Basic Pool or Spa Plumbing Systems. To understand the basic plumbing system of a pool or spa, you must follow the path of the water. The water from the pool or spa, not both at the same time, enters the equipment system through a main drain on the floor, through a surface skimmer, or through a combination of both main drain and skimmer. It travels to a three-port valve (if there is no spa, there will be no such valve) and into the pump, which is driven by the attached motor. From the pump, the water travels through a filter, then to the heater, and back through three-port valves to the pool or spa return lines.