My setup uses 3 way jandy valves for pool only, spa only or spillover operation. (they are automated by my aqualogic control system) in pool only mode the drains and the returns go to the pool, in spa only mode the drain and return to to the spa, in spillover mode the dains in the pool are on and the returns to the spa are on. I have a heatpump that brings my spa to 104 degrees in about 15 minutes from pool temp in spa only mode during the winter here in N FL and allows separate settings for pool and spa temp. In spillover mode the spa is at pool temp (85 degrees) I usually do not operate the spillover unless I have has a lot of spa use, the waterlevel in the spa is low, or if I am adding chemicals. Not sure how you could plumb it if the spa is not physically located right next to the pool and has a spillover but it probably can be done.
Bookmarks