A heating and air conditioning device includes a warm air path that leads through a heat exchanger to a mixing chamber and to outlets of the heating and air conditioning device, and a cold air path that leads from the evaporator to the mixing chamber and the outlets of the heating and air conditioning device. A warm air duct is located at an end of the warm air path and an entrance to the mixing chamber. A cold air duct is located at the end of the cold air path and the entrance to the mixing chamber. One or more openings are formed on one of a side wall of the warm air duct and a side wall of the cold air duct so that air can flow from the cold air path into the warm air duct and from the warm air path into the cold air duct.