My mom has a gas furnace in her condo. Directions on hot water heater says handles down for heat. When I put the handles down water drips from the large vents or ducts (not sure what they are called) above. No water drips if the handles are up for air conditioning mode though . Why does this happen?

If this is hot water heat there could be a leak in the hot water coil or other device like a air vent.