This diagram shows the three basic operating modes of a sand filter.
Pool Main Drain
The main drain is built into the deepest area in the pool to allow some flow of water from the bottom of the pool through the circulation system and filter. This is not the drainage unit for the pool but acts instead as a floor suction device.
The automatic skimmer skims the water surface catching floating debris before it becomes water logged and falls onto the pool floor. It has a weir or "flap" that adjusts automatically to fluctuating water levels, allowing only the top layer of water to flow over it. The skimmer basket needs to be emptied regularly (even several times a day in fall) to ensure continued effectiveness.
Automatic Pool Water Filler
An automatic pool filler connected directly to your water system will add water to your pool as soon as it drops from its set level. It is very important not to let the water level of a pool get too low as the pump can run dry and damage or even burn out the motor.
These are inlet fittings that are usually placed on the opposite end of the pool to the skimmer. Through them water is allowed to re-enter the pool after it passes through the filter. If pointed upwards they will create surface circulation that moves floating debris towards the skimmers.