What does the bottom drain do in a swimming pool?

Answer 1

The main thing a bottom drain does in a swimming pool is help the pool water circulate better and more evenly stabilize the water temperature. Without a bottom drain, the skimmer is the only place water gets out of the pool to the filter system. If the return jets are aimed toward the surface intending to sirculate the floating debris toward the skimmer, as most home-owners aim them, the water near the surface circulates around the pool but the cooler water pretty much stays near the bottom of the deep end. With the filter pulling some water in from the bottom drain, the cool water being removed from the bottom will cause the cooler water to be replaced with some of the warmer surface water causing the water in most of the pool to be pretty close to the same temperature. Also, if there is debris causing cloudiness in the water, if you add clarifier, the clouding matter in the water will settle to the bottom of the pool. With a bottom drain, some of that cloudy debris might be able to be trapped by the filter to help clear the water. But that's no guarantee since often what's making the water appear cloudy will be too fine for the filter to be able to trap so trying to run with water flowing through the bottom drain can sometimes result in cloudy water flowing back into the pool through the returns causing the water to look even worse. In which case, best bet is some drop out followed by leaving system turned off for 12 - 24 hours, then vacuum to waste to pump all the cloudy crud out of the pool through the waste line.