[dmoose]

I live in snow country as well so when I updated my boiler for home automation I connected the existing controller that has worked for 20 years to the NC side of each relay. This way if the HA controller fails it falls back to the old less sophisticated but near bullet proof analog control system. I dialed those thermostats down 10 degrees below the HA setpoints. If the HA system fails the relays are inert and the old system keeps the house from freezing.

I suppose if it fails with the relays energized it's going to get pretty hot inside, but in the meantime if I have issues with the HA system I can just turn it off and use the old thermostats. This was great while I was debugging my design.