The main reason for a pool drain is due to the TDS (total dissolved solids) in the pool water. In the summertime, temperatures reach 100 – 118 degrees. Combined with low humidity and windy conditions the evaporation rate in your swimming pool is greatly excelled. When water evaporates, only H2O dissipates The TDS remains in that body of water. The pool water is continuously replenished with fresh water supply through the automatic water fill. The water coming into the pool has a TDS factor. Thus, there is a rise in TDS.
If you find that hard to believe, do this. Fill a quart container with water at 900 ppm (parts per million). Evaporate 1/2 of the water from the container. You now have 1/2 of a quart of water with the TDS factor of 1800 ppm. Fill the container back to it’s original level with distilled water which is (0 TDS), and you will find out that you now have your original quart of water at 900 ppm TDS. However, the water that is being used to replenish pools is not 0 TDS. In fact, it could have a TDS factor as high as 900 ppm.
Now, perform the same experiment as before. Only this time, fill the container with it’s original level with water having 900ppm TDS. You will now have 1 quart of water with a TDS factor of 1350 ppm.
This is exactly what happens in your swimming pool. In addition, everything that enters the body of water in the pool: dirt, leaves, chemicals, and everything else has a TDS Factor which contributes to the overall rise of TDS in the pool water.
When the TDS level of your pool rises, subtle problems start to occur. Calcium deposits at the water line, brown manganese stains, salty taste to water, irritation to skin and eyes, and the list goes on. As the TDS rises higher and higher these problems amplify to a point where the swimming pool will begin to deteriorate. At high levels of TDS, calcium will begin to replace the plaster in gunite pools. This will expose itself as a dime sized circle on the plaster. At this point the calcium deposits at the water line will have grown to resemble a stalactite and manganese stains may be very evident.
That is exactly why you should not let the TDS level rise to these levels. The solution given by the pool industry at this time would be to drain and acid wash the pool and bead blast the tile. After the pool cleaner has finished his job the most important thing a swimming pool owner should do is check chemicals once per week and backwash once per month as a regular pool maintenance program.