The answer to 'why is my heat exchanger blowing cold air' is that there is likely an issue with your heat exchanger. The heat exchanger is a critical component of your HVAC system that transfers heat from the combustion process to the air that is circulated through your home. If the heat exchanger is not functioning properly, it can cause cold air to be blown instead of warm air.
Some common causes of a malfunctioning heat exchanger include cracks, corrosion, or blockages that prevent the heat from being effectively transferred. This can be caused by age, poor maintenance, or other issues with the HVAC system. If the heat exchanger is blowing cold air, it's important to have it inspected by a professional HVAC technician as soon as possible, as a faulty heat exchanger can be a safety hazard.