Radiators were a common heating method for a long period of time and are still used in newer residential applications today. With the help of a boiler, pump, valves, and various thermal-mechanical or digital accessories; hot water is distributed through pipes to heat specific rooms or "zones". Since the whole house isn't heated constantly a boiler only operates to maintain the heat of the water in the tank when the load falls. If this is installed you likely have no heating duct work.