Abstract: An economizer is provided for building heating systems and the like, of the type having a forced air furnace with adjacent combustion and heat exchange chambers, and a hot water reservoir or heater. The economizer comprises lower and upper heat transfer coils shaped for positioning adjacent the combustion and heat exchange chambers respectively of the furnace. An insulated standpipe interconnects the two coils, and the outlet of the upper coil is communicated with the inlet of the water heater. During use, water flowing through the economizer is heated in the lower coil and flowed to the upper coil to transfer additional heat to the air flowing through the heat exchange chamber, and simultaneously preheat water fed into the water heater.