Sorry to revive an old thread, but I have a similar idea. I plan to plumb a line from my air compressor (with a pressure regulator)
into the irrigation system before the zone valves. If the temp got below 40F I’d love to have the controller shut a valve supplying the water, open a valve to supply compressed air, then open each zone for 30 seconds to blow the water out then close all valves. Ideally, the system could either reset itself when the temp reaches a certain point or it could be manually reset.
Could this be done? That would be full proof freeze protection.