The answer to 'air duct sealing leaking water' is that the most common cause is a breach or hole in the ductwork, which allows water to leak out. This can happen due to aging, damage, or poor installation of the ducts. Symptoms include visible water leaks, water stains, or mold growth around the ducts.
To fix this, the first step is to locate the source of the leak. This may require inspecting the ductwork visually or using a duct leakage test. Once the leak is found, the ductwork will need to be sealed using a mastic sealant or metal tape designed for HVAC applications. Extensive damage may require replacing sections of the ductwork.