I have a Climatemaster Tranquility 27, 2 ton, running in open loop. I had some water flow issues that are now resolved, that resulted in low water temp lockouts. The last time, the water coil in the heat pump was frozen and took a while to thaw. The low water temp trip is 30F. Is it normal for the water to freeze in the coil after tripping? Why did it do it only the last time out of 4 similar events? Should I be worried about a bad temp sensor?