a microwave oven has a cooking chamber in which an electric heater is provided to perform convection cooking as an alternative to high frequency cooking . heat from the heater is circulated within the cooking chamber by a heat - circulation fan situated behind an apertured back wall of the cooking chamber . a cooking chamber disposed behind the heater houses a motor for driving the heat circulation fan and also for driving a cooling fan disposed in the cooling chamber . the cooling fan draws in cooling air through cool air apertures formed in the rear of the oven . hot exhaust air from the cooking chamber is discharged through exhaust holes formed in the rear of the oven , next to the cool air apertures . in order to minimize the travel of hot exhaust air from the exhaust holes to the cool air apertures a guide duct mounted on the rear of the oven directs the exhaust air from the exhaust holes upwardly to a discharge outlet at the top of the duct .