Most dual-flush toilets use a drop valve, which sits underwater at the bottom of the tank.
Instead of relying on siphoning it uses gravity to do the job, which means less water for every flush.
But that valve can easily be stuck open by mineral deposits or other debris, causing the toilet to fill continuously.
Plus: “The volume of water loss is getting bigger every day as more people refurbish and retrofit their older toilets and as we build more homes, so we’re actually adding a problem.”