try again. However, some clogs are more serious (such as foreign objects like toys) and will require the assistance of a plumber, so if you can’t get the toilet to work, you might need to call in a plumber.

    Leaks

    If your toilet has a leak, there’s a good chance the siphon won’t flush properly. There are a few types of leaks it might have, but the most common types are damage to the tubing making up the siphon, and leaks from the tank.

    If the problem stems from a small hole or leak in the toilet itself, the toilet won’t be able to form a vacuum. When this occurs, it’ll effectively prevent the toilet from flushing properly. The repair varies based on the location of the problem and its severity. If you’re unsure whether the problem is fixable, consult your local plumber.

    Alternatively, it could be a leak stemming from the tank. For example, maybe the flapper isn’t covering the drain hole in the tank properly, resulting in a constantly running toilet and a tank that cannot fill. To remedy this problem, you’ll need to replace the flapper (if it’s damaged) or reseat the flapper (by adjusting it or lengthening the chain).

    Toilet flushing mechanism

    Image Credit: Adobe Stock

    Weak Flushes

    Your toilet needs enough water from the tank to push the water level over the high point of the siphon and trigger a vacuum effect. If there isn’t enough water flowing down from the tank, the toilet might not be able to flush at all, as the water level won’t surpass the high point.

    In many cases, this problem is the result of a weak flush. The tank might not have enough water in it to facilitate a strong flush to whisk waste away. Generally, weak flushes stem from one of two issues: water level problems and flapper issues.

    If the water level is set too low, you’ll need to adjust it to ensure the toilet receives enough water to create a siphon. The process of adjusting the water level varies based on the model you have, so refer to manufacturer instructions for assistance.

    Alternatively, it could be a problem with the flapper. The flapper is the part that covers the drain hole in the tank. If the flapper is damaged or not seated right over the hole, it’ll allow the toilet to run constantly. When you attempt to flush the toilet, there won’t be enough water in the tank to facilitate a strong flush.

    To correct this issue, you’ll need to examine the flapper for damage and replace it if necessary. Or, if everything looks okay, lengthen the chain connected to the flapper to ensure it seats right.

    While the mechanisms in a toilet are surprisingly straightforward, many problems can develop, each stemming from a different toilet component. The video below discusses several simple DIY toilet maintenance tips to keep your toilet in tip-top shape and correct common problems. 

    How Do You Replace a Toilet Siphon?

    If the tubed portion of your toilet, where the siphoning action takes place, becomes damaged, you may or may not be able to repair it. When you’re dealing with significant damage, like larger holes and cracks, restoring the toilet to ensure it can create a vacuum is complex.

    However, hairline fractures are easily repairable with plumbing or porcelain epoxy. You’ll need to clean the area around the crack thoroughly and dry it before applying the epoxy. The instructions vary based on the type of epoxy you use, so be sure to follow the manufacturer’s instructions for mixing and application.

    Replacing a Toilet Lever Flush Siphon

    It’s important to note that the toilet siphon this article discusses differs from the toilet lever flush siphons, which is located in the toilet’s tank (upper reservoir).

    Installation of a toilet lever flush siphon

    Image Credit: Canva

    These siphons usually take between one to two hours to repair; here’s how:

    1. Find the toilet lever flush siphon in your tank.
    2. Check if it is one piece or two.
    3. Purchase a replacement (by either a one-piece or two-piece unit).
    4. Turn off the water to the toilet with the shut-off valve near the floor.
    5. Empty the toilet bowl by manually flushing it by pulling the plug up inside the tank or by using the handle if it still works.
    6.  Unscrew the water inlet valve on the toilet.
    7. Unscrew the plastic nut that holds the siphon beneath the toilet cistern.
    8. Unhook the handle hook from the flush lever and remove the old siphon.
    9. Inspect the plastic diaphragm at the bottom of the new siphon and ensure it does not have any holes.
    10. Install the new siphon in the same area you removed the old one from the cistern.
    11. Screw the plastic nut back beneath the cistern.
    12. Reconnect the water supply to your toilet.
    13. Turn the water supply valve back on and check for leaks.
    14. Allow your toilet to refill with water and flush it a few times while checking for leaks. 

    Final Thoughts on Toilet Siphons

    Given toilet siphons’ integral role in a properly functioning toilet, they’re well worth maintaining. Of course, like any fixture or product, your toilet siphon might encounter issues now and again, so repairs might be necessary here and there.

    The setup might be damaged or outdated, but regardless of what is behind the siphoning problem, it’s essential to repair it to ensure your toilet works properly. Use the tool below to find a qualified plumber near you.

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    FAQs About Toilet Siphon

    What is a toilet siphon?

    A toilet siphon is the part of a toilet responsible for transporting waste and water in a toilet bowl to the sewage system. The mechanism is simple, as it relies on siphoning to work. Once the water clears a certain point in the drainage system, it creates a vacuum, effectively pulling waste and water up from the toilet bowl and down into the sewage system.


    How does a toilet siphon work?

    Toilet siphons work by raising the water level in the short leg of the tubing beneath the toilet bowl. Once the water surpasses the short leg and clears the high point, it drops down the long leg. This creates a vacuum that pulls waste and water from the toilet bowl into the sewage system.


    What’s the purpose of a toilet siphon?

    The purpose of a toilet siphon is to pull water and waste from the toilet bowl and transport it to the sewage system.


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    Jonathon Jachura

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    Jonathon Jachura is a two-time homeowner with hands-on experience with HVAC, gutters, plumbing, lawn care, pest control, and other aspects of owning a home. He is passionate about home maintenance and finding the best services. His main goal is to educate others with crisp, concise descriptions that any homeowner can use. Jon uses his strong technical background to create engaging, easy-to-read, and informative guides. He does most of his home and lawn projects himself but hires professional companies for the “big things.” He knows what goes into finding the best service providers and contractors. Jon studied mechanical engineering at Purdue University in Indiana and worked in the HVAC industry for 12 years. Between his various home improvement projects, he enjoys the outdoors, a good cup of coffee, and spending time with his family.

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    Roxanne Downer

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    Roxanne Downer is a commerce editor at Today’s Homeowner, where she tackles everything from foundation repair to solar panel installation. She brings more than 15 years of writing and editing experience to bear in her meticulous approach to ensuring accurate, up-to-date, and engaging content. She’s previously edited for outlets including MSN, Architectural Digest, and Better Homes & Gardens. An alumna of the University of Pennsylvania, Roxanne is now an Oklahoma homeowner, DIY enthusiast, and the proud parent of a playful pug.

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