The recirculation of an aquatic system serves to continuously deliver chemically treated and filtered water, at specified temperatures and flow rates, to provide a clean, safe, and enjoyable environment for patrons. Recirculation systems are in almost all cases running 24/7 to meet a specified turnover rate, the allotted time for the recirculation system to process every gallon of water located in the pool.
Piping, an essential factor of the recirculation system, can exist in multiple forms throughout the recirculation system. All below grade piping is typically concrete incased schedule 40 PVC or Schedule 80 non-encased PVC. Once the recirculation piping makes it above grade, it is recommended that schedule 80 PVC be used or, when dealing with heater piping, CPVC may be considered. 
The pool piping shall be sized to provide code compliant velocities. Most codes also require all exposed pool piping to be equipped with color coded flow directional arrows at thirty (30) inch intervals. A piping diagram shall be included in the mechanical room which should detail the purpose and role of each pipe from the main drain(s) all the way to the inlets.
Pools can have two (2) different types of inlets, wall and floor. Wall inlets can be associated with lower cost, due to the piping not going below the pool shell, but floor inlets provide more favorable benefits. Floor inlets, when provided at equidistant locations across the floor, provide a more uniform distribution of filtered water as well as a “sweep and clean” movement across the pool floor. When racing lanes are present in a pool, inlets shall be placed in the lanes to prevent staining of the pool plaster or tile. When determining the number of inlets needed for a pool, the total flowrate should be divided by the number of desired inlets which will give the individual flowrate of each inlet. For floor inlets a flowrate between 35GPM – 40 GPM is recommended. When the inlet flow rate is on the high side, closer to 40 GPM, the floor will most likely contain less debris and dirt. It is advised that floor inlets shall not be placed close to main drains to prevent filtered water from being sucked back into the recirculation system.
Main drains shall be located at the deepest part of the pool on a flat surface. Each recirculation pump shall pull from multiple main drains with each main drain being designed to handle 100% of the recirculation rate. All main drain grates must be certified VGB complaint with flow rating clearly labeled on the grate. Main drain spacing shall be designed to the governing code, or a minimum of 4 feet apart when the distance is not specified. Main drain sumps come in a variety of sizes and materials, but stainless steel or poured concrete are most commonly used. Hydrostatic relief valves shall be installed at the bottom of each main drain sump. The valves provide a means of equalizing differences in the hydrostatic pressure within and under the pool, thus stabilizing the pool shell when abnormal sub-surface hydrostatic pressure occurs. This can happen when the pool structure is subjected to high ground water conditions that can literally float the pool shell out of the ground when it is empty.