Wille, I can't say for sure since I can't see your equipment layout, but if it is like mine, I have one water in and one water out of the heater. Changing from pool to spa is done with the manual valves that are downstream of the heater. I fully open the spa in and spa out, and I fully close the pool in and out. That was what the pool builder said to do, and it works. Just be sure to change them back afterward.