The answer to 'why is my heat exchanger blowing cold air' is that the heat exchanger in your HVAC system may be malfunctioning or damaged. This is a serious issue that requires professional attention, as a faulty heat exchanger can be a fire hazard and can also lead to the release of dangerous carbon monoxide gas.
Some common causes of a heat exchanger blowing cold air include cracks or holes in the exchanger, a blocked or restricted airflow, or issues with the burners or ignition system. Symptoms may include inconsistent or fluctuating temperatures, unusual noises from the furnace, and a yellow or flickering pilot light.