The answer to 'why is my heat exchanger not turning on' is that there could be several common causes, including a faulty thermostat, issues with the ignition system, or problems with the blower motor. The heat exchanger is a critical component of your furnace that heats the air circulated through your home, so if it's not functioning properly, your heating system won't work.
Some common symptoms of a malfunctioning heat exchanger include the furnace not turning on at all, the blower running but no heat being produced, or strange noises coming from the furnace. If you're experiencing any of these issues, it's best to have a professional HVAC technician inspect your system to diagnose the problem and recommend the appropriate solution.