Abstract: A hot water heat exchanger system for a forced air furnace includes a radiator having a water inlet and a water outlet. A first water conduit is mounted in fluid communication between the water inlet and a hot water outfeed line of a hot water tank. A water pump is mounted on the first water conduit for urging hot water from the tank into the radiator. A second water conduit is mounted in fluid communication between the water outlet and a cold water infeed line into the tank. The radiator is mounted into a plenum of the furnace so that air forced from the furnace into outflow ducting of the furnace by operation of a furnace fan in the furnace flows through the radiator, wherein the fan is actuated by a temperature sensor in the furnace. The pump is thermostatically controlled by a first thermostatic sensor. A furnace heater of the furnace is thermostatically controlled by a second thermostatic sensor.