The answer to 'why is my heat exchanger freezing up' is that a frozen heat exchanger is usually caused by a lack of airflow over the coils. This can happen due to a clogged air filter, a blower fan issue, or a refrigerant leak. A frozen heat exchanger can damage the HVAC system, so it's important to address the issue quickly.
Some common symptoms of a frozen heat exchanger include poor heating performance, ice buildup on the coils, and reduced airflow. If you notice these issues, try changing the air filter first. If that doesn't solve the problem, you may need to call an HVAC technician to diagnose and repair the issue.