Thank you for writing. Most overflow pool / spa combinations with one filter and water pump, send water to the spa jets and pull water from the pool during the day. Then when you want to use the spa, you change (2) 3 ways valves to pull water from the spa drain and send water to the jets stopping the overflow to only heat the spa. For every 100,000 BTU's of heat, you will get about an 18 degree temperature rise for every 500 gallons in the spa per hour. I would get with a local pool guy to go over everything as there are so many ways to pipe these things that we don't want to turn a wrong valve and drain the spa or the pool. It is depressing when you have a high water table and you drain the pool and it "pops" out of the ground.