A boiler can be used with an air-to-water heat exchanger, similar to an A-coil used in your plenum for a/c.
I presently heat our home with an add-on forced-air furnace, a Fire Chief, inside the house. It's sheet-metal ducted just like any natgas or fuel oil unit would be. I'll be adding an indoor wood-fired gasification boiler in a year or two once all the buildings are completed.
