The answer to 'why is my heat exchanger not heating' is that the heat exchanger may be malfunctioning or damaged. The heat exchanger is a critical component in your furnace that transfers heat from the combustion process to the air that circulates through your home. If the heat exchanger is cracked, corroded, or not functioning properly, it will prevent your furnace from effectively heating your home.
Some common symptoms of a failing heat exchanger include uneven heating, strange noises coming from the furnace, and higher than normal energy bills. In some cases, a malfunctioning heat exchanger can also be a safety hazard, as it may allow dangerous combustion gases to leak into your home. If you suspect an issue with your heat exchanger, it's important to have a professional HVAC technician inspect and diagnose the problem as soon as possible.