Just wanted to show you guys the new control system that I finished up over last weekend. She's a bit rough around the edges but fully functional. My apologies in advance for the lack of pictures as all I have right now is the one below. The system is driven by an Arduino USB board. The LCD you see displaying the temperature readings is one of SparkFun's serial enabled LCD's (very easy to hook up and use). The temperature sensors the board gets its readings from are the Dallas 1-Wire devices. Right now I have nine temperature sensors that the Arduino gets its readings from. Eight of the temperature sensors are located on the storage tank with three of them being on top, two in the middle, and three on the bottom. The last temperature sensor is mounted on the supply line off the boiler. Right now the logic that the board uses is pretty simple and rudimentary. When the board senses that the temperature at the top of the storage tank is above the usable water temperature it will activate a solid state relay for the load circulator. If the water temperature is below the usable temperature it will activate a solid state relay for the propane furnace that I have for backup and open the circuit for the load circulator. This way the next time the house calls for heat it will use whichever system the board has decided is best, with the boiler system obviously being the priority. The board also controls the loading the storage tank based on the temperature sensor on the supply and the temperature sensors in the middle of the storage tank. If the temperature sensor on the supply is more than 5 degrees warmer than the middle of the storage tank or is above 175 degrees the board will activate the solid state relay for the boiler circulator. I'm still not sure if this is the best way to handle the differential loading of the storage tank but it is better than what I had, which was nothing, and I can always fine tune it in the future.