For a little more info. The weir door has a float in it so that it will float just under the surface of the water when water is flowing into the skimmer. That provides two functions. One is to improve the skimming action at the surface of the water and the other is that it helps to prevent a vortex in the skimmer that would suck air into the plumbing. That same float will 'close' the weir when the water stops flowing.