I had experience with exactly what Tom is describing. During the ice storm of 1998 I had left my house in Maine to return to PA the day before and had done all the normal setbacks that I would do including setting the thermostats to 55. Then the next day the storm hit and then subsequently the power was out for close to a week. I had a caretaker who kept checking the house and just to check he things completely he left a thermometer on the basement floor. It never went below 35 or 40, I cannot remember the numbers but it did not freeze. The house was tight for its day (2.4 ACH@50) and well insulated so no damage was done. That house had a hydronic heating system so turning off the main water supply would mean no make-up water for the boiler. So I always left it on and had no trouble. I would turn off the boiler mate water heater but otherwise always left the plumbing on. I never had a broken pipe.