After our first winter we ended up with a cracked flat plate even though I was very careful to drain it any time the temperature was below freezing. I know my flat plate was cracked because I went to check it one day as it was getting cold again and when I opened the valve I got coolant not water draining out.
After installing a new flat plate, not a fun job. I did some experiments to try to figure out what had gone wrong. What I discovered is that when draining it is necessary to open the hot water faucet to allow air into the system so the water drains out. I knew this, what I didn't realize is there is no check valve in the faucet so that if the faucet is left open when the cold is turned on water will feed backward through the faucet into the hot water system. We spend a week camped in weather that was ranged from the low 20's during the day to the teens at night an I am sure at some point we had the hot water faucet open and inadvertently filled the heat exchanger with water that then froze.
I am posting this so hopefully others will learn from my mistake.