The main drain for a pool is a suction port that should be located at the deepest point on the pool floor. A main drain is responsible for aiding in the circulation and filtration of the pool water. Having a properly working and dedicated main drain line can also allow you to fully drain your pool if necessary.
Main drains on older pools are often attached to the skimmer lines back to the equipment. When this configuration of the plumbing exists, a vacuum plate is necessary in order to vacuum to pool. A vacuum plate will block the port in the skimmer that the main drain line is attached to and isolates the skimmer to provide the suction necessary to effectively vacuum the pool.
As of December 27, with the passage of the Virginia Graeme Baker Act, all public pools should be equipped with anti entrapment/anti vortex main drain covers. Public pools with a single main drain require the addition of a Safety Vacuum Release. A vacuum release system is capable of providing vacuum release at a suction outlet caused by a high vacuum occurrence due to a suction outlet flow blockage. Pools with a dual main drain system, two main drains measuring 36” apart (three-feet from center to center) or more are exempt from the addition of a Safety Vacuum Release system.
Once suspended material in a pool drops below the level of the skimmer(s), the only way for it to find its way into the filter is through the main drain. When a pool does not have a working main drain it simply settles to the bottom where it will sit until vacuumed out. While main drains do assist in the circulation and filtration of a pool, many pools have a disabled or capped main drain line.
In the Austin metro area which includes Round Rock, Georgetown, Cedar Park and Pflugerville, soils shrink because of persistent drought and then swell after a heavy down pour. This constant movement could break or compromise the main drain line. The location of the line makes it expensive and unfeasible to repair. The end result is the capping of the main drain line.
Main drains in the Austin Metro area often become blocked due to the tremendous amount of debris that our trees rain down every spring and fall. While this is very annoying, it is easily avoidable by maintaining the pool more regularly during extreme drop out. Even with vigilant pool maintenance, main drains will become blocked. A blocked main drain is not the end of the world, all that is needed is a stiff CO2 blast of the line.
A CO2 blast is done from the pump back to the pool. The main drain line is isolated from the rest of the plumbing and then a specially tipped hose is inserted in the pump suction port. The hose is attached to an 1800 psi CO2 tank, which is slowly opened to begin to pressurize the line. When enough pressure is applied to the blockage, the blockage is flushed back into the pool clearing the line.