The answer to 'air duct sealing blowing cold air' is that air duct leaks can cause cold air to blow from the ducts instead of warm air. This is a common issue that can happen when the seals around the air ducts become damaged or deteriorate over time. The leaks allow cold air to escape the ducts before it reaches the vents, resulting in uneven heating and cooling throughout the home.
To fix this, the air ducts may need to be resealed using mastic sealant or metal tape. This is a job that can often be done DIY, but it's important to properly identify and seal all the leaks for it to be effective. If the issue persists or the ductwork is difficult to access, it's best to call a professional HVAC technician to inspect and repair the ducts.