We had a cold snap up here. Eleven below zero (Fahrenheit) for the low. While we were gone for the weekend, the house did fairly well. It only got down to 55F. So that was fine. But then Monday morning when it was really fridgid, the solar panel fluid froze. It sounds like it might not have gotten solid, but it was slushy enough the pump would not move it. Once the sun got up, the panels got hotter than 257F, but somewhere else in the system was still too cold. The plumber came by and back pumped until the system defrosted, and then he added more anti-freeze. There is an expansion tank in the system, so we probably did not damange anything. Tuesday morning, it froze again. So the plumber came by and added more anti-freeze. The weather has changed and warmed up radically, so that’s where we are now.