Thought I would throw this into the mix and see if it has any traction. The thought is that in at least some, if not many, wood boiler applications the system returns quite hot water to the boiler. Might there be an advantage to using a heat pump to "process" that return hot water to provide additional heat btu's for space or hot water heating with resulting cooler return water going back to the boiler? Cooler return water to the boiler would increase boiler hx efficiency. And a heat pump could pull lots of heat out of "hot" boiler return water. I don't know if a heat pump can work at a water source of, say, 140F to deliver heat at, say, 180F, with then cooler water back to the system. Perhaps a hair brained idea.