There are doors in the distribution case the distributes the air out of the vents. One of those doors is called a blend door. The dir that determines cold it hot is called a blend door. these doors can break and bind causing then to be stuck in one position not allowing cold air to come out of the vents. These doors are often controlled with electric motors. The most common cause is defective blend door motors. There is also a heater valve under the hood that allows or restricts engine coolant to the heater core. If this valve its stuck open that would also cause hot air to come out of the vents. This is probably a bad heater valve. A trained technician should be able to Determine the cause for you.