Today’s low flush toilets


By Simon Blake

Few plumbers who were in the business 15 years ago will forget the problems when low flush (six-litre) toilets were introduced in this country. The problems got so bad that people were going out of their way to buy the older 13-litre models. New designs and test procedures have largely put an end to those issues.

“Since the early 2000s, the industry figured it out. They figured out how to design the shape of the bowl, the trapway, the tank and how the water moved through the system in order to remove the waste and get it transported down the drain line much more efficiently,” remarked Robert Zimmerman, director, engineering, sustainability at Kohler Co., Kohler, Wisconsin.

In fact they figured it out so well that it was a relatively small step to go from six litres to 4.8 litres per flush, which makes up most of the market today.

A new test method provided a big push. Introduced in 2002, the Maximum Performance (MaP) Test used simulated human waste made from a soybean mixture that duplicated real world conditions, unlike previous tests.
“Instead of being a pass/fail, it was an actual rating,” reported engineer Bill Gauley, who along with John Koeller created the MaP Test. Consumers could go on the MaP website and check the flush performance of the toilet they were considering. Manufacturers wanted to score high. “Because the scores were published, manufacturers started really trying to improve,” added Gauley (Bill Gauley Associates Ltd., Acton, Ont., formerly with Veritec Consulting, Mississauga, Ont.).

The other key was that Gauley and Koeller capped the performance requirement at 1,000 grams so that manufacturers didn’t have to chase a constantly moving bar, as HVAC manufacturers have to do with efficiency requirements.

“It became an unofficial standard for water closets,” added Frank Leone, regional manager wholesale, Ontario and Atlantic, for American Standard Brands, Mississauga, Ont. It was so successful that lower volumes were tried. “We started, in the lab, reducing flush volumes by 20 percent to 4.8 litres (1.28 gallons) per flush to see how that would work. We found the new (lower flush volume) designs still worked fine.”

The U.S. Environmental Protection Agency (EPA) used the results to establish its WaterSense standard that requires toilets to flush 350 grams of waste at no more than 4.8 litres.

“But the products just kept getting better and better, so now we have 4.8 litre toilets that will flush 1,000 grams,” noted Gauley.

“We use that test internally for quality control as well as product development. It really has become an important part of how performance is measured,” said Zimmerman.


The Canadian-made Proficiency line uses a unique flushing system and only three litres of water.

How low can it go?

Originally, low flush meant six litres or 1.6 gpm per flush. Today there are toilets on the market that flush with half that amount, which begs the question: just how low can it go?

“The industry isn’t uniform on this, but it’s my opinion that four litres is about as low as you are going to want to go for toilet flush,” said Zimmerman. “The plumbing is a system – it’s not just the toilet and waste – it’s all the other inputs of clear water that are coming into your drain that have also been reduced. There is a minimum flow rate that makes the system work, but there’s no absolute number that you can say ‘this is it.’”

Gauley expects the next big push will be to go from 4.8 litres flush to 4.0 litres/1.1 gallons. There is already a new MaP category – MaP Premium – for toilets that flush 600 grams with four litres or less. “We know it’s difficult to get down to three litres, but four is not a problem. And we knew that people wouldn’t sacrifice performance to get more efficiency.”

“That’s where people that are trying to conserve are going to,” noted Leone. “(The MaP Premium) is more important than even the WaterSense certification,” he added. Some municipalities, concerned about exceeding their water treatment infrastructure capacity, are already mandating MaP Premium toilets in new construction. “That’s what drives conservation in countries like Canada where there is an abundance of water.”

Gauley expects that with virtually all toilets meeting the WaterSense standard, it is likely that the EPA will reduce flush volumes to four-litres/1.1 gallon, while maintaining a 350-gram volume.

Leone believes the WaterSense standard is too low because the average man can excrete 250 grams and then, when one adds paper, the total waste volume can exceed 350 grams. “Today, with anything under 500, people don’t consider it a good performing toilet.”


