Not all in-ground pools have skimmers. How then is the water drained from the pool and re-circulated?
Pools that do not have skimmers normally have what is called an overflow system. A channel is constructed around the edge of the pool and inside this channel there are drains (normally called gutter drains) which collect the water and convey it to a balancing tank (surge tank). This is a collecting vessel, which can be made of fibreglass, plastic or concrete depending on the size and nature of the application, and which is used to “balance” and compensate for the water leaving the pool through the drains (which is not always flowing at a constant rate as it is in a skimmer pool). This balancing tank should have a water-level regulator connected to a water-supply line. With this system, when the water reaches a predetermined low level, a valve will be activated which allows water from the supply line to fill the tank. This is to ensure that there is always sufficient water for the pump to operate correctly without sucking air. The water is pumped through the filtration system and back out through the return inlets (wall inlets around the side of the pool) - or, more used in new hotel- and public pools, bottom inlets. This system involves a lot of suction, but if the pool is built and operated correctly, there is virtually no risk of suction holding somebody against one of the drains. The only way the plumbing system could exert this sort of suction is if there were only one open drain. As already mentioned above, in a safe pool, there are always multiple main drains as well as several skimmer drains, so if somebody or something blocks one drain, the pumping system will pull water from one of the other drains. This eliminates the suction on the blocked drain. Many swimming pools also have one or two vacuum points, which are only used in pool cleaning. Normally, these vacuum points are connected to the main filtration line although they may have their own separate pumping system. Automatic pool cleaners or other cleaning equipment can be attached to these vacuum points by means of vacuum hoses – these use the suction created by the pump to suck water (together with any dirt or sand in the pool) in much the same way as an ordinary vacuum cleaner works (with air). Apart from automatic cleaners depending on the pool pump, there are also automatic cleaners with own pumps, operating on an electronic circuit and working under water.