Can you give a little bit more information about your system? What type of A/C Unit, I assume it doesn't control heating, since you mentioned a furnace. Is it single zone? Does one unit control multiple zones? From what I can muster together, it could be that the fact that "there is no pressure," assuming you mean static pressure, means that the fan in the unit can't overcome the pressure in the duct to push air to your storage room, causing the relative humidity to build up, creating the psychrometric chart nightmare, causing an overwhelming amount of condensation. If that's the case, you need a new unit. But I don't have all the facts I need to discern a correct hypothesis.