The answer to 'why is my heat exchanger not turning on' is that there could be several common causes, including a faulty thermostat, a problem with the furnace's control board, or issues with the heat exchanger itself. Some key symptoms to look for are the furnace not turning on at all, the blower fan running but no heat, or strange noises coming from the furnace.
If the heat exchanger is not functioning properly, it's important to have a professional HVAC technician inspect and diagnose the issue. Attempting to repair or replace the heat exchanger yourself can be dangerous, as it involves working with gas lines and high-voltage electrical components. A qualified technician will be able to safely identify the root cause and recommend the best solution, whether that's repairing the existing heat exchanger or replacing the entire furnace.