What is the failure mode of a Nest these days? i.e. if it crashes or otherwise has a failure what mode does it default to? This isn't a purely theoretical concern as I know someone who had an older Nest fail and they said it defaulted to just being off. I'm sure this is a rare event (and could theoretically happen with any thermostat or the furnace itself of course) but living in the Northeast and traveling a lot I hesitate to introduce anything into my furnace control system that could increase the potential probability of failure.