The answer to 'why is my heat exchanger freezing up' is that a frozen heat exchanger is typically caused by a lack of airflow over the coil. This can happen for several reasons, such as a dirty air filter, a problem with the blower fan, or a refrigerant leak. A frozen heat exchanger can lead to the furnace shutting off, and if left unaddressed, it can cause further damage to the HVAC system.
Some common symptoms of a frozen heat exchanger include poor airflow, the furnace cycling on and off, and ice buildup on the coils or vents. To address the issue, you'll need to identify and fix the underlying cause, such as cleaning or replacing the air filter, checking the blower fan, or repairing any refrigerant leaks. In some cases, the heat exchanger itself may need to be replaced, which is best handled by a professional HVAC technician.