Water in heating ducts is more common than you might imagine. Some houses built on top of concrete slabs have the heating and air conditioning ducts buried in the ground under the concrete. Water can enter these heating ducts when it rains heavily as the water table starts to rise and pushes water up into the soil under the slab and then into the ducts. It is easy to remove the water and to permanently stop the water from entering the ducts. You have to divert the ground water away from the house with a special outdoor trench drain.