I had a radient heat system in southern utah and also had solar pannels. I had 10 pannels and the water in the winter would heat up to 140 deg and the summer would heat up to 210+. I also had a boiler and a water to water heat exchanger tank the solar water would heat. The problem was the floor would cool down the water too much to heat the house very much. It would take the edge off the cold in the house so the heater or boiler wouldn't come on as much. If my house had more direct sun and there wasn't a big hill in my back yard I might have gotten better results. From what I heard was that you need alot of pannels and a big tank to store the water.