Toilets have come a long way. Kohler uses a canister-type flush mechanism for better efficiency.

Different approaches

The only widely available three-litre toilets on the market are the Proficiency line from Hennessey & Hinchcliffe in Mississauga, Ont. Launched in 2009, all models flush 800 grams in MaP testing.

“It was a pretty big leap,” remarked Hinchcliffe & Hennessy general manager Jerrad Hennessy. It uses a unique passive air pressurized trap-way that starts an immediate siphon without depending on water entering the bowl. As the toilet tank refills, it pushes the air out of an airtight capsule that is connected directly to the trapway, where it creates pressure – less than a few psi. When the user flushes, the air is sucked back into the capsule, creating a vacuum in the trapway and an immediate siphon.

“Three litres of water are effectively used to clean and scour the bowl since our tests have shown that the vacuum created by our BSB flush system alone will flush the toilet contents without any additional water,” said Hennessy.

Also on the leading edge of water conservation, American Standard Vormax technology uses twin flappers. Typically, a toilet uses about 70 percent of the water to start the siphon, with the remaining 30 percent to clean the bowl. Vormax technology reverses that, producing a strong siphon with 30 percent while keeping the bowl spotless with 70 percent of the water flowing from a single jet, sending the water around the bowl in a cyclone effect.

Kohler has gone away from flappers altogether, using a canister type “flushing engine”. A cylinder lifts straight up so that the water comes in from 360 degrees, allowing a quicker transfer of water from the tank while straightening the flow and reducing turbulence.

Drain line carry performance

As toilets flush with lower volumes and low-flow faucets and showerheads are installed, there has been concern that slopes and diameters of existing drains, which were designed for larger volumes of water, might not adequately carry the waste to the sewer main.

“The codes often don’t reflect that the water volume going down the drain may be half what it was years ago,” noted Zimmerman.

In new construction, engineers can design for lower flow, but not so in retrofit. However, he noted that when toilets went from six litres to 4.8 there were very few reports of problems.

Increasing the drain slope doesn’t necessarily help – tests have shown an increase from one percent to two percent grade makes the water flow past the waste rather than pushing it.

The Plumbing Efficiency Research Coalition, an organization created in 2009 by plumbing manufacturer and regulatory organizations, conducted extensive testing on low flush toilets in commercial applications that, among other things, showed that reducing pipe size from four inches to three inches to increase flow speed did not offer improvement, in large part because paper products were more likely to clog the system. The tests also noted a significant drop in drain-line carry performance when the toilet flush volume was reduced from 4.8 litres to 3.8 litres. Full test reports for studies conducted in 2012 and 2015 are available on the group’s website at

Canadian code authorities have been looking at this as well, reported Ralph Suppa, president of the Canadian Institute of Plumbing & Heating (CIPH). As a result, the most recent edition of the National Plumbing Code maintained six-litre toilets for retrofit applications because they have proven to work well. In new construction, however, the maximum flush volume has been reduced to 4.8 litres.

MaP test 2

The more, uh, realistic MaP test helped the industry move forward and gave consumers – and plumbers – a reliable way to compare toilets.


Educated customers

Today home-owners have often done their research before shopping for a toilet, including checking MaP test results, noted Ginnette Charbonneau, Mondeau showroom manager for Boone Plumbing and Heating Supply in Ottawa. “The ones that we recommend to customers are those that achieve 1,000 on the MaP test,” she added. And so many models today achieve 1,000 grams per flush that the customer doesn’t have to choose between efficiency and style.

Plumbing wholesalers tend to stock brands with a proven track record. “They’re really well made. Many of the ones that we deal with have a five or even ten-year warranty,” she added.

At the MaP testing facility today, it’s rare to see a bad toilet among the thousands tested, said Gauley. “With most of the legitimate manufacturers, the products are far superior; we rarely get a bad fixture in now where as initially (in 2002), half of them were substandard.”

And that, for the plumber, means that callbacks for poorly functioning low flush toilets have become a thing of the past too.


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