Just a thought Amanda,
It sounds as though the rig is garage kept and residual moisture freezes in the lines when not in use.
I don't have a single tier set up but thought perhaps if there was a way to blow the system out with compressed air after brewing it might help in keeping things from freezing up.
Glad to hear your day today went well!
Fortunately I have thought of that!
Since it is (insulated) garage kept, I store it with all of the valves open, including the drain valve.
The freezing from a week or so ago was when the rig was in use, after I had to use the main drain valve, and when it was exposed to 8F temps.