The answer to 'heat exchanger smells bad' is that a bad smell coming from the heat exchanger is typically a sign of a serious problem that requires professional attention. This could indicate a cracked or damaged heat exchanger, which can allow dangerous combustion gases like carbon monoxide to leak into your home. A bad smell may also be caused by a buildup of dust, dirt, or other debris inside the heat exchanger.
You should never try to inspect or repair the heat exchanger yourself, as this can be extremely hazardous. If you notice any unusual smells coming from your HVAC system, it's crucial to shut it off immediately and call a qualified HVAC technician to diagnose and fix the issue. They have the proper tools and training to safely inspect and repair the heat exchanger.