The answer to 'why is my heat exchanger blowing hot air' is that there is likely an issue with the heat exchanger itself. The heat exchanger is a critical component of your HVAC system, responsible for transferring heat from the combustion process to the air that is then circulated through your home. If the heat exchanger is damaged or malfunctioning, it can cause hot air to be blown into your home, even when the system is not actively heating.
Some common causes of a heat exchanger blowing hot air include cracks or holes in the heat exchanger, a blocked or restricted airflow, or a problem with the blower motor. These issues can be caused by normal wear and tear, lack of maintenance, or a more serious underlying problem with the HVAC system. It's important to address the issue promptly, as a malfunctioning heat exchanger can be a fire hazard and may also release dangerous carbon monoxide into your home.