Low-flush WC problems

A number of factors affect the reliability, from the manufacturer, the design of the fixture, flushing method, and how long fixtures have been sold at the particular flow rate, so it’s difficult to recommend a lowest flush volume. For commercial flush-valve fixtures (those without their own tank) we've seen issues when the porcelain bowl is made by one company, and the plumbing valves are made by a different company. Better results can result from using the same company for both bowl and valve. The first round of models made for the lower flow rates tend to have more problems than the second generation, when problems get figured out. In tank-style toilets, the factors than can affect performance include the diameter of the trapway, whether the toilet uses siphonic action or “wash down” method, (which is more common in Europe), and whether there is pressure assistance or power assistance to aid the flushing action. In the USA, the low range of flush volumes is 1.1 to 1.28 gallons per flush, or 4.2 to 4.8 liters per flush, but others may have more experience with the 2-4 liter range you mention in other countries. For ratings on the Maximum Performance (MaP) testing of many residential and commercial fixtures in the US see, http://www.cuwcc.org/MaPTesting.aspx .

I would also check the "GreenSpec Products" shown in the sidebar to the right on this page. (Clicking through to view detail on any of the products listed will require a separate membership to GreenSpec, which is published by BuildingGreen, also the makers of LEEDuser.)GreenSpec has a great screening of products that not only perform well in terms of water efficiency, but they also flush the **** down the toilet!