I also have a pool/spa combo but I heat my spa with the gas heater only when we use it. I let the overflow take care of chlorinating the pool and spa together. I think you are looking for a higher free chlorine concentration in a typical spa because bather load is higher due to smaller water volume, but with an overflow 3 hours a day, you are greatly mitigating this issue. You are not constantly recycling the spa water but you are moving fresher, lower bather load water, back into the spa during pool mode. I am no expert, but I do have a technical background and I don't think the chlorine difference you are looking for is a huge issue. The biggest benefit of the swg in this situation is that WHILE you have heavy bather load in the spa, you are getting the higher chlorination just while you need it, when it is in spa mode.