Rainwater collection tank freezing
I have constructed a rainwater collection system at my mountain home in western NC. I use it to water my plants and garden while I am there and when I am away. When I am away, I worry about my 275 gallon, above ground tank, freezing and bursting if the temperature drops below the freezing point. I want to add a thermostatically controlled drain to empty the tank when the temerature drops below freezing. I can use an irrigation valve on a line that I can tie into my overflow drain line, whereby it will open at about 30 degrees and drain the tank. If the temperature increases, the valve will close and the tank will be allowed to refill when it rains again. My watering of plants is gravity fed with a timer control. My queston is:
What is the best/most economical/recommended type of thermostat control device available to operate the irrigation valve? It must sense the outside temperature and be able to operate a standard irrigation valve based on outside air temperature. The irrigation valve is closed unless a voltage is applied to a solenoid, which opens the valve.