Luigi,

There are several ways to do a pool and spa combo. If the spa is a stand alone (fiberglass or concete), you need to separate filtration systems, one for spa and one for pool. That means two of everything...

With a spillover spa, you share the filter, and pump. While the pool pump is running the water from pool returns to the spa and spills over. That keeps your spa clean and you dont have to worry about controlling the chemicals in two bodies of water.

You can have a one pump system or two pump system.
With one pump system the water is drawn from pool main drains and returned through pool inlets. Some of the water is also returned through the spa creaing your spillover. To use the spa two valves are turned, either manualy or with a controller such as Jandy, and the suction changes to spa maindrains and returns trough the spa jets. The spillover stops and now you can heat your spa. With a two pump system, the pool pump draws from pool main drains and returns trough inlets in pool and spa and a second pump is for spa jets.

Hope this helps.

Zina
www.pooldesign.net