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 in your HVAC system that transfers heat from the combustion process to the air being circulated through your home. If the heat exchanger is cracked or damaged, it can cause hot air to be blown into your home instead of properly heated air.
Some common causes of a malfunctioning heat exchanger include normal wear and tear over time, corrosion, cracks from temperature changes, or a blockage preventing proper airflow. Symptoms may include hot air blowing from the vents, strange noises coming from the furnace, or a burning smell. If you suspect an issue with your heat exchanger, it's important to have a professional HVAC technician inspect and diagnose the problem, as a cracked heat exchanger can be a serious safety hazard.