I have a boiler system and I have had some problems with it when it was -40 outside. It does not take long for the house to cool down in that kind of weather. I was a little panicked and wished I had a second source of heat. Since that time I have installed 6 electric heaters for backup heat in the rooms where the water lines are located. Of course electric backup is not very effective when the power if off. The safest backup heat is a wood stove but it is a lot more involved and costly to set up. Wood heat is also a lot dirtier and requires a lot of effort. I recommend some sort of backup just in case. The same can be said for a regular furnace but in my experience a boiler is more likely to go down then a furnace.