Lots of variables here that we still don't know. There's a big difference between dropping to 21 for 10 hours and dropping to 29 for 2 hours. For the former, I would definitely want the furnace running. For the latter, I would just get up early and run some water. Don't forget to leave cabinet door open under the sinks.As Danemayer pointed out, a generator (and plenty of propane) should relieve your worries.