I have a toilet flapper with an adjustable float on the chain. As I move the float down the chain, more water is released from the tank per flush. Why is this? How does the mechanism work?

  • What does it do? - it allows manufacturers to sell 'low-flow' toilets, that after you adjust the float, have half a chance of actually flushing something.
    – Mazura
    Jul 19, 2019 at 19:50

2 Answers 2


Buoyancy holds the float afloat. Suction holds the flapper down.

When you break the suction, the float can do what it's been trying to do the whole time: get to equilibrium, which is the top of the water level. Only when there's enough chain hanging will it lower the flapper down to where it seals.

Once it seals with even a tiny amount of water and has suction again, you'll have to push the handle down a lot harder than the float is trying to lift; the float only has enough buoyancy to lift the flapper valve, not break the suction.

The height of the float determines how much water can be left over in the tank before it re-seals. The less water left in the tank, the more water down the drain.

  • This being the accepted answer makes me believe that the question really belonged on Physics.SE... Jul 19, 2019 at 11:03
  • But then there'd be a bunch of LaTex in the way of the OP understanding what part the concept of suction plays in a toilet. "Why does the flapper close?" - gravity. Why does it stay there (even when the float is lifting)? - suction. Why does jiggling the handle make it stop leaking? - the last time it closed, it was off center, but suction still holds it there. Jiggling it, momentarily breaks the suction enough for it to re-seat, and flow will close it again; jiggle too hard and it will lose all suction and flush (because the float can defeat gravity, but not suction).
    – Mazura
    Jul 19, 2019 at 19:42

Pushing the float down the chain makes the distance between the float and the flapper less, therefore the same is true of the distance between the water level and the flapper. That means more water is released before the flapper is lowered into its sealing position.

  • I understand that this is what happens, but why? Why does the flapper close when the water level in the tank reaches the level of the float?
    – Például
    Jul 18, 2019 at 20:16
  • 1
    @Például The water level in the tank does not “reach the level of the float”, the water level and the float level always stay the same because the float is floating on top of the water. Therefore the float is always at the water level.
    – Alaska Man
    Jul 19, 2019 at 1:19
  • I now understand that the float is just something that helps guarantee that you get a certain amount of water out. When you set the float at a certain level the float will continue to exert an upwards force until the water goes down to that level. At that point the float is floating on the surface and no longer pulling the flap upwards. However, it’s up to gravity to pull the flap down. If you set the float at a certain level, flushing should release at least the amount of water from the level of the float to the fill level, but it might release more. That depends on gravity and the flapper.
    – Például
    Nov 30, 2023 at 3:04

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