Abstract: A plate-type air-to-air heat exchanger is provided with a control device disposed directly adjacent the corner of the heat exchanger located between the cold air inlet and the warm air outlet. The control device includes a swingable baffle plate positioned adjacent the corner for partially closing off the cold air inlet directly adjacent the corner to deflect the inflowing cold air away from the corner to prevent build up of frost or ice. The baffle is controlled by a motor which in turn is preferably controlled in response to the outside air temperature.