The answer to 'heat exchanger not heating' is that the most common causes are a cracked or damaged heat exchanger, a problem with the blower motor, or a malfunctioning thermostat. Symptoms include the furnace not producing adequate heat, strange noises or smells coming from the unit, and inconsistent heating throughout the home.
If the heat exchanger is cracked or damaged, it will need to be replaced by a professional HVAC technician. This is not a DIY repair as it requires specialized tools and expertise to safely remove and replace the heat exchanger. Other potential solutions include cleaning or replacing the blower motor, checking and replacing the thermostat, and inspecting the ductwork for any blockages or leaks. While some basic troubleshooting steps can be done by a homeowner, it's generally best to call in a licensed HVAC technician to diagnose and repair issues with the heat exchanger.