Forget what you know about traditional HVAC, because when it comes to its system design and mode of operation, radiant heating shares little in common with older, increasingly outmoded systems like forced air. In the case of forced air, the furnace blows heated air into the living spaces indirectly, via bulky metal supply ducts. Radiant heating, meanwhile, relies not on air, but on something much more effectively controlled—water. In a home heated by a radiant system, boiler-fired water pumps through a network of tubes set into panels installed beneath the floor. The tubes heat the panels, and the panels, in turn, radiate warmth into the home from the ground up.