Here are the facts.
Our upstairs bathroom is the only plumbing we have in the upper level. Kitchen and half bath on lower floor and laundry tub and washer taps in basement. So, one hot copper 1/2 pipe goes upstairs through the exterior wall and the hot line goes through an interior wall, near the exterior wall.
We have gone away for the last two years for a week or two during the winter, and turned the heat down to around 60 F while we were gone with no freezing problems.