Abstract: A method of preheating combustion air being supplied to a furnace, and a furnace including the means for preheating the combustion air. The method comprises circulating a portion of fluid from a fluid stream first in non-contact heat exchange relationship with the flue gas for heat collection, then circulating this portion of fluid in non-contact heat exchange relationship with the combustion air for heat donation thereto. The rate of circulation and temperature of the portion of fluid is controllable substantially independently of the rate of flow and temperature of the parent fluid stream. The furnace comprises means to heat a portion of a fluid stream and includes means to circulate, at a controllable rate and temperature, such portion of fluid from a fluid stream through an adjunct loop connected to the fluid stream for collection of heat from the flue gases and donation of heat to the combustion air.