McDonnell & Miller water feeders open if the boiler's water line drops to the feeder or cut-off combination's feeder operating point, which is near the bottom of the gauge glass, the feeder will then open mechanically and add water to the boiler. How much water will enter the boilers depends on several things, but there will always be enough to keep the boiler operating at a safe minimum water level. Once it has added the right amount of water, the feeder closes.