The problem with using a heat pump to heat the spa is that it takes too long. Heatpumps are efficient at maintaining pool temperature but lousy at quickly raising temperature.

I use a heatpump to keep the pool water at 85 and a gas heater for when I want to use the spa. I have a 400K BTU propane heater that takes about fifteen minutes to raise the spa temp temp to 100 degrees. A heat pump would take several hours - if not more.

Good Luck