How Does a Siphon Work?

A siphon is a tube that lets liquid flow upward, above the surface of the origin reservoir, and then downward to a lower level without using a pump. It uses gravity and atmospheric pressure to create a continuous flow of liquid.

Today, we’ll explore the science behind siphons and how they’re used in everyday plumbing fixtures. Ask This Old House plumbing and heating expert Richard Trethewey also explains more about siphons and how they help toilets flush in the above video. 

The Science of Siphons

Siphons work by creating a pressure difference that moves liquid against gravity. Understanding the basic mechanics of this process helps explain why siphons are so useful in different applications.

How Siphons Work

A siphon has two main parts: a short leg and a long leg. The short leg sits in the liquid source, while the long leg reaches a lower point where the liquid drains.

When the siphon is primed (filled with liquid), the weight of the liquid in the longer leg creates a vacuum that pulls more liquid up the short leg and over the bend. This process keeps the liquid flowing as long as the siphon stays primed.

For a siphon to work, a few things must happen:

1. The discharge end of the tube must be lower than the liquid source.
2. The tube must be completely filled with liquid to start the siphon action.
3. The siphon’s peak can’t be too high, or the liquid will evaporate and break the flow.
4. The siphon tube must stay airtight to maintain a stable vacuum.

Breaking the Siphon

A siphon will keep flowing until one of the following happens:

• Air enters the tube and breaks the vacuum.
• The discharge end goes above the liquid source level, disrupting the pressure balance.
• The liquid source runs out.

Everyday Uses of Siphons

People widely use siphons in both everyday life and specialized industries. Their simplicity makes them helpful in many situations.

Household Uses

You’ll find siphons in homes for tasks like:

• Unclogging sinks or bathtubs when plungers won’t work.
• Draining aquariums, water beds, and other household items involving large quantities of liquid.
• Transferring liquid from large containers to smaller ones without spilling.

Industrial Applications

Industries use siphons for:

• Controlling water levels in hydroelectric dams for safe and efficient energy production.
• Draining large tanks or reservoirs without heavy machinery.
• Moving chemicals in manufacturing processes where careful control of liquids is important.

Emergency Situations

Siphons can also help in emergencies, such as:

• Providing water in areas without electricity, such as in remote locations or during infrastructure failures.
• Removing floodwater from basements quickly and affordably.
• Transferring fuel when pumps aren’t available, such as during equipment failures or power outages.

Siphons in Plumbing Systems

One of the most common places you’ll see a siphon is in your plumbing system, specifically in toilets. Understanding how siphons work in these fixtures can help you troubleshoot common issues.

How Toilets Use Siphons

In the video, Trethewey explains how a toilet flushes using a built-in siphon. The short leg is the visible part of the bowl, and the long leg is hidden within the porcelain fixture. Here’s how it works:

1. Water from the tank rushes into the bowl through the flush ring and siphon jet.
2. The siphon jet pushes water up the short leg of the siphon.
3. Once the water level reaches the top of the siphon, it begins to flow down the long leg.
4. This action creates a vacuum, pulling the rest of the bowl’s contents with it.
5. The flush continues until air enters the system, breaking the siphon.

Advantages of Siphon-Based Toilets

Toilets that use siphons offer a few key benefits:

• They flush effectively without needing electricity or complex mechanisms, keeping utility costs down.
• They have fewer moving parts, which means fewer breakdowns and less maintenance.
• They still work during power outages or water supply issues, making them reliable in all conditions.

Emergency Toilet Flush

As Trethewey shows, you can flush a toilet using a siphon even when the water supply is off:

1. Fill a bucket with water.
2. Pour the water quickly into the toilet bowl.
3. The sudden flow of water will activate the siphon, and the toilet will flush without needing the tank.

Advantages and Maintenance of Siphon-Based Plumbing Fixtures

Siphon-based plumbing fixtures, like toilets, offer several advantages beyond just flushing. They’re efficient, low-maintenance, and cost-effective. Since these systems don’t rely on electricity or complex machinery, they’re more sustainable and often last longer.

To keep siphon-based fixtures working properly:

1. Regularly clean the siphon jet to prevent clogs.
2. Avoid flushing nonbiodegradable items that can block the siphon.
3. Fix any leaks quickly to maintain proper water levels.

Are Siphon Toilets the Only Option?

While siphon-based toilets are the most common in American homes, there are a few alternatives that some homeowners prefer:

• Pressure-assisted toilets: These use air pressure to push water into the bowl, delivering a stronger flush. They’re ideal for areas that need more flushing power, though they tend to be louder than other systems.
• Gravity-fed toilets: These use the natural force of water falling from the tank to flush the toilet. They rely on a simpler mechanism and are generally quieter but may not be as powerful as pressure-assisted models.
• Dual-flush toilets: These offer two flush options for liquid and solid waste and have gained popularity because of their water efficiency. They still use a siphon mechanism but with modifications to allow for the two flush options.

Each option has its pros and cons. If you’re happy with your siphon-based toilet, there’s no need to switch. But if you’re looking for something with a different flushing mechanism, understanding these alternatives might help guide your decision.