A radiant heating system includes an air-to-fluid heat exchanger, a radiant heating loop and a pump and conduits to circulate heating fluid between the air-to-fluid heat exchanger and the radiant fluid loop. The air-to-fluid heat exchanger is installed within a hot air duct connected to a conventional forced air furnace; preferably in a section of the hot air duct located near the forced air furnace with an expanded cross-section. In operation, the apparatus uses heated air supplied by the forced air furnace to heat the heating fluid circulating through the apparatus. Some of the heat from an air flow supplied by the forced air furnace passing through the hot air duct is transferred to heating fluid in the air-to-fluid heat exchanger, installed within the hot air duct. This heating fluid is then circulated through a radiant heating loop to heat a desired area.