Is it like the two black 2" hoses on the left and right in this pic:
If so, those direct air from the main axial cooling fan through your heat exchangers. When the heater valves are open, the air continues up to the cabin. When closed, the hot air is exhausted to atmosphere below the car.
If your hose is missing, cap off the outlet next to the fan until you have repaired it. A spray can cap works perfectly. Your engine's cooling system is losing air out the open hole (not good). Hope this helps.