A process and apparatus for heating fluids, particularly for heating cooking oil within a deep fat fryer. A fuel-air mixture is introduced and combusted within a combustion tube, preferably of either a power burner or a pulse combustor. The flue gases are discharged from the combustion tube, into a plenum. A baffle arrangement is used to direct the flue gases from the plenum through a passageway, creating flue gas flow in one direction over a primary portion of an outer surface of the fluid container, and then in a counterflow direction over a secondary portion of the outer surface of the fluid container. The flue gases are then discharged from the passageway into a flue stack. According to the process and apparatus of this invention, flue gas temperatures are reduced to about 200.degree. F.-400.degree. F., nitrogen oxides can be reduced to 10 ppm or lower, carbon monoxide is reduced to 4 ppm or lower, and the overall thermal efficiency of the fluid heater is increased to approximately 80-85 percent.