Washdown vs. Siphonic toilet

Siphonic vs Washdown Toilet. Which Is Better?


Siphonic vs Washdown Toilet – A Comparison

Siphonic vs Washdown Toilet

Siphonic vs Washdown Toilet. The Pros & Cons

When you are unfamiliar with how a toilet works, it is fair you think all toilets flush the same way. But the fact is, they don’t. Not to the Americans and Europeans at least, and they don’t see eye to eye in the way a toilet flushes.

Though toilet flushing is no rocket science, it has seen some major changes in recent years. Change that allow toilets to use a lot less water and at a higher efficiency to dispose waste.

But one thing hasn’t changed. That is, water is still being used as a medium to clear waste. And that’s because it is still the best and the cheapest means.

No Rocket But Still Science

A toilet flush works by using the force of rushing water to push waste out to the septic system.

So it’s a matter of how to recreate this fluid energy with the least amount of water. And manipulate it for the highest efficiency in waste clearing. And all this will not happen without taking a different perspective in toilet design.

But whatever design may be, the modern World still uses the 2 most common toilet flushing systems. They are the siphonic and washdown flushing system.

What are Siphonic Toilets?

Siphonic Toilet TrapwayTypically, siphonic toilets will have a long and narrow reverse ‘P’ or ‘S’ shape trapway. With one end as the inlet in the bowl, and the other connected to the drain pipe underneath the bowl.

This trapway design is intentionally meant to create a siphon. And that’s exactly how a siphonic toilet will work.

How Does A Toilet Siphon Works?

A flush begins with pulling the lever or pushing a flush button. Then the flush valve opens letting the tank water flows through into the bowl.

In the case of a siphonic toilet, typically you will see the water in the bowl rises and then subside rapidly into the bowl outlet. What happen is, the water is flowing out faster from the tank than it tries to exit the bowl. This is usually due to a larger flush valve diameter than the trapway.

As water exits through the trapway, it displaces the air inside to form a vacuum. Then when it flows over the kink in the trapway, that’s when the siphon begins. You can tell from the water in the bowl that a siphon action has started when it stops rising and begin to subside rapidly.

The siphon, at its optimum state is so strong, heavy solid wastes get sucked out with the water. You may see a swirl with some siphonic toilets, but the working principle is the same.

Towards the end of the flush you can hear a gurgling sound, that’s when the vacuum is broken and stop the siphon. The bowl is filled with the remnant water, and the refill in the tank begins. All ready for the next flush.

That’s basically how a siphonic toilet works.

New Siphonic Toilets With Greater Focus On Details

As you can see, the siphon is central to a siphonic toilet. In the past, before the US Energy Policy Act of 1992 (EPAct) kicks in, siphonic toilets use 3.5 gallons of water per flush.

The act regulates water volume for toilet flushing to a maximum of 1.6 gpf (gallon per flush). That’s more than 50% of saving on water for toilet flushing. On a national level, that’s a lot of water we are talking about.

So how does a drastic reduction of flush volume can improve the efficiency of a flush toilet?

With only 1.6 gallons of water to play with, you just have to do things differently. From the valve opening right down to the trapway design, everything has to change and improve.

Rethinking Toilet Design

There’s a whole lot of focus on details with the small water volume in mind. So the coordination of the various toilet parts has to be precise. In particular, the flushing and bowl design need to work on to get more done with much less water.

And competition for market shares are also key drivers for greater innovation. The result is a highly efficient flush toilet we see today. This is evident with the host of fancy names given to toilet flush systems. Like Vormax, Double Cyclone, Dual-Max, AquaPiston…etc.

Even the bowl and trapway are not spared either. The new toilets will have bowls and trapways fully glazed for smoother water flow and to prevent stains. And you may have heard of EverClean Surface or the CeFIONtect Finish. They are propreitary glazes developed by American Standard and TOTO respectively. They are a class above the conventional glaze.

So, if you are still using the old 3.5 gpf toilet, this is the time for a change. And it won’t cost you an arm or a leg to make that switch. You get better performing toilet that save water and money.

Characteristics of Siphonic Toilets

It’s easy to identify a siphonic flush toilet. From the appearance, the S-shape trapway is clearly visible. But if the toilet is a skirted or concealed trapway design, you can look at the water level in the bowl. Which typically is high above the bowl outlet and measure about 9″ x 8″ in surface area.

This water in the bowl is to seal off the noxious gases in the sewer pipe. Also to minimize waste from sticking to the bowl surface. The water surface area of a siphonic toilet is usually more than twice the size of the washdown model.

Speaking of bowl streaking, some toilet models will have a special surface finish. Such as the SanaGloss (or CeFIONtect) glaze from TOTO and American Standard’s EverClean Surface. They are designed to counter such problem.

And with the fully glazed trapway walls, it helps with smoother flow and prevent clogs.

Siphonic toilets are prone to clog because of its narrow and winding trapway. Despite that, they are the power flush toilets that are preferred by most Americans.

Water Surface Area

Water Surface Area


  • Large water surface area
  • High water level in bowl
  • Long and narrow S-shape trapway
  • Longer bowl base


  • Cleaner bowl with lower probability of skid marks
  • Powerful and quieter flush
  • Save water
  • Odorless


  • Prone to clog due to long and narrow trapway
  • Bigger in size (longer bowl base to accommodate the long trapway)
Siphonic vs Washdown Toilet

Siphonic vs Washdown Toilet Trapway

What Are Washdown Toilets?

Washdown toilets, or European toilets are seldom seen in American homes. You will understand why as you read further.

The way it works is simple. It uses the weight and gravitational flow of water from the tank to flush-out waste from the toilet bowl. It can’t create the siphon action.

In another words, wastes are push out by sheer pressure of the water gushing out from the tank. Its short and wide trapway allows waste and water to exit the bowl easily.

The trapway diameter of a washdown toilet is easily twice the size of the siphonic model. Usually around 4 inches wide and much shorter. And due to its short trapway the water level in the bowl is low, resulting in smaller surface area. And because the trapway is short and wide, the washdown toilets are relatively clog free.

Most of the washdown toilets come with push button dual flushing system. It has 2 flushing capacity, a full flush and a half or partial flush. The half flush is usually 1.0 gpf or lower and useful when you only need to clear liquid waste. The full flush is of course meant for removing solid waste.

Here’s Why Americans Are Not Ready For The Washdown

Perhaps we never will.

Washdown toilets may have the advantage of being clog free, they are not great at bowl cleansing. And bowl streaking is a problem because of the small water spot, which causes skid marks to form easily.

Odor is also a problem with the washdown toilet especially on heavy load. As the wastes are not fully submerged in water, smell can quickly fill the air. So your toilet needs to be well ventilated and having an exhaust fan is a good way to keep the toilet fresh.

Another shortcoming is the need to flush twice on a heavy session. Not that it can’t clear solid waste efficiently, it’s the waste smears on the bowl surface. Often you will need a second or third flush to get a clean bowl.

Washdown Toilet

Characteristics of Washdown Toilets

They are shorter in length (front to back) obviously. The modern washdown toilet uses push-button flush instead of trip lever, and they are usually dual flush. But that’s not necessary always the case. There are models which are still using the trip lever, even for dual flush toilets.

The trapway is designed to be wide and short so waste can easily get push over the weir and straight into the drain pipe. And because of this, the water level is just at the brim of the bowl outlet. Hence, the water surface area is just as big as the bowl outlet (typically about 4″ x 5″).


  • Short body length
  • Short and wide trapway
  • Small water surface area
  • Skirted design (There’s no trapway to concealed)


  • Saves water (dual flush)
  • Efficient flushing
  • Clog free
  • Compact in size (shorter bowl base)
  • Lower cost


  • Odor
  • Loud and noisy flush
  • Small water surface area – Bowl streaking that leads to unsightly skidmarks
  • Often need to flush more than once for heavier load or to remove sticky waste

Regardless of toilet types and design varieties, this are the 2 toilet flushing systems the world is using. Both has its merits and flaws, you will need to weight them out and decide which one can best serve your needs.

End of the day, it all voice down to personal preference. Especially for a toilet, you can’t get more personal than that.

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  1. As a kiwi with US relatives I visit a lot and hate US bumsplasher toilets. The ones in public lavs (dont get me started on the lame euphemism “bathroom” – It’a the toilet/dunny/bog/lav. There is no f..king bath in there) are worse as the water trap is at testicle level. The number of these hideous contraptions I have seen blocked defies counting. Just another bit of yank Exceptionalist nonsense.

  2. I have made the exact opposite experience with never 1.6 GPF symphonic Toilets in the US while I was on holiday at the east coast. Many of the new 1.6 GPF toilets were not able to wash out the entire contents with a single flush, sometimes it needed more than two flushes. Some of these toilets even had problems with non solid waste or a single sheet of toilet paper. Only toilets with high tank or flushometer were more reliable. In contrast, my Keramag Allia Paris toilet with 6 liters or 1.6 GPF flushes even larger loads down. It is very rare that I have to rinse several times. Skid Marks are indeed a problem, but they can also be avoided. Newer toilets here in Europe have a special surface coating which makes it easy to remove soiling with a single flush.

    1. Author

      Hi Andy. Thanks for sharing your experience.

      I supposed your Keramag Allia Paris is a wall hung toilet with a 4 inch (100mm) trap/drain pipe? That’s awesome. 🙂

  3. I’ve been to europe a couple times and never understood why it seemed like their toilets used huge amounts of water. Now i know why. But this article never mentions the volume of water differences for each design? European toilets waste a lot of water and those types of toilets would never work in drought prone areas.

    1. Author

      Hi E.

      Regardless of design, the maximum water use for toilet flushing is 1.6 GPF in the United States. Europe will have a different rule on water usage for toilet flushing.

      “In 1992, the United States Congress passed the Energy Policy Act of 1992, which mandated that beginning in 1994 common flush toilets use only 1.6 US gallons (6.1 L).” Reference – https://en.wikipedia.org/wiki/Flush_toilet#US_standards_for_new_toilets

      So all toilets, including the washdowns, that are sold in the US will have to meet the maximum 1.6 GPF. However, in states like California, Colorado, Georgia and Texas only 1.28 GPF max. are allowed to be sold.

      Hope that answers your question.


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