The backup system will incorporate some form of a wood fired stove and a propane or electric boiler.
Wood is the logical choice for our secondary heat source.
With a combination of solar and wood, our heating bills should be next to nil. And I'm sure I've missed a few. I have pretty much researched them all to death. They all use cast iron or stainless steel to enclose a firebox around a water jacket. The water is pumped underground to the house to heat domestic hot water, radiant floor systems or retrofitted forced air systems.