How dual flush toilets and its valves work

A dual flush toilet is designed to conserve water by offering two different flush options. It uses a dual flush valve, which allows the user to choose between a full flush for solid waste and a half flush for liquid waste. This system is more efficient than traditional toilets because it reduces the amount of water used with each flush.

The mechanism allows you to choose between two levels of water usage with the push of either the full-flush or half-flush button. When the smaller button is pressed, it triggers the half flush, releasing less water. When the larger button is pressed, it triggers the full flush, releasing more water. Inside the tank, the flush valve controls the amount of water released based on the button pressed.

Because it can use less water for liquid waste, dual flush toilets are considered to be more environmentally friendly. From July 2009, the government mandated that all plumbers in Singapore have to install dual flush low capacity flushing cisterns.

Gravity operated dual flush valve mechanism explained

Gravity operated dual flush valves consists of a cylindrical valve body fitted with two separate floats attached to a central seal. The full-flush float sits at the bottom of the valve and the half-flush float is positioned midway up the valve.

When either flush button is pressed, the corresponding float and the central seal is lifted to allow water to flow from the cistern tank into the toilet bowl. When the water level drops below the level of the lifted float, the float descends and mechanically pushes down the seal to stop the flush and let the tank refill for the next flush.

How much water does the half and full flush use?

Starting 1 January 2023, flush valves were mandated to have at least two ticks under the MWELS programme. A two tick rating meant that the flush volume had to be between 3.5 to 4.0 litres for a full flush and 2.5 to 3.0 litres for a reduced flush.

In SS636:2018, every flushing cistern serving a water closet pan (commonly refered to as a toilet bowl) shall be of such a design as to give a dual-flush of two different volumes such that the volume of the full flush does not exceed 4.5 litres and the volume of the reduced flush does not exceed 3.0 litres.

Half flushes are not really half

The more accurate terminology for half flush is reduced flush as the half usuually uses mroe than half the volume of water of a full-flush, so the term "half-flush" is not accurate.