Furnaces heat air
A furnace heats air and uses a blower and duct system to distribute that air through supply registers. Return ducts bring air back to the equipment.
Boilers heat water—or create steam
A hot-water boiler circulates heated water through piping to baseboards or radiators. A steam boiler distributes steam to radiators. These systems do not depend on a central air-distribution blower.
Neither is automatically better
The right system depends heavily on the existing home. A house already built around hydronic or steam distribution may have very different replacement choices from a house with usable ductwork.
Replacement decisions
Before changing system type, consider the distribution system, cooling needs, electrical service, venting, fuel availability, installation cost and the condition of existing piping or ductwork.