I've been inspired by various passive solar projects. One of my favorite project ideas is to supplement my hot tub heating with a solar water heater. I've seen various totally passive designs, and some with just pumps, but these don't seem to deal with the possibilities of cooling your water by pumping it through the collector on cold days, etc. I have used temperature controllers before, but most tend to have a single sensor. This project would need two sensors, with logic that only pumps water through the collector when it would be efficient to do so. There are pre-made solar water controllers out there, but most are intended for supplementing water heaters.
Any advice on where to get started sorting out the options for a solar hot tub heater that can sense the appropriate temps?