I brewed this morning. It was -20° F. When it came time to chill, I ran the wort through my plate chiller full speed, with all valves wide open, powered by my Chugger pump. The carboy filled up in about 2 minutes, and the wort was 64º F. But it was so cold that the regulator on my burner froze up and I couldn't adjust my flame. Also, the foam on top of my bucket of Star San froze and was crunchy. The hoses got stiff and my hands were freezing, but it all worked out and the beer is now happily fermenting